This is default featured slide 1 title

Productivity shows a decline in automative repair shops

Output per hour of persons employed in these shops fell at an annual rate of 1.2 percent between 1972 and 1986, reflecting a larger increase in employee hours than in output John G. Olsen and RICHARDS B. CARNES

Output her hour of all persons(1) in the automative repair shop industry(2) decreased at an average annual rate of 1.2 percent between 1972 and 1986. During this period, productivity in the private nonfarm business sector rose at an annual rate of 0.8 percent. The overall productivity decline reflects a 3.0-percent average annual increase in output and a corresponding larger growth in all person hours of 4.3 percent. (See table 1.)

Despite increased efficiencyd in some specialty repair shops, overall productivity for the automotive repair shop industry has declined since 1972. Factors contributing to this decline include a large influx of new establishments and workers in the industry, a shortage of adequately trained mechanics, the introduction of more complex cars and tricks, as well as the effect of several recessions in the U.S. economy during the 1972-86 period.

The output per hour rates for automotive repair shops varied substantially from year to year. Since 1972, annual increases in productivity have occurred in 6 years, ranging from 0.5 to 8.7 percent. Declines in productivity occurred in 8 years, the largest in 1982 when output per hour dropped 6.3 percent.

The auto repair shop industry is affected by cyclical changes in the economy. During periods of economic contraction, output in the automotive repair shop industry slows or falls, and productivity tends to decline. During an economic downturn, industry output may grow because of maintenance and repair expenditures on older motor vehicles, but be offset by declines in disposable income and new motor vehicle sales. This would slow the rate of growth in industry output or lead to a decline in output. Cyclical influences on the automative repair shop industry can be seen by examining subperiod trends.

From 1972 to 1974, productivity in the automotive repair shop industry increased at an annual rate of 2.1 percent, as output rose 5.8 percent and all employee hours grew 3.7 percent. Reflecting a general downturn in the economy, productivity declines 5.6 percent in 1975, as output dropped 0.2 percent. Between 1975 and 1979, output per hour fell slightly, 0.6 percent per year, as hours (5.3 percent) grew faster than output (4.6 percent). The recession of 1980 and 1981-82 had a more adverse effect on the industry than the previous recession. From 1979 to 1982, productivity experienced its largest decline, dropping 4.2 percent per year, as output fell at an annual rate of 3.1 percent. Automotive repair shops shared in the economic recovery that began in 1983. Productivity grew 2.5 percent per year between 1982 and 1986, as output rebounded at a 9.0-percent annaul rate and hours increased 6.3 percent per year.

Industry structure

The automotive repair shop industry consists of establishments primarily engaged in the repair of automotive tops, bodies, and interiors; repairing and retreading automotive tires; automative painting and refinishing; general automotive repair; and specialized automotive repair, not elsewhere classified, such as fuel service (carburetor repair), brake relining, front-end and wheel alignment, exhaust system (muffler) repair, radiator repair, and glass replacement and repair. Automotive repair shops compete in a broad service and parts market. The automotive service market is heterogeneous in its structure, ranging from new car and truck dealers and self-service fleets to gasoline service stations and independent repair shops. In addition, a large number of motor vehicle owners perform some or all of their own repairs. Automotive repair departments maintained by establishments engaged in the sale of new automobiles are classified in retail trade, as are gasoline service stations (where sales of merchandise, including fuel, exceed repair receipts).

The automotive repair shop industry is characterized by a large number of small firms. In 1972, there were an estimated 127,203 establishments operating in the industry. By 1982, the industry had grown to 179,093 establishments. Almost half of these establishments had payroll in 1982. The number of paid employees in establishments with payrolls averaged 3.6 in 1972, 3.9 in 1977, and 4.1 in 1982. Many of these establishments are owned or operated by partners or sole proprietors. In 1982, partners and proprietors made up almost 80 percent of the ownership of all establishments and accounted for more than 60 percent of all persons in the industry.

In 1986, the industry generated $32.0 billion in receipts with a work force of about 780,000. Small establishments accounted for the majority of industry receipts. More than 75 percent of all automotive repair shops with payrolls had sales of less than $250,000 in 1982.

Output and demand

In spite of several economic downturns, overall output of the automotive repair shop industry increased 3.0 percent per year between 1972 and 1986. In comparison, over the same period, output for the private nonfarm business sector increased an average of 2.5 percent per year.

Industry output growth reflects, in part, increases in the number of motor vehicles in operation. Passenger cars in operation increased at an average annual rate of 2.0 percent between 1972 and 1986. The number of trucks in operation also increased over this period, growing 5.7 percent per year.(3) An increase in the average age of cars and trucks also has contributed to the growth in output. The median age of passenger cars has grown steadily from 5.1 years in 1972 to 6.9 years in 1985. The median age of trucks grew from 6.0 years in 1972 to 7.6 years in 1984.(4)

The industry’s output growth generally paralleled the trend for the overall economy. Between 1972 and 1979, the industry’s output rose at an average annual rate of 3.9 percent. Output increased in 6 of the 7 years over this period, falling only in 1975 with the downturn in the economy. In 1979-82, output declined an average of 3.1 percent per year. Recessionary conditions in 1980 and 1981-82 contributed to the weak demand experienced during this period. Reflecting the general economic recovery since 1982, output experienced a sharp turnaround, rising at a 9.0-percent annual rate from 1982 to 1986.

Auto repair shops have boosted their share of the automotive service and parts market during the last 10 years, increasing from 25 percent in 1976 to nearly 28 percent in 1985.(5) New car dealers, who have enjoyed the largest share of the service and parts market, and gasoline service stations declined during this period. The percentage of sales of the automative service and parts market (including tire sales) during 1985 was as follows: franchised new car dealers, 33 percent; automotive repair shops, 28 percent; gasoline service stations, 8 percent; tire, battery, and accessory dealers, 22 percent; mass merchandisers, 7 percent; and all others, 3 percent.

Between 1972 and 1986, the numbers of full service gasoline stations fell from 226,459 to 120,150, a 47-percent decline.(6) Self-service stations ahve taken their place. Large corporations, chain organizations, and franchise operations are claiming some of the business that the full-service stations are giving up. Specialized auto repair shops have takend a large part of the muffler and brake repair businesses. In addition, “quick lube” shops are increasing their share of the oil change market.

The distribution of industry receipts by type of automotive repair shop showed little change between 1972 and 1982. In 1982, about 44 percent of all industry receipts were generated by general automotive repair shops, more than 26 percent by top and body repair shops, and almost 30 percent by other automotive repair shops. The distribution of establishments by type of operation, however, experienced a slight change over this period. General automotive repair shops accounted for 61 percent of all establishments in the industry in 1982, compared with 56 percent in 1972. However, this group’s share of total industry receipts declined slightly over this period, falling from 45 percent in 1972 to 44 percent in 1982. This trend reflects the continuing entry of small establishments into general automotive repair. Although top and body repair shops and all other automotive repair shops’ share of total industry establishments dropped between 1972 and 1982, their proportion of total industry receipts increased slightly over this period. These trends reflect the growth of franchised operations among specialized auto repair shops.


Total employment in the automotive repair shop industry grew steadily from 415,700 in 1972 to 780,300 in 1986, at an average annual rate of 4.6 percent. In comparison, the private nonfarm business sector experienced a 2.1-percent rise in employment over the same period. The hours of all persons in the industry increased at a slightly lower annual rate of 4.3 percent because of a small drop in average weekly hours. The hours of nonsupervisory workers, for example, declined slightly from 39.9 in 1972 to 38.6 per week in 1986. Hourly earnings of nonsurpervisory workers in automotive repair shops averaged $8.17 in 1986, compared with $8.76 for the total private economy and $8.16 for the total service sector.

While the number of self-employed workers remains very high, establishments with paid employees have increased since 1972. Employees accounted for almost 60 percent of all persons in the industry in 1982, compared with 59 percent in 1977 and 56 percent in 1972. Self employed workers dropped slightly, from more than 40 percent of all persons in 1972 to less than 38 percent in 1982. There was little change in the number of unpaid family workers; they accounted for about 3 percent of all persons in 1972, 1977, and 1982.

The decline in the number of full service gasoline stations has contributed to the large growth of employment in the industry between 1972 and 1986. Since the early 1970’s, full service stations have declined in number, as oil companies have consolidated small stations into larger self-service facilities. As a result, the number of automotive repair shops has increased to absorb the former service station operators and repair services.

The industry’s work force is dominated by persons in mechanic, installer, and repairer occupations, who made up almost 51 percent of total employment in 1984 (the latest year for which data are available).(7) Within this occupational group, automotive and motorcycle mechanics represented the largest category, accounting for 27 percent of employment in the industry. Automotive body and related repairers, the next largest category, accounted for more than 16 percent of the industry work force. Bus and truck mechanics and diesel engine specialists made up another 3 percent of employment.

Factors affecting productivity

One factors affecting productivity growth in the automotive repair shop industry is the small size of many of the shops. Small firms have limited resources in capital, personnel, and meterials. Although there are little data of capital investment for this industry, it is clear that the small average size of establishments in the industry makes it difficult for the average firm to invest in new capital equipment, such as computerized diagnostic equipment. Automotive repair shops also have limited access to manufacturers training programs and data services.

The addition of more than 50,000 establishments and 200,000 workers to the industry between 1972 and 1982 has also influenced the movement of productivity over the period studied. The apparent ease of entry and exit from the industry has led to increases in the number of establishments and workers in the industry, even during periods of general economic recession. The large growth in employment, together with high separation rates for some occupations, has affected the overall experience level of workers in the industry. Garage- and service station-related occupations, which include some workers in the automotive repair shop industry, had high separation rates, as measured by the percent of workers leaving these jobs in 1981 and 1983.(8) This influx of new firms and less experienced workers has contributed to the small, average size of industry establishments and the decline in output per hour during this period.

Another factor affecting productivity between 1972 and 1986 has been the introduction of more complex engineering in the design of cars and trucks. Automotive technology has changed significantly since the early 1970’s. During the 1970’s, advances in automotive engineering were concentrated in the areas of safety, emissions, and fuel economy. Although these changes included some improvements in serviceability, such as longer intervals between oil changes and ignition system tuneups, they have increased the complexity of many repair jobs. The downsizing of cars to reduce fuel consumption, along with the addition of numerous electronic components, has turned some routine maintenance jobs into major operations.

In the 1980’s, the general trend has been toward the introduction of more subsystems of increasing sophistication and complexity. The increasing use of computer microprocessors in newer vehicles to control engine performance, transmission, and the suspension systems has also changed the complexity of repair work. According to a recent industry study, 83 percent of the mechanics surveyed indicated that newer cars are more difficult to repair than 10 years ago because this complexity makes it more difficult to pinpoint problems.(9) The skill and equipment mix generally found in small automotive repair shops often are ill suited for such sophisticated diagnosis and repair work.

According to some automotive service industry analysts, a shortage of adequately trained mechanics also has affected industry productivity. Technological innovations have occurred so rapidly, particularly in automotive electronics, that it is difficulty or impossible for many small repair shops to keep up with these changes. Small and medium sized repair shops often cannot afford to let mechanics take time off to learn the latest technology. As a result, worker skills are not keeping pace with new automotive technology.

Outlook for productivity

Long term prospects for demand in the overall automotive service industry should be good, as the automobile continues to play a key role in transportation. Future output growth will reflect further increases in the number of vehicles in operation and a modest rise in vehicle miles traveled. It is unclear, however, what share of the market will belong to the automotive repair shop industry. A smaller market share will reduce any opportunity for future productivity gain, especially for smaller operations unable to purchase needed capital equipment.

New automobiles are expected to continue to incorporate even more complex technology. Auto manufacturers, for example, plan to use onboard computers to chemically analyze oil, fuel, and radiator coolant, detect wear and tear in mechanical parts, and electronically readjust the engine to compensate. With the growth of computerized and fuel injected motor vehicles, new cars will require more sophisticated electronic diagnostic service.

Because of the shortage of mechanics with advanced training and the need to keep up with rapid technological innovations, some industry analysts foresee a decline in the percentage of service work performed by auto repair shops and a growth in business for new car dealers and large chain stores. Some new computerized diagnostic equipment developed by auto manufacturers, for example, will only be available to authorized dealers. Another trend affecting industry output has been the lengthening of service contracts by new car dealers on new cars and trucks. These extended warranties may lower demand for some specialized auto repair shops, such as engine rebuilders. To improve productivity and to remain competitive, automotive repair shops need to invest in new equipment, and provide advanced training.

The occupational composition of the work force for the automotive repair shop industry is not expected to change significantly during the next decade. Based on projections by the Bureau of Labor Statistics, the proportion of mechanics, installers, and repairers is expected to increase from almost 51 percent of industry employment in 1984 to about 52 percent of industry employment in 1995. Within this occupational group, automotive and motorcycle mechanics are expected to grow from about 27 percent of industry employment in 1984 to nearly 29 percent in 1995. The share of employment among automotive body and related repairers is expected to fall slightly during this period. Administrative support occupations, including clerical, are projected to decline from 10 percent of industry employment in 1984, to about 9 percent of industry employment in 1995. This trend reflects, in part, a greater use of computers in the industry in the future. The availability of more affordable and more powerful personal computers has made the technology feasible for small shopowners. Among other functions, computers will be used to perform recordkeeping and administrative functions formerly done manually.


(1)All average rates of change are based on the linear least squares trends of the logarithms of the index numbers.

(2)The automotive repair shop industry is classified as SIC 753 in the 1972 Standard Industrial Classification Manual and its 1977 supplement issued by the U.S. Office of Management and Budget.

(3)Based on statistics published in Ward’s Automotive Yearbook (Detroit, MI, Ward’s Communications, Inc., 1987).

(4)Motor Vehicle Facts and Figures, 1986 (Detroit, MI, Motor Vehicle Manufacturers Association, 1986).

(5)NADA Data for 1986 (McLean, VA, National Auomobile Dealers Association, 1986), p. 10.

(6)Based on statistics published in Natioinal Petroleum News Factbook, 1987, p. 123.

(7)Bureau of Labor Statistics, data for 1984-95, National Industry Occupational Matrix.

(8)Occupational Projections and Training Data, Bulletin 2206 (Bureau of Labor Statistics, 1984), p. 10; and Occupational Projections and Training Data, Bulletin 2251 (Bureau of Labor Statistics, 1986), p. 18.

(9)”Mechanics Struggle to Keep Up With Engineering Advances,” The Washington Post, June 14, 1987, p. D7.

TABLE: Table 1. Indexes of output per hour of all person and related data, automotive repair shops, 1972-86 (1977=100)

TABLE: Output per
TABLE: Year hour of all Output per Output Hours of all All
TABLE: persons person Persons persons

Prevent corrosion of car parts completely

Nobody wants the owner of the car and saw the car looks boring. It can also be very rusty. A beautiful view and can arise if the media is usually in the use of clean and free of corrosion. To keep the car from the permanent risk of erosion, there are some tips that can be applied. The first part is of course always a car wash on a regular basis. But, do not wash dirty. It should be right on target. One way is always dry some parts usually flooded. Including small folds can be difficult.

Furthermore, it will be more dangerous rust because cars are typically used in areas containing salty seawater. Or even in the region, has a ratio of water mixed iron. If you are interested in real time and can also use the liquid antioxidant as 3M, Tuff-Kuti dinol, Barracuda.

But there is a constant enough to proven tips. Any use of marine paints or Agatha Navy. Usually, this item is used for charging. Nature is to protect the paint against the risk of corrosion. That is, if you are spoiled to have to paint the bottom of the device alone.

Traffic police diameter Supplies

“Trailer” is associated with the issue of lifting or carrying your car or home. Provides the main difference trailer with crane system is integrated vehicle or trailer on the instruments in a single unit and ready to work almost anywhere, while usually cranes working in factories. When using a crane outside the factory, and parts of the crane to make the procedure time. Can tow trucks and other vehicles of small dimensions taken almost anywhere, and the lack of interchangeable parts to facilitate different types of, towing. Tow truck can lift and offer many things at once, without any damage, in less time portable crane.

Technology Focus – Morgan Cars: The high-technology future of the Classic British Sports Cars

A recent visit to Morgan Cars prompted to write this article. I was really interested to see how the mix of classic heritage skills is being blended with high-tech Engineering to produce really great, desirable cars. This is a picture you can see mirrored at other strong British automotive brands (JLR, Aston Martin, Bentley Motors etc.).
Morgan Cars is a high value brand, associated with the traditional, high quality craft skills needed to create a classic British sports car – one that creates excitement and enthusiasm for the driver. Thus, buying and owning a Morgan is a really special and personal experience, knowing that you have invested in a vehicle that has been designed, created, Engineered and manufactured with exceptionally high precision and care. This has been the Morgan tradition and hallmark for many years!
Times are changing though, legislation in relation to safety and exhaust emissions are the main drivers for technological developments in Automotive Engineering. Customer expectations are high with respect to performance, drivability and emission compliance – and Morgan has no exemption here! So the question is – what is Morgan doing to meet these challenges. The answer is that Morgan is investigating a number of technologies to investigate and meet future challenges. A considerable undertaking when you consider that one of the constraints is to retain the heritage and tradition of the brand and the marque!


In general, light weighting concepts have a number of benefits – advanced materials have superior stiffness, providing improved handling chassis. The lighter overall weight reduces inertia – improving acceleration and cornering performance. However, a real benefit in fuel consumption (and reduced emissions) can be gained by reducing vehicle mass as much as possible (whilst maintaining structural integrity). Morgan has successfully experimented with magnesium for body structures – which is the lightest structural metal available (30% less dense than aluminium). The use of sheet magnesium for vehicle structural applications requires hot-forming, increasingly being adopted by premium car manufacturers, as this process can produce large, complex body panels. Morgan intends to adopt the newly developed technologies (produced by an experimental project) on its next generation of premium sports cars.
Magnesium has significant benefits for manufacturing car body panels – mainly its strength combined with light weight, to reduce vehicle mass

This is a general term used in Automotive Engineering covering numerous applications of applying electric drives and motors in order to provide power for accessories, or traction – but only when needed (for example electric power steering). Morgan has experimented with the option of a full electric power train, a project known as the Plus-E. An electric sports car with a five-speed manual gearbox, designed by Morgan with the support of British technology specialists Zytek and Radshape. This was developed as a concept vehicle to test market reaction, but the radical new roadster could enter production if there is sufficient demand.
The Plus-E electric concept vehicle – could be coming to a Morgan showroom near you soon
This vehicle combines Morgan’s traditional look with high-technology construction and a power train that delivers substantial torque – instantly at any speed! This is combined with a manual gearbox to increase both touring range and driver engagement. The Plus E is based on an adapted version of Morgan’s lightweight aluminium platform chassis with power provided by a new derivative of Zytek’s 70kW (94bhp) 300Nm electric machine (already well proven). The power unit is mounted in the transmission tunnel and drives the rear wheels through a conventional five-speed manual gearbox. However, the system has sophisticated electronic controls to synchronise the motor speed and torque during shifts, to provide a seamless gear change with minimal interruption of traction for a perfect gear shift. The combination of multi-speed transmission and high torque e-machine allows operation of the motor at maximum efficiency, for as much of the time as possible, whilst providing the best possible performance for the driver experience. The project is future oriented and encompasses the exploration of alternative transmission types (CVT, DSG) as well as different battery chemistry options.


Morgan employs state-of-the-art Engines, supplied by leading Automotive Manufacturers. These power units are integrated into the overall Morgan chassis and then calibrated to adapt them to the unique character of the Morgan vehicle. The power units are selected specifically to incorporate the latest technologies for emissions reduction and engine efficiency. For example, the V8 power unit employs direct injection – this technology improves efficiency at part load due to the fact that no throttling of the engine is needed to control power output (it’s controlled by injected fuel quantity). In addition, knock resistance (knock is a limiting factor for the efficiency of a gasoline engine) is improved by advanced fuel injection systems that use high pressures, to provide a well prepared fuel/air mixture – this is advanced technology but, the current state of the art engines now include downsizing or down-speeding concepts in order to reduce friction losses and operate the engine at maximum efficiency for as much of the time as possible. Engines with high specific power outputs, based on turbo-charged/boosted concepts, are expected to dramatically increase their market share within the next five years. Further down the pipeline, engines will evolve again to meet ever changing and more challenging targets – technologies such as Variable valve lift, Variable compression ratio and variable ancillary drive systems (oil and coolant pumps) will become mainstream, in addition to energy recovery (thermal and kinetic – as used in Formula 1 from this year) – this area is very promising technology to improve overall power train efficiency.

Energy recovery – Formula 1 technology that could be used by Morgan cars to improve the efficiency of the overall power train. Thermal heat recovery is also applicable (now applied in Formula 1 cars)


The transmission and the engine have to be considered and optimised together in order to provide a harmonised power unit that delivers the performance expected by the driver, combined with meeting legislative demands. Manual transmissions are common with 5 or 6 ratios. However, in the near future, more ratios are needed, in conjunction with automation of shifting and control, in order to keep the engine operating in the optimum fuel consumption range. It is suggested that 10 speed transmissions will be needed and common place (in DSG form). This could be combined with an electric machine for low power requirements at low speed. Electrified transmissions with electronic control can be used to reduce fuel consumption and emissions in several ways – The e-machine can provide power at low or zero speed where the combustion engine is very inefficient. The shift control strategy can be combined with engine control to give the optimum shift point for maximum efficiency. Also, the electric machine can be used to provide seamless shifting and constant tractive power. There are a number of transmission concepts available and in use, but no clear leader. If integrated into a Morgan Cars power train, the transmission concept chosen will have to support the performance and driveability that matches the marque!
The Eva GT – tomorrows Morgan available today! High technology, advanced design, stunningly attractive!

A combination of future technologies has been combined in the Morgan life car project. This prototype received a rapturous response and according to some sources, Morgan has decided to take it from a prototype to a fully-fledged production vehicle. There have been some changes to the original brief, making the car more practical, while retaining the revolutionary features that made LIFE car unique.
The proposed vehicle now includes a super-efficient, series hybrid drive train, developed using some of the country’s best universities, making use of the wealth of knowledge in their research departments. The drive train will power a vehicle that epitomises Morgan core value of innovation. The use of sustainable lightweight materials will ensure that not only is the vehicle fuel efficient, with a low carbon output, but that at the end of its very long life, it will be easily recyclable. The goals set are for a vehicle
  • 1000 mile range
  • Ultra lightweight (sub 800kg)
  • 15 mile EV range
  • 0-60mph in 7 seconds
  • ~£40,000 Price

The Morgan life car project – Next generation of Morgan sports car combing light weighting with an advanced powertrain.


There is no single technology that will secure the future for Morgan, or any other manufacturer. Even the mainstream manufacturers are gambling with a combination of low carbon technologies in order to meet or achieve current and forthcoming requirements. It could be considered that Morgan cars, as a manufacturer of ‘niche’ vehicles does not need to lead but just follow industry trends. However, that is not the Morgan way! Even though production volumes are low (compared to the mass market), innovation and technology are within the Morgan DNA. As the only remaining, true British manufacturer, Morgan takes its responsibility to be a leader very seriously. A clear example of this is the position Morgan takes in this area, with many research projects and collaborations with leading universities, who can undertake the research task and produce tangible technology that can be ported into production by Morgan.
There is no doubt – Morgan is a leader in pushing the boundaries of design and technology for Classic British sports cars, and will continue to do so for many forthcoming generations.

Combustion Pressure measurement for efficient Engine Diagnostics

Measuring cylinder pressure in an engine, in order to establish the combustion efficiency and losses is a well-established, widely used technique. In fact, it goes back to the days of steam engines!
The in-cylinder pressure, with respect to crank angle is important for understanding the rate of energy release, and to be able to understand the amount of work done in the cylinder prior to transfer to the crankshaft, allowing losses to be established. However, these measurements normally require sophisticated equipment with specialised sensors. In addition, the engine normally requires some level of modification or adaption in order to be able to access the required measured parameters (cylinder pressure and crank angle). For these reasons, cylinder pressure measurements are normally the reserve of research and development environments.


Figure 1 – Cylinder pressure plotted against volume giving the classic diagram from which the Indicated mean effective pressure is derived

Figure 2 – Pressure vs. crank angle – gasoline engine running in knock condition

In theory though, measuring pressure in the cylinder for diagnostics is quite feasible these days, this is due to the reasonable cost of high speed measurement and recording equipment available to the after-market for sensor and actuator signal measurements (e.g. oscilloscopes). These are normally applied specifically for fault diagnosis of vehicle electronic systems, but these devices are easily capable of measuring a signal from a cylinder pressure sensor, of a suitable type, installed in the engine cylinder.
Another more recent development is the availability of sensor technology of appropriate durability, with scalable output ranges, that come with appropriate non-intrusive adaptors which allow the sensor to be installed into the cylinder in place of the spark plug. This technology has facilitated a trend towards examining pressure traces, in diagnostic procedures, in order to reduce the amount of time spent getting to the root cause of difficult to trace faults, especially those which generate non-specific or misleading fault codes.
But why does the cylinder pressure trace help us? and what are we actually looking at? Also, how we can interpret the data effectively to make good diagnostic judgements. A good place to start is the system set-up…


Figure 3 – Overview of system configuration for cylinder pressure measurement (Source: LHM Engineering)

The diagram above provides a system overview. Basically, the scope hardware is connected to a PC and this is the data acquisition system. The transducer is remotely mounted (from the cylinder) and produces an analogue voltage in response to the pressure applied to it. This pressure comes from the combustion chamber via a pipe and adaptor which takes the place of the spark plug. The diagram below shows an actual installation ready for measurement. Note that the target cylinder must be ‘disabled’. Normally this can be achieved by disconnecting the electrical connector to the fuel injector (where 1 cylinder is to be disabled – running test). For a cranking test, the CPS (Crankshaft position sensor) should be disconnected which prevents starting of the engine in full (normally no fuel or spark).


Figure 4 – A typical installed sensor, connected to the engine via a pipe/spark plug adaptor (Source: LHM Engineering)

For this type of measurement, we have to consider the boundary conditions as there are 2 main limiting factors to consider with respect to the acquired data for diagnostics.
1.We have to remove the spark plug and measure the motored pressure curve – so we need to motor the engine – either with the other cylinders firing, or via motoring with the starter motor. This provides a motored pressure curve from which much information about engine health and general condition can be gained. However, it’s clear that firing is not possible and hence the engine cylinder is not working under it’s true thermal and loaded conditions
2.The data is sampled in the time domain – that is, the scope will sample with a regular sample rate with respect to time, not engine position (although this can be derived subsequently). Time based sampling alone means that the engine position and cylinder volume can only be estimated from the raw data. Hence accurate calculations that would involve cylinder volume are not really possible (they would be too inaccurate), an example of these calculations would be the Indicated mean effective pressure (IMEP) which gives a measure of the work done in each engine cycle. Due to this, the data which is measured is only suitable for calculating direct results – those which are derived from the raw data alone – for example, the peak pressure value, or the rate of pressure rise.
However, the motored curve can be extremely useful and can tell us a lot about the general condition of the engine. When motoring, the engine cylinder it effectively becomes a simple air compressor and expander (rapid compression machine), of course, this is not a very useful type of machine but by examining the pressure curve, we can establish an idea regarding how efficient the engine behaves in this mode of operation, and that’s important because in between the compression and expansion part of the engine cycle, the four stroke engine is effectively a pump, expelling the burnt gases and drawing in the fresh charge, this part of the engine cycle, known as the gas exchange, is essential for efficient and effective combustion – optimising this part of the engine cycle is of high interest to engine developers. It worth noting that most instability and variation is related to the combustion event itself – when measuring a motored cylinder, there is none of the errors or variation relating to combustion, therefore the repeatability of the motored curve is excellent and there are a number of useful metrics that can be derived from this curve to assist diagnostics
Curve analysis

Let’s look at the raw pressure curve with respect to the motored engine cycle. The diagram shows the full cycle, with phase markers:

Figure 5 – A full engine cycle, separated into each engine stroke phase with markers (Source: LHM Engineering)

You can see clearly each of the four strokes. Compression and expansion during the motored curve is wasted energy – some of the energy is converted to heat in the process and subsequently lost (rejected to the surrounding engine thermal mass) – there is no work done but we can establish the peak cylinder pressure from this curve as a metric for general engine condtion. It’s obvious that any significant cylinder leakage (via the piston rings, head gasket or valves) will reduce the peak value generated, this will be obvious in a cylinder to cylinder comparison – however, the whole cycle curve can give us much more information about the possible reasons for reduced compression, as opposed to just indicating that reduced compression exists


Figure 6 – Full cycle pressure curve

Note – The compression and expansion ratios are design factors of the engine optimised to give the best possible efficiency from the engine and combustion system design – therefore a loss of compression due to worn components gives a considerable loss in engine cycle efficiency



Figure 7 – The high pressure part of a motored cycle – this should be presented as a nice,
smooth curve with good symmetry – in this diagram, two completely separate measurements on different engines are overlaid – both curves can be considered as showing a ‘good’ condition engine
Relative loss of compression pressure is not just due to leakage factors – it could also be due to engine throttling or inefficient breathing due to worn valve gear components or incorrect valve timing or clearances. These breathing problems normally impact on the pressure curve dynamics. You can see from a typical measured curve that there are resonance effects during the gas exchange part of the cycle.


Figure 8 – Similar to the above diagram, but focussed on the gas exchange part of the cycle, as before though, the measurements are very similar in form. The resonance during gas exchange can clearly be seen on both curves – this diagram shows that even different engine display similar, common characteristics on the motored curve

When using a remote sensor (i.e. a sensor connected to the engine via a pipe). It is likely that oscillations are generated due to the air passage between actual the sensor membrane and the in-cylinder air volume. However, these flow dynamics should not vary significantly between cylinders on the same engine – as the sensor and pipe, as well as the cylinders should all be the same (more or less) with respect to dimensions and physical properties. Therefore, any small difference on the curves will be due to the flows within each cylinder and can thus be used for diagnostics. In particular, it is worth studying the baseline of the pressure curve, plus the amplitude and frequency of the resonance. However, try to be sure that when making measurements between cylinders for comparison, that the cylinder conditions are as similar as possible, in particular with respect to engine speed and cylinder temperature during the measurement.


Figure 9 – Pressure sensor installation directly into the cylinder for R&D measurement applications


Figure 10 – Two separate measurements compared – one made using a scope and remote sensor (lower), one using an in-cylinder, direct installed pressure sensor (upper) – you can see much less pressure wave resonance where the sensor has no connecting pipe, as in the latter case – however, the pressure wave dynamics can still be useful in diagnostic procedures

In addition to high–pressure measurements (i.e. within the cylinder) a useful approach is to monitor low pressure effects – specifically, in the exhaust and inlet. With a suitably calibrated sensor, the pressure dynamics, pre and post combustion chamber, can be easily gained and are useful to help on the diagnostic pathway! In terms of the diagnostic process. It is very worthwhile to try and measure the low pressure effects first, as installing the sensor for this task is easier and less effort – this helps to gain some insight to the root cause of a problem with lower initial effort. The low pressure dynamics can also highlight breathing issues and flow issues, in addition, by measuring other signals and using them a phase markers  (for example, a cylinder specific ignition pulse), cylinder specific related  issues can often be identified.

Figure 11 – Inlet and exhaust measurements – in this case highlighting a problem with a specific cylinder (Source: Pico Automotive)

The diagram above below comes from a diagnostic procedure where a cylinder misfire was apparent but the root because not that clear. In this case the manifold pressure and exhaust pressure were measured and as you can see from the inlet trace, a cylinder specific issue could be seen on the signal. This allows the diagnostic technician to know that there was a problem with one of the  cylinders, with the breathing on the inlet side. The root cause in the end being a valve clearance issue. There are similar case studies in the public domain that highlight the value of using pressure measurement to support diagnostic studies looking for classical mechanical faults, which cause an electronic failure mode or warning via OBD system (the OBD system is often considered to be able to identify electronic related failures only – often though the root cause can be a mechanical issue).


In conclusion, its clear that pressure measurement can support efficient diagnostics, whether the failure is electronic or mechanical. The equipment available for this measurement technique is now easily available and reasonably priced. If you buy the kit, keep it handy in the workshop and practice making measurements on a regular basis. You will build up knowledge and be confident to carry out the process whenever needed. In addition, regular use can shorten diagnostic time, increase efficiency and shorten return-on-investment time after purchasing the kit.
However, you will still need a well-defined process to support your diagnostics in this specific area. A suggested approach, using a Picoscope or similar could be:

1. Collect, identify and clear any fault codes

2. Carry out a compression test to establish mechanical health as an initial test – ideally using an non-intrusive method – note any cylinder specific effects or deviations greater than 10% between cylinders

3. Measure inlet pressure and examine closely the dynamics – using a reference pulse check if any cylinder specific issues correlate with the compression test data

4. Measure the exhaust pressure and pulse – check dynamics as above

5. If a deficient cylinder is identified, instrument the cylinder with pressure sensor and measure some traces (disable cylinder firing). Analyse raw curves

6. If in any doubt, measure some pressure curves from another cylinder for comparison during analysis – compare peak pressure values, plus the pressure wave dynamics during gas exchange – in particular pressure pulses resonances and equalisation ramps

Following this process should take you from the least intrusive method, through the more involved procedures, but in the right order. So that if you uncover the problem ‘on the way’ then you don’t need to proceed further – unless of course you have the time to spend to validate your findings! This approach will ensure that you get to the root cause a quickly as possible – ensuring an efficient process and a ‘fast-time-to-find-fault’.

More information
For more background, take a look at the information below:

  • LHM Engineering – an excellent application note covering “Understanding Running Compression”
  • Pico Automotive – Excellent case study by Steve Smith – “Subaru cylinder misfire”
  • Autoelex – An overview of engine indicating and combustion measurement – “Engine Indicating technology”
  • SAE – A book on combustion pressure measurement – “Engine Combustion: Pressure Measurement and Analysis”

Building Your Own Computer – Fixing Cars

During the early days of the PC, assembling your own computer was easy to do and saved money. Today, these are disposable and inexpensive consumer items. Are cars going the same way?

I started playing with computers when I was 13 years old, back in 1973. Back then, we programmed FORTRAN and BASIC on time-shared mainframes, or with punchcards. As I grew up, the computer industry grew up as well. And by the time I was and adult, things that seemed like science fiction in 1973 were a reality.

During the early days of the PC, it was typical to buy a computer as a series of components and then assemble them, install an operating system, format the drive and start working. We’d peruse catalogs and magazines for the latest and greatest in processors, memory, hard drives, display cards, and the like, and then design a system to our liking and build it. Or we’d look for used components on the cheap and build a system on a budget.

Sometime in the mid-1990’s this model became obsolete. Sure, today some hobbyists like to build computers – but they are doing so as a hobby, not to save money. When you can buy a complete machine, assembled, for less than $500, it makes no sense to build your own. And I realized this around 1995, when you could buy complete DELL systems, with monitor, for about $500 apiece, or less than 1/3 the cost of the last computer that I built myself.

Since then, I have not built a computer, but rather just bought them. Granted the skills I learned from building computers has helped me in maintaining them and repairing them (e.g., replacing a broken display card). But for the most part, I don’t open the cases anymore except to vacuum out the dust. It just isn’t cost-effective to build your own computer.

Cars are rapidly going the same way. Until recently, it was cost-effective to repair an older car and keep it running. However, increasingly, cars are becoming the type of inexpensive consumer good that, once it reaches a certain design life expectancy, it is time to throw it away and start over. The labor cost of repairing an older car, once it is totally worn out, far exceeds that of buying a newer one.

Many cars today, particularly Japanese ones, are designed to give a good service life of 150,000 to 200,000 miles, for about 10-15 years, and then fall apart all at once. At that stage of the car’s life, it may need a number of major repairs, and many minor things may be broken as well. Since the book value on the car is almost nothing, it makes no sense to put more money into it.

As with any consumer product, there is an “end game” and a time to call it quits. Trying to rebuild a car from scratch, like trying to build your own computer, makes no sense at all.

Now when a car gets to be a certain age, generally over 20 years old, people may attempt to “restore” them in one form or another. Interesting cars, performance cars, or other unique rides, are often fun to restore and drive around – as a hobby, of course. As practical transportation, such cars often are unworkable, as they struggle to even run on today’s fuels. An old car may seem like fun, until you realize it has no air conditioning, power steering, remote keyless entry, airbags, or even three-point shoulder harnesses.

And of course, such “collector cars” are worth less if you drive them any appreciable distance. Just parades and car shows – that’s it.

I recently sold two BMWs in my collection. They are nice cars and the people who buy them should get some good service out of them. However, due to their age (13 years) they are not worth very much – maybe $6,000 or so – which is a far cry from their $40,000 price tags when new.

As I have noted time and time again, the best bargain in a car is to buy one that is 1-3 years old and still under warranty. They can cost 1/2 to 2/3 of the price new, and depreciate little for 5 years or more. Once they go beyond a certain point, however, some repairs may be needed, and the value drops off dramatically.

Thus, for example, I paid $21,000 and $16,500 for these two BMWs when they were about 5 years old. If I had sold them at about the 10-year mark, I may have gotten over $10,000 for each of them. But “hanging on” to them for another few years dropped the value nearly in half again.

This probably was not a good bargain for me, as I hardly drove them in the interim, and certainly could make do with less car in my life.

And that brings up another conundrum with regard to cars. They are like fresh fruit and need to be eaten. You can’t “save” a car by not driving it just as you can’t “save” a bunch of bananas by not eating them. A car ages whether it is driven or not. When you hear about people putting 300,000 miles on a car, usually it is done within 10 years or so. These are folks who drive a lot, not keep their cars a long time. Lots of highway driving does not put a lot more wear on a car than driving it short distances to the store.

As a result, “low mileage” is not really big deal with regard to an older car. You may still have just as many wear and aging issues as a higher mileage car. The lower mileage car may just look better.

And it also means for you as a user, that not driving a car doesn’t do much in terms of preserving value. You are better off buying one car and driving it a lot than to own two cars and drive them less.

Historically, I have enjoyed owning a number of cars – usually having at least 2-5 in my collection at any given time. But it is an expensive hobby, and a time-consuming one, as you need to do a lot of work to keep such cars in top shape.

Moving forward, we are going to downsize to two cars, or even perhaps one. Our driving needs have dropped from a combined 30,000 miles a year (typical of most Americans) to less than 15,000 miles a year. From a cost-effectiveness standpoint, owning one car makes sense.

And, just as I have given up on building computers, I will probably give up on rebuilding cars. I may do an oil change now and then, and perhaps a brake job, but beyond minor repairs, I think trying to “rebuild” a worn-out car is not cost-effective.

And just wait until the Chinese start selling cars over here….

Diagnosing Car Electrical Problems

Most car electrical problems, like the problems in any electrical circuit, can be traced to the power supply – in this case, the battery.

Note:  See my companion articles on Mysterious Electrical Gremlins and Understanding the Check Engine Light.

On online car forums, you often see postings about car electrical problems. And you may hear questions about car electrical problems from friends and acquaintances. In almost every case, the scenario is the same, and the cause the same, and yet people end up spending 2-3 times more money than they have to, in order to fix the problem. As I will note in a subsequent blog entry, people are often irrationally afraid of repair bills, and as a result end up spending hundreds, if not thousands of dollars more in repairs, in an attempt to avoid a repair.

The typical car electrical problem starts out as follows:

“I went out to start the car this morning (boy was it cold today!) and when I turned the key, it just clicked – nothing. It was fine yesterday, so it can’t be the battery. I had a friend jump-start the car and it ran fine after that, so it can’t be the battery. The car ran fine all day and started OK after work so it can’t be the battery. I went to the mall and when I came out, it did the same thing again! I hads someone jump start the car and it ran fine, so it can’t be the battery. There must be a short circuit somewhere draining the battery! Can anyone help me?”

The common thread on these postings or inquiries is always the same – the questioner immediately dismisses the battery as the problem, when in fact it is often the most likely problem. Once they put these blinders on, the problem becomes “mysterious and unsolvable” merely because they irrationally discard the most common solution.

Car batteries are much better today than they were even a few years ago. I’ve had batteries last as long as 9 years in a car, which is amazing. Only a few years back, the “60 month” DieHard car battery from Sears was considered the top of the line. Technology has improved over time.

But car batteries can fail at any time, even when brand new. And although they can last as long as 9 years, typically most fail in the 5-7 year range, if not before. And cheaper batteries might not last even that long.

The failure mode of car batteries appears sudden, which is something that confuses people. Most folks’ experience with batteries is based on flashlight or other consumer product batteries. A flashlight battery fails slowly over time, getting dimmer and dimmer, until one day, it is a mere flicker and then goes out. Consumers think that this gradual failure mode applies to car batteries as well, which is why they dismiss the battery as the problem when the car “suddenly” fails to start.

A new car battery, fully charged, should be putting out about 13.5 Volts. Yes, it is a 12-volt battery, but if it actually is at 12 volts, it is partially discharged – actually mostly dead. At 11.5 Volts it may not start the car. Below 10.5 Volts it is basically dead and may cause all sorts of weird things to happen in the car (car alarm going off, etc.).

So the level of charge and voltage is not linear. It is not like the battery is half-charged at 6 Volts and fully charged at 12. More like half charged at 12 volts and fully charged at 13.5 Most consumers don’t get that, and if they are smart enough to put a voltmeter on the battery, wonder why the car won’t start when the battery is showing 11 volts, which they think of as 11/12ths charged. 11 volts is pretty much dead.

The other thing consumers fail to realize about car batteries, is that they don’t have a lot of depth of charge, and this depth becomes shallower over time. As a car battery ages, it may hold little more than a surface charge. There is enough charge to crank over the engine and start the car – once – before the alternator kicks in and recharges the battery, which in turn will now have just enough charge to re-start the car.

On the first cold day of the year, where the engine needs to crank a little harder or longer, or the first time the driver leaves the dome light on for a few moments too long, this surface charge isn’t enough to start the car, and “suddenly” the battery appears to fail. “Gee, it worked yesterday,” the owner thinks, applying his flashlight battery experience to the situation, “it should work today!”

If you have a fine ear for such things, you may notice that, just prior to failure, the starter motor is cranking a little slower than before. But the change is very gradual and most folks don’t notice it in time. On the other hand, you will notice how the starter motor spins much faster once you replace the battery, as the contrast is noticeable. But it is not like in the movies where the car cranks slower and slower – at least anymore. Modern starter motors stop cranking at all, once the voltage drops below 11.5 or thereabouts.

Thus consumers are so quick to dismiss the battery as the source of their troubles, as the car doesn’t crank at all, but merely “clicks”. The consumer thinks this “click” is an indication of some sort of mechanical trouble, and often blames the starter motor. However, when battery voltage drops down to about 11.5 volts or less, there is not enough juice to turn the starter motor. The clicking you hear is the starter solenoid pulling in, a sound you don’t normally hear as it is normally drowned out by the sound of the engine turning over. But chances are, if you hear a “click” you have a dead battery.

So how to you diagnose and fix these problems? As I noted in the header of this piece, in any electrical circuit or device, the most common failure mode is in the power supply. So when diagnosing problems in any electrical circuit, the first thing an Engineer does is check the power supply. In this case, the battery.

With the car off, your battery should be at 13.0 to 13.5 volts, once it is charged. If it won’t hold a charge, well, end of story, buy a new battery. Anything from 12.5 to 13.0 Volts is suspect, and if you are having problems and the battery is of any age at all, I’d replace it. Below 12.0 volts, your battery is likely shot.

With the car running, by the way, the voltage should be 14.0 to 14.5 Volts, which also confuses a lot of consumers. A battery won’t charge unless the voltage applied to it is greater than its basic voltage (otherwise current won’t flow). So to charge a battery that is nominally 13.5 volts, you need 14 to 14.5 volts to make things happen. If the car is less than 14.0 volts when running, you may have an alternator problem – but more about that later.

Most auto parts stores or auto centers will test your car battery for free – either in the car or with it removed. Such testers will test the battery under load as well, and provide a more complete diagnostic than mere voltage level.

However, car batteries are cheap (usually less than $100) and if they are more than 4 years old and you are having problems, it is a good idea to just replace the battery to eliminate this as a potential problem source in your diagnostics. This is particularly true if you live in a cold climate and winter is coming. If your car has an older battery and winter is on its way, it might not be a bad idea to buy a new battery, just so you don’t get stuck somewhere on a cold night. It is cheap insurance and less costly than calling a tow truck.

You can install a battery in a car yourself pretty easily, but with modern cars, there are some considerations to take into account. Some car radios (such as in BMWs) have anti-theft codes that need to be entered after the power has been removed, so make sure you have these codes before removing power to the car.

In many cars, self-learning computer systems may have to “re-learn” themselves when you put in a new battery. So, for example, in my Ford pickup, the transmission will shift poorly for the first 50 miles after installing a new battery.

Most modern cars will also set off the check engine light if voltage drops below 10.5 volts, as “low voltage” may be one of the error codes. Moreover, low voltage may trigger spurious error messages as well. If your car has been jumped a number of times, chances are, you’ll get a CE light, which will need to be reset.

Other flaky things may happen if your battery is disconnected. My BMWs, for example, set off the brake and ABS lights when power is removed. Once the car rolls a few feet, the lights reset themselves. There are some folks who “transplant” batteries into a car, using a jumper cable or charger, to keep power to the car when the battery is being replaced, to avoid problems. The only problem with this approach is that the positive (+) battery cable is “hot” when disconnected and can spark if allowed to touch ground.

And speaking of which, when disconnecting a car battery, always do the negative cable first. When connecting, do the negative last. If you disconnect the positive first, you may shock yourself if you are touching the positive terminal (with your wrench) and your body touches a ground on the car body. Similarly, if you connect the negative first, when you connect the positive, you increase the chance of electrocution.

Thus, if you are at all uncomfortable or unfamiliar with the process, have a mechanic replace the battery. People who are not handy with tools should avoid the process.

The second problem with the “it can’t be the battery” people have is that oftentimes their battery problems morph into alternator problems. It has happened to me, so I know. My truck sat for long periods of time between use and when I went to start it one day, it wouldn’t start. I jumped the battery and it worked fine. Over a period of a month, the car stereo and other electronics pulled down an older battery to the point where the truck wouldn’t start.

The problem was, once I jump-started the truck, the alternator had to pick up a huge load to recharge a totally dead car battery. For the old generators and alternators of the 1950’s and 1960’s, this was not a big problem. But today, with weight a paramount concern for carmakers, alternators will not put out sustained high currents, without overheating. A modern alternator may be rated for 100 amps or more – but only intermittently, not continually.

As a result, my truck battery went dead again. And when I bought a new battery, the problem persisted. The new battery appeared not to be holding a charge. I checked the system voltage and realized that my alternator wasn’t charging. In fact, the windings had melted together and shorted out. (It should be noted, that when it comes to “there’s a short somewhere”, the alternator is usually the culprit. A shorted alternator can drain a car battery, even with the ignition off).

Lesson learned, I bought a rebuilt alternator. On the box were explicit instructions NOT to try to charge a dead battery with the alternator – they just can’t handle the load! I was lucky, others are not as lucky. Some folks continue this route, running a brand new battery dead again and again, and then smoking new alternators one after the other. Several alternators and batteries later, they sell the car, convinced it has an “unsolvable” electrical problem.

You see, in addition to being basically dead at 11 volts, a car battery is not a deep-cycle battery. As a result, you can destroy a brand new car battery (or severely shorten its life) if you leave the lights on and run the battery totally dead a few times. Even a 2-3 year old battery can be toast, if it has been totally discharged more than a few times.

So the bad battery leads to a smoked alternator which in turns allows the new replacement battery to go dead, which in turn smokes a new alternator, and so on.

(Any by the way, an alternator, unlike a generator, requires a battery in the circuit in order to work properly. If you disconnect a battery from a car while it is running, you will likely “fry” the alternator in short order – like immediately.)

To avoid this problem, fully charge a car battery before installing it. Test the alternator as well as battery (again, most shops will do this for free, or you can check the output voltage as noted above). If you must jump start a car, leave the cars connected for a while to allow both alternators to charge your battery. Don’t run your car for a long period of time, but rather shut it down after 15 minutes to let the alternator cool. But avoid jump-starting if at all possible.

In modern cars, jump-starting can cause all sorts of problems. Electrical systems as noted above, can do flaky things when voltages go low. Jumping can cause spikes in voltage which are just as bad. And every time you jump-start a car is an opportunity for someone to reverse the cables, which causes all sorts of bad things to happen.

Most cars, by the way, have an “idiot light” or voltage gauge. The idiot light, usually in the shape of a battery, comes on when system voltage is below 14 volts or thereabouts. Thus, when you turn on the ignition, the light comes on, as the car battery is at 13.5 volts. When you start the car, the alternator kicks in, and the voltage goes up to about 14.5 volts, and the light goes off. If the “battery” light comes on, chances are, you have an alternator problem, ironically.

Voltage gauges work the same way, but most are not accurate enough to read a proper voltage level very well. Some older cars have Ammeters, which show the rate of charge. When the alternator is working, it may show a positive amperage, representing the current charging the battery. When accessories are turned on, the amperage may drop or go negative, representing current draining from the battery. Such gauges were interesting to have during the early history of the automobile, when charging systems were fragile. But often the connections on such gauges cause more problems than the good they do. For this reason, I do not recommend “aftermarket” gauge sets, as they add little in terms of useful information and can cause electrical problems down the road.

Presuming you have eliminated the battery and alternator as the problems, where do you go from here? Connectors are the second largest failure mode in a car, and the first connector you should check is the battery terminals. Terminals can come loose (or be improperly installed) or become corroded. Clean and tighten the terminals and see what happens. A neighbor recently had this problem. His mechanic couldn’t get the negative terminal to reach, and attached it at an angle. As a result, it was making only intermittent contact and the car would “mysteriously” not start sometimes. loosening the cable so it would reach and then tightening the contact fixed the problem.

Cars left for long periods of time will discharge their batteries. Most cars can sit for about a month before the various computers and radio gear discharges the battery. If you let a car sit for any length of time, a battery tender may be a good idea. These chargers keep the battery topped up and compensate for the minor current drains from all the accessories in the car. I own four of these types of chargers and use them, as I tend to let cars sit.

Note that if you remove a battery from a car, boat, or tractor, you should put it inside, in a heated space, and not on a concrete floor. Batteries can freeze, particularly when discharged, and this freezing usually destroys them. A battery that appears “puffed out” probably froze at some time. Folklore suggests that you should not leave a battery on a concrete floor, although it is not clear whether there is any science behind this. There may be two reasons for this folklore. First, a concrete floor may be cold, and transfer heat away from the battery more quickly, allowing it to freeze more readily. Second, plate material can slough off a battery if it is exposed to vibrations, and a concrete floor may more readily transmit such vibrations to the battery. Whether it is folklore or not, I put my batteries on a wooden shelf in the winter and not in use.

So, you’ve replaced the battery, tested the alternator, checked the battery terminals, and you are still having problems. Now what? The odds of a real “short somewhere” are actually very small. Wires sitting by themselves and not moving (rubbing or abrading) don’t suddenly decide to shed their insulation and short out. Automotive designers are aware of the difficulties of vibration when wires pass through bulkheads and provide grommets and bushings to compensate. And wiring harness portions going to doors and trunks are extensively tested prior to production to insure that they do not break or short as they flex at the joints.

Sometimes errors do occur. For example, in some BMW 3-series coupes and sedans (E36) the trunk wiring harness had a tendency to short out. Most of such cars had a new harness installed under warranty. But such shortfalls are few and far between. These are the rare exception, not the norm.

The easiest way to test for undue electrical drains is to insert an ammeter in line with the negative terminal of the car battery and the ground cable. The current draw should be on the order of milliAmps (check the service manual to be sure) from the various computers in the car (engine, radio, SRS, ABS, transmission, etc.). If it is larger than this, pull fuses one at a time and note the change in current draw. If one circuit seems to be pulling more amperage than normal, you can than trace this one circuit for problems, rather than just pulling at wires willy-nilly. Again, note the precautions above, and make sure if you disconnect power from an electrical device (such as your radio) you won’t need a special code to reactivate it.

Aftermarket radios tend to draw more power, even when off, than factory components. Many of these types of radios are installed after the car is a few years old and the battery is getting worn. The combination of an aftermarket radio and an older battery can trigger an electrical problem. And again, the solution is usually a new battery (hopefully replaced before the alternator is shot as well). So often, you hear of these “mysterious short somewhere” pleas right after some youngster put a boom-boom stereo in his old hoop-de.

And speaking of which, some car stereos and nav systems can draw a LOT of power. My new Pioneer AVIC-Z2 is on a 20-amp circuit. The owners manual explicitly states that the radio should NEVER be run unless the engine is running. It draws that much power. And my experience has shown than running the radio even for a half-hour, can pull down the battery enough to the point where the car may not start.

Folks who make a hobby of huge radios and subwoofers (and we all love pulling up to a light next to them, don’t we?) often install a separate battery to power the accessory equipment. The standard car battery can easily be drained flat by large aftermarket subwoofer amplifiers and other audio, video, and navigation accessories. If you plan on putting in a bad-ass stereo, you might want to consider an auxiliary battery as well.