Computer Controlled Cranking Circuits: Part 5

A New Automotive Era Began in Model Year 1982

Every GM car produced in model year 1982 contained a new electronic device called an Engine Control Module or ECM for short. The ECM controlled fuel mixture, spark timing, EGR, TCC and other functions to reduce emissions and maximize fuel economy. The author of this document, yours truly, set up the ECM remanufacturing facility in Dallas Texas and began to rebuild ECM’s for General Motors on January 2, 1982 following three weeks of school covering ECM remanufacturing processes.

A Sharp Contrast

A new host of problems for automotive and truck technicians began to emerge as ECM “electronics” became the controller of some of the automobile’s mechanical/engine systems. Up to this point, technicians evaluated mechanical system failures by “smell, touch, shake, rattle and roll” and observing the wear and tear on mechanical components.

Electronic circuit failures are different. There is seldom anything to observe with an electronic circuit other than the fact a circuit controlled by the ECM either works or does not work. Meanwhile the ECM “looks” fine. Then suddenly for no apparent reason it stops working then starts working again. Meanwhile the outward appearance of the circuit and components look perfectly fine. In the world of electronics, we call this an "intermittent problem" and many factors come into play causing electronics circuits to work then not work. We will discuss some of these factors as we continue.

Electronic technicians are used to the frustration caused by intermittent electronic circuit problems and have devised a few troubleshooting techniques over the years to pass on. Automotive and truck technicians must learn to deal with intermittent problems. Everyone who works with electronic circuits (radios, TVs, computers) has the same problem and there is no easy solution to this dilemma. Intermittent problems come with the territory of electronics in the vehicle.

The ECM became a whole new world for automotive and truck technicians to consider. The ECM contains thousands of semiconductor components, such as various types ofdiodes, transistors of every type and those mysterious components called integrated circuits. All these solid-state components are arranged and soldered to a circuit board housed in a metal container. Numerous wires exit the metal container and connect the ECM/PCM circuits to mechanical systems on the vehicle. Electronic components can be grouped into three general semiconductor categories.


Diodes in a circuit allow electron current to flow in only one direction, not both directions. When a solid-state diode becomes “OPEN” in a circuit no electron current flows through the diode. When a diode becomes “SHORTED” electron current can flow in both directions. In either failure mode, “OPEN” or “SHORTED,” the bad diode causes the electronic circuit to not function properly. Diode failures in electronic circuits can also cause the failure of a perfectly good transistor or integrated circuit from unsuppressed voltage spikes.


Transistors are used to control the amount of electron current or switch current ON or OFF. A “Load,” could be a lamp, DC motor, solenoid, or relay. When a transistor is “ON” it controls the amount of electron current through the load causing the load to operate. When a transistor is “OFF,” electron current through the load is stopped and the load does not work. These devices are called “switching transistors” because they switch electron current ON or OFF. High current switching transistors can fail “OPEN” so the circuit does not work at all or they can fail “SHORTED” and the circuit works all the time.

The most difficult problem to diagnose with a switching transistor occurs when the transistor is intermittent. Sometimes the transistor works and sometimes it doesn’t. It may look fine to the naked eye. If that transistor can be identified by testing, it must be replaced. Other types of low power transistors are used to create and pass digital electronic signals through circuitry. They also can be “OPEN,” or “SHORTED” or operateintermittently.

Integrated Circuits:

Integrated Circuits, abbreviated as IC or ICs (plural) may contain only a few transistors or millions of transistors in memory ICs that store digital information as a series of “1” (one) or “0” (zero). Integrated circuits are mounted on a substrate and encased in an epoxy type material with numerous pins sticking out from the sides of the case. The IC pins are soldered to circuit components on the circuit board to complete a circuit. If one of the millions of transistors in an IC goes bad the integrated circuit may fail to function properly and must be replaced. If IC replacement is not practical or possible (most of the time) the entire computer must be replaced. Integrated circuits can also become “OPEN,” or “SHORTED” or operate intermittently which causes great frustration. In this day and age of “throw-away computers” a new computer must be installed to restore circuit operation.

Although in the 1980s automotive computers could be repaired by hand (failed parts replaced). The present small size of electronic components is almost impossible to replace on a circuit board by human hand. Robots are programmed to place these tiny components on a circuit board with precision when the circuit board is first produced. In most cases, it’s cheaper to build a new computer with a robot than to attempt to repair a defective computer by troubleshooting the circuits and replacing defective components by hand.

There is seldom anything to observe with the naked eye when an electronic circuit has failed other than the fact a circuit controlled by the ECM either works or does not work. Then suddenly for no apparent reason it starts working again. In the world of electronics, we call this “OFF-again/ON-again” syndrome an “intermittent problem.” Electronic technicians are used to the frustration caused by intermittent problems. It’s part of life in the world of electronics. Automotive and truck technicians are learning this the hard way like everyone else. We all have the same intermittent problem and there is no easy solution to this dilemma.

An intermittent problem can be caused by a crack in a circuit board developed from vehicle vibration or a “cold solder joint” that intermittently makes and breaks contact with the loose leads of an electronic component intermittently contacting the copper trace on the circuit board. Sometimes this intermittent failure mode can be duplicated by a gentle "Tap Test" of the ECM case. A control unit sensitive to vibration must be replaced.

Another cause of an intermittent problem can be excessive heat or excessive cold and then the circuit sometimes returns to normal operation when the temperature extreme is removed. Both temperature extremes can cause a semiconductor to permanently or intermittently fail. During the days of humans repairing ECMs, a technician would struggle to duplicate an intermittent problem by applying external heat to the circuit board with an infrared lamp to warm up the semiconductor components. When the failure occurred the semi-conductor components were sprayed with “Freeze Spray” designed to quickly cool down the heated electronic components. An intermittent semiconductor component would immediately begin to operate again when hit with the coolant spray thus identifying a component with an intermittent problem.

A Not So Obvious ECM Problem

In the early days of computer control (early 1980s) I remember a luxury car that had a history of ECM failures. Each time we went into the newly failed ECM to see what was wrong we found cold solder joints on the circuit board. Why did just this car have this problem? The dealership sent us the car to diagnose the problem. What we found was something quite surprising.

The generator charging voltage was running about 17.5x V instead of a more normal 13.8xV-14.xxV. In the hot Texas summer the ECM was being exposed to higher than normal operating temperatures and a higher B+ voltage causing circuit components to get so hot that the solder connections on the circuit board would soften during vehicle operation then re-solidify when the engine was turned off and the circuit board cooled down to ambient temperature. Over a short period of time the heating up and cooling down created intermittent solder connections on the circuit board.

Ever since then I have been adamant in teaching technicians to test and monitor the charging voltage during engine run. I also have written several training programs with details on how to check the charging voltage on a vehicle. You may have heard about this test in FIRST THINGS FIRST flip charts. The test only takes 60 seconds.

Below in figure 5-01 shows our by now well-known cranking circuit where the starter relay is now controlled by a PCM (Powertrain Control Module). The PCM, formally known as an ECM, performs the function of the P/N and Start switches in “B.C.” days (Before Computers). The relay coil connects between Pins 86 and 85. An electron current through the relay coil energizes the relay and Pin 30 connects to Pin 87 applying B+ to the Starter Solenoid.

Fig. 5-1 PCM Controls Starter Relay

The PCM contains many circuits. In Figure 5-01 the role of the PCM is shown and only the circuit in the PCM that controls the starter relay is shown. In some schematic diagram formats the lines surrounding the PCM might be shown as a dotted line to indicate there are other circuits in the computer not shown or used in the starter relay circuit.

The PCM circuit in our illustration contains two diodes, an NPN transistor - Q1 and a circuit designated as U1.

Diode D1 is a Polarity Protection Diode that prevents damage to PCM circuitry should a jump battery be connected in reverse polarity by accident of course. The polarity diode does not allow reverse current to flow through the PCM circuits which could smoke all solid-state components.

The diode across the transistor Q1 is not numbered but protects the transistor from the energy dump created when the relay powers down. It is called a “spike suppression diode.”

Crank Input is a DC command voltage (5 volts or 12 volts) from the Ignition Switch in the CRANK position. The voltage present tells the computer to “please” activate (turn ON) transistor Q1 to energize the relay to power the Starter Solenoid.

When Q1 turns on, electrons flow up from ground and enter Q1 emitter, (Q1 emitter connects to ground) then flow through the transistor and exit Q1 collector (the collector connects to PCM-Pin 8). Electrons leave the PCM at Pin 8 and travel by wire to Pin 85 of the relay.

Q1 electron current flows through the relay coil exiting the coil at pin 86 and flowing to B+. The relay is now activated and closes the relay contacts. Relay pin 30 moves from relay Pin 87A to Pin 87 to provide B+ voltage to activate the Starter Solenoid. The Starter Solenoid Pin G being hard wired to ground allows current to flow through the Starter Solenoid which closes the heavy-duty contacts and activates the starter motor.

Circuit U1 represents all internal PCM circuitry. Think of U1 as a portion of the computer’s memory and brain circuits that control various functions. This is where the computer’s program is stored and used to control, in this case, the cranking process by turning Q1 ON providing electron current through the relay coil.

U1 can be programmed to limit the time the starter solenoid can crank the engine should the engine not begin to run. Other inputs to the PCM (not shown) will contribute information to the computer’s brain in the event of a long crank.

U1 can also be programmed to prevent cranking when battery voltage is below a threshold because injectors are not pulsed when battery voltage drops below 9.0V.

Herein Lies A Problem

An onboard computer is programmed to do certain things by computer programmers. What has the PCM “brain” been programmed to do and how does it respond to its program? What I came to learn in the early days of repairing ECMs/PCMs is that the computer program dictates how the onboard computer thinks and works which I like to describe as the computer’s “personality.” All vehicles of the same make and model will have the same personality because all run on the same software program. If a brand of car has a different engine it will be programmed differently resulting in a unique personality for that engine computer.

What is Computer Personality?

In the early days of computer control I would hook up a lab scope to the O2 sensor and watch the O2 sensor voltage swing through rich up to 0.9 V and lean down to 0.1 V. Any adjustments on the vehicle that could affect fuel mixture could be adjusted to their proper setting by observing the reaction of the waveform as the mixture trace line on the scope went to rich (trace swings high) to lean (trace swings low) and from lean back to rich. Make and model of cars had a different action of swinging from rich to lean or lean to rich. The differences were not dramatic but you could see similar vehicles all behaved the same way while a different make or model of a vehicle might have its own slightly different lean/rich waveform. These subtle differences are a reflection of acomputer’s personality.

Any function an onboard computer is programmed to perform will contribute to the overall personality of that particular make and model vehicle. The more the circuitry the more complex the personality of the control unit. The more you work on the same type of vehicle with the same engine the more you become aware of each computer’s personality. Many of you already recognize this phenomenon in the vehicles you service the most but you may not have called it “computer personality.” The more programming that is involved in the operation of a control unit and the more functions on the vehicle it controls will contribute a great deal to that control unit’s unique and sometimes complex personality.

This particular ECM/PCM generation of ECM, Figure 5-01 controlling a starter relay, is an example of computer control and it is still in use today. This function by itself is not nearly as sophisticated as president onboard control units are today with the complex circuitry and vehicle functions they control. Expect significant differences in today’s onboard computers personality. Watch for it!

The more you know about each computer’s personality the better you are able to diagnose various driveability problems. If you work on whatever comes in the door it takes a little longer to recognize each vehicle’s unique computer personality.

Next time we will look at various ways this computer-controlled cranking circuit can fail in the electronics of the circuitry and offer some electronic testing techniques to diagnose the problem.

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