Computer Controlled Cranking Circuits: Part 7

What is going on inside the PCM?

Ever wonder what is going on inside an on-board control unit? How do they do what they do? How does a tech troubleshoot these circuits? In Part 7 a sample of some of the digital circuitry from our on-board control unit controlled cranking circuit, utilizing some PCM circuitry is shown. This discussion will help in understanding what a technician should do when the decision must be made to replace a control unit. Figure 7-1 below is a schematic of some of the inner workings of the PCM circuitry that controls the Starter Relay. The partial schematic of PCM circuitry shows three electronic circuit components.

Transistor Q1:

Q1 Driver transistor provides electron current through the relay coil to turn the relay ON. Q1 is a NPN transistor that controls the relay coil from the ground side. Notice the small arrow drawn on Q1’s “e” (emitter) wire lead is pointing outward in the transistor symbol indicating the transistor is a NPN (Not-Pointing-iN).

(A PNP transistor is used to control the relay coil from the voltage side at Pin 86 and the arrow is Pointing iN.)

Fig. 7-1 Logic circuits inside PCM


To the left of Q1 is a circuit labeled “U1” which controls Q1. It is part of the PCM’s “CPU” (Central Processing Unit), or the “Brain” of the PCM that makes all the important decisions according to the PCM’s software program. The stored operating program is what you change or edit when you “flash” a control unit.

AND Gate:

To the left of U1 is a digital logic circuit called an “AND Gate” that provides an output on Pin 4 when all inputs are present. For (3-input) AND Gate U2 to provide an output signal at Pin 4 (which connects to U1’s input), requires 3 inputs to appear at the same time to AND Gate Pins, 5, 6 and 7. That’s why it is called an AND Gate. It takes an input to Pin 5 andan input to Pin 6 and an input to Pin 7 at the same time before an output appears on Pin 4.

Logic Legend

There are two categories of logic circuits, Positive Logic or Negative logic. The logic level of a “1” or “0” must be defined according the logic circuits used. Our Logic Legend shown in the upper left of Figure 1 describes Positive Logic for our circuit because a “1” logic level is a positive voltage level of either 5V or 12V and a “0” logic level is zero volt. Our circuit works with positive logic so let’s continue with that understanding.

Digital logic circuits work with two conditions simply called a “1” (say one) and a “0” (say zero). It is common to write the logic level of a “1” or “0” on a digital schematic diagram to express the “logic level” which is the voltage present at that point in the circuit at a particular moment in time. Then a technician measures the “logic level” (voltage) present at each point in the digital circuit to verify the logic circuit is working correctly (has the correct voltage at each point) or has a problem (voltage doesn’t match the logic level at each point). Pins on an “IC” (integrated circuit) or the wire connected to the pin are labeled with a “1” should have 5V or 12V present and wires labeled with a “0” should be 0V (zero volt) present.

Logic levels present at the connector pins of an on-board control unit can be 12V for a “1” and zero volt for a “0.” An exception could be a sensor that provides a 5V signal rather than a 12V signal to a computer, as we show in Figure 1. Inside the on-board control unit a logic “1” is most often 5V and 0V for a “0.” An exception would be the logic level designation for driver transistors such as the voltage on Pin 8 of the PCM which is the collector of Q1. When Q1 is OFF the collector is a logic HI 12V (a “1”) and drops a logic LO (“0”) when Q1 is ON.

Logic circuits are very logical if you think logically.

PCM Inputs

An on-board control unit must have input information from sensors to make decisions. The PCM has three required inputs to activate the Starter Relay in Figure 7-1 above.

P/N switch input is a logic level "1" or 5V indicating the vehicle is in PARK or Neutral. A "1" appears at Pin 6 of U2. U2 Pin 4 output is "0" and Q1 is OFF.

BRAKE switch input is a logic level “0” indicating the brake pedal is not depressed. A “0” appears at Pin 7 of U2. U2 Pin 4 output is still “0” and Q1 is OFF.

START switch input is a logic level “0” indicating the START button is not depressed. A “0” appears at Pin 5 of U2. U2 Pin 4 output is still “0” and Q1 is OFF.

Q1 remains OFF and the Starter Relay is not ON (not energized). The vehicle is parked.

Fig. 7-2 Logic levels of "1" and "0"

In Figure 7-2 above, the vehicle operator initiates the cranking process by depressing the brake pedal.

BRAKE switch input is now a logic level "1" indicating the brake pedal is depressed. A "1" appears at Pin 7 of U2. U2 Pin 4 output is still "0" and Q1 is OFF.

Now AND Gate U2 has two “1” inputs on Pins 6 and 7. Since U2 Pin 5 is still a logic level “0” there is no output at U2 Pin 4. A 3 input AND Gate requires all three inputs at the same time to generate an output at Pin 4. Q1 remains OFF and the Starter Relay is OFF (not ON).

In Figure 7-3 below, the vehicle operator presses the START button.

START switch input becomes a “1” indicating the START button is depressed. A “1” appears at Pin 5 of U2.

U2 Pin 4 output becomes a “1” since all three inputs are now a “1.” This causes U1 to turn ON and place a small DC voltage of approximately 2V on the base (b) of relay driver transistor Q1 which turns ON and allows an electron current to flow through the relay coil which becomes an electromagnet.

Notice the electromagnetic field created around the relay coil and the movement of the movable contact creates an audible CLICK.

Fig. 7-3 Logic circuits turn ON Q1 to control relay

The movable contact connected to Relay Pin 30 with B+ becomes magnetized and moves under the influence of the electromagnetic field to contact Pin 87. The B+ voltage on Pin 87 supplies B+ to the Starter Solenoid (not shown in this schematic). The engine cranks.

As soon as the engine begins to RUN the vehicle operator releases the START Button which removes the “1” from U2 Pin 5 as shown in Figure 7-4.

The AND Gate U2 no longer has three inputs at a logic “1” so it changes the “1” at U2 Pin 4 from a “1” to a “0.”

Fig. 7-4 Logic levels change to turn Q1 OFF

U1 sees a “0” at its input and turns OFF which removes the 2V from the base of Q1. Transistor Q1 turns OFF and stops electron current through the relay coil.

The engine is running.

The Aftermath

The engine is running but shutting down the Cranking Solenoid and the Starter Relay causes powerful electrical energy dumps as the electromagnetic fields across both coils collapse.

The Starter Relay has a spike suppression diode connected between Pins 86 and 85. The diode absorbs the collapsing electromagnetic field to protect transistor Q1 from being destroyed which would knock out the PCM.

The starter solenoid also has a diode connected across the coil to absorb the collapsing electromagnetic field. If this energy dump were not suppressed by the diode there is a good chance that the energy dump would travel back into the electrical system and possibly damage sensitive computer memory circuits. It has been known to happen causing the adaptive memory information inside the PCM to be erased. The vehicle would eventually relearn the adaptive strategy after about 25 miles of driving with poor engine performance until relearned.

Identifying and Confirming a Defective Control Unit

Technicians are not required to open up a control unit and repair internal circuitry such as logic circuits and transistors. Yet these components do fail requiring a new computer to get the vehicle going again. Before a technician should condemn a control unit follow some simple troubleshooting steps to confirm a control unit must be replaced.

  1. Confirm the control unit has B+ on all pins connected to B+ power with the engine running.
  2. Confirm the control unit has a good B- ground on all ground pins with the engine running. Always remember computer grounds need to be an exceptionally good ground. Whereas electrical circuits are considered to have a good ground when the ground voltage drop is 0.10V (100 mV), computer grounds should have a voltage drop of 0.05V (50 mV.)
  3. Confirm the proper voltage or signal is present at the control unit’s appropriate input pins that are necessary to control the circuit in question.
  4. Measure the voltage or signal on the control unit’s output pin (PCM Pin 8 in our example) to determine if the computer is functioning properly by the proper change in voltage that should occur.

Let’s say the problem with this vehicle is the Starter Relay does not click and the vehicle does not crank. Measure the voltage on PCM Pin 8 or at the Starter Relay Pin 85 which ever is easiest to access. If the relay is not energized, the voltage should be B+. (If the voltage on Pin 85 is 0.0V at this time the relay coil is open or there is no B+ on pin 86.) That’s not a PCM problem. It is a problem in the wire harness.

When the PCM gets all three inputs the voltage on Pin 8 should drop to a low voltage of about 0.80V as the PCM circuitry does its job to control the Starter Relay. The voltage drops from B+ (battery voltage) to about 0.80V because that is the voltage drop of transistor Q1 when Q1 is supplying electron current through the relay coil.

If the voltage at PCM Pin 8 or Starter Relay Pin 85 stays at B+ when the PCM has all three inputs, it is a reasonable conclusion that PCM internal circuitry is defective. The PCM can now be replaced with an expectation the problem is resolved.

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