Computer Controlled Cranking Circuits: Part 6

What Do You Test and When?

An electrical circuit operates with voltage applied, electron current flowing and circuit components that have the correct resistance to keep the electron current moving at a safe level.

When a circuit is performing correctly, voltage readings, electron current measurements and resistance values around the circuit are normal. Most technicians never bother to check these values in a circuit when it is working correctly.

"Why should I do that, you ask?" It is practice using your DMM and at the same time learning what the normal voltage readings are throughout the circuit and what is the normal level of electron current. Resistance measurements also indicate the normal resistance value of critical components in a circuit. This is all vital information in troubleshooting a faulty circuit. An electrical problem in a circuit will certainly affect voltage readings around the circuit and the amount of electron current flowing. When faulty readings are compared to known good readings, you are well on your way to determining the problem in the circuit.

When a circuit develops a significant problem that affects circuit performance is the time most technicians begin, for the first time, to check voltages, check electron current and maybe the resistance value of a critical component without any knowledge of what the normal readings should be for each of these exercises. As a result, technicians cannot be certain a voltage value, or electron current measurement found in a faulty circuit is normal or abnormal indicating the problem in the circuit.

The Solution

When you have repaired an electrical circuit, that is the time to take a few critical voltage readings to establish what normal voltage readings exist in a good circuit.

In the case of DC motor circuits, it would certainly be advisable to check the amount of electron current flowing through the DC motor circuit when the DC motor is operating properly. Use your current clamp to quickly measure the electron current. What? You don’t have a current clamp. GET ONE! Current clamps can quickly be connected to a circuit to measure the electron current flowing during normal operation.

Sampling voltage and electron current readings in repaired circuits will begin to educate you about what good readings look like. This will build up your store of electrical knowledge that will be useful when testing another faulty circuit for the first time. You will begin to accumulate knowledge about good readings in a circuit – you are learning valuable information needed to troubleshoot electrical problems.

Understanding Computer Control

The addition of the PCM circuit in Figure 6-01 to control the starter relay introduces numerous issues that must be considered in the diagnostic process. Take a few minutes to look over the schematic diagram.

Fig. 6-1

First notice the addition of the PCM and its place in the cranking circuit. We have the luxury of showing the components in the PCM relating to starter relay control.

Transistor Q1 in the PCM is a NPN (solid-state) transistor switch and connected to the relay Pin 85 to control the relay on the ground side of the relay coil. The voltage side of the relay coil is hard wired to B+ through Fuse F12. The PCM provides the same function as the P/N and Crank Switches.

Q1 turns ON to operate the relay and drives an electron current through the relay coil. Electrons travel up from “ground” (the source of all electrons) and enter PCM Pin 55, travel through the turned “ON” transistor and exit the PCM at Pin 8, travel to the Relay Pin 85, flow through the relay coil and exit the Relay at Pin 86 as electrons race to B+.

1. What is required for the PCM to operate the Starter Relay?

First and foremost, the PCM must have good B+ voltage on all of its B+ wires in the PCM connector (only Pin 6 in this schematic, others not shown). The PCM must also have a good ground circuit voltage drop of 0.05V on all PCM ground Pins 48 and 55. When these critical PCM circuit inputs (good B+ voltage and good B- ground) are present, the PCM is ready to operate the cranking circuit on command. These electrical values are determined by measuring the voltages (back probing) at the PCM connector pins. Make sure your DMM is grounded at -BATT to obtain the most accurate voltage readings, especially on the ground pins.

2. When does the PCM operate the starter relay?

Immediately upon receiving the proper voltage input on the three INPUT pins (1) Crank Request, (2) P/N and (3) BRAKE (pedal depressed), a DC voltage appears at each pin at the same time. The DC voltage could be a +5 volt or a +12 V input. You can’t be sure until you test this voltage in a known good circuit if the manufacturer doesn’t provide this information.

Internal circuitry in the PCM combines these three inputs (using an AND Logic Gate) which allows the PCM to activate circuit U1 to put a DC voltage on the base of Q1. This small voltage is called “forward bias” and Q1 allows sufficient electron current through the relay coil to energize the relay. The B+ voltage at pin 30, from fuse F11, is connected to Pin 87 through the CLOSED relay contacts.

It any one of these 3 inputs are not present the PCM will not activate Q1. The PCM requires all three to appear on their pins at the same time.

3. How does the PCM operate the starter relay?

Transistor Q1 supplies enough electron current to energize the starter relay when told to do so. If transistor Q1 is good the voltage on PCM Pin 85 will drop to about 0.8 V.

If transistor Q1 is OPEN the starter relay never energizes. Pin 8 of the PCM remains at 12 V.

4. What do you do first when the PCM does not operate the starter relay?

First place the electrical system in CRANK request, Gear shift in P/Nposition and the depress BRAKE the brake pedal.

Does the relay click? YES – NO. Put your finger on the relay case to feel the vibration when the relay clicks. I like to use a big screwdriver placed on the case of the relay and hold the handle of the screwdriver to my ear so I can hear the relay clicking.

YES - Relay Clicks

This means the PCM is doing the job to energize the starter relay. Check the voltage at starter relay pin 30 and then at starter relay Pin 87. Both pins should indicate B+ is present.


If Pin 30 does not have B+ check Fuse F11 and the voltage on both sides of the fuse.

If no or low B+ at Pin 30 look for a voltage drop on the wire feeding fuse F11 and the wire going to Pin 30.

If Pin 87 does not have B+ immediately recheck Pin 30 again to verify B+ is still present at Pin 30 with the relay contacts CLOSED.

If Pin 87 does not have B+ (but Pin 30 has B+) then the relay contacts are bad. Replace the relay with an identical relay.

NO - Relay Does Not Click

Verify that the PCM has good B+ voltage on all of its B+ wires by measuring the voltage at each B+ pin. Next verify that all ground circuit pins have a voltage drop of 0.05V on all PCM ground pins. Then measure the CRANK request pin, P/N pin and BRAKE pins for the proper voltage. You should measure at least 5 V on some vehicles and it may be as high as 12 V depending on the voltage level used in this vehicle.

Replacing a Relay - CAUTION

Do not try any old relay you have laying around to see if it clicks. It may not be configured (pin arrangement) the same as the original relay. A spare relay that has the same pin arrangement may have the wrong relay coil resistance.

If the relay coil resistance is too low, transistor Q1 could be damaged which would require a new PCM. A second reason do not use a spare relay laying around is the component across the relay coil for spike voltage suppression could be missing in your spare relay. That would cause a voltage spike when the relay powers down which would damage the driver transistor in the computer.

Some relays use a resistor for spike suppression and some relays use a solid-state diode. A replacement relay with the same part number as the original relay avoids all these potential problems that could damage a control unit. Just don’t use a spare relay that isn’t the same part number.

For safety sake always use an original replacement relay part number any time you are replacing a relay you suspect is defective. There are several reasons for using an exact replacement relay which would require much more explanation than is possible to provide in this brief article.

Notice the schematic diagram below in Figure 6-02. The internal circuitry of the PCM is not shown. The PCM is represented by an empty box.

Fig. 6-2 PCM control circuits not provided.

This can lead to some confusion because it’s hard to determine how the PCM is controlling the relay. From our understanding of relays, it should become obvious that the PCM is controlling the ground side of the relay coil.

Relay Coil Pin Voltages

When the relay is commanded “OFF” the voltage on Pin 86 and Pin 85 is B+. It is very important to remember that taking voltage measurements in a computer control circuit requires the DMM to be grounded at -BATT, the negative terminal of the battery which is the best ground for the DMM and results in the most accurate voltage readings.

When the PCM turns the relay ON, the voltage at Pin 86 remains at B+. But the voltage on Pin 85 drops to about 0.8 V. This is the voltage drop of the driver transistor inside the PCM. This reading is very important to analyze. When the transistor driver begins to deteriorate, the voltage on Pin 8 begins to rise slightly when the relay is “ON.” If the voltage exceeds 1.0 V on Pin 8 the driver transistor is going bad and may not last much longer. There’s no way to know how much longer the transistor driver will function properly. It may last for years because it has a very short “ON” time since it’s only “ON” when cranking the vehicle.

Recent developments in solid-state driver transistors has progressed over the years so that the normal voltage drop of a solid-state transistor driver at 0.8 V has been reduced to as low as 0.3 V.

It is a good idea to check pin voltages on a relay’s coil circuit when the relay is “OFF” (not energized) and when the relay is “ON” (energized) and notice how the voltage on the pins change. Pay particular attention to the voltage on the side of the relay coil that goes to the control unit to learn what the voltage drop is of the transistor driver circuit in the PCM.

I would also recommend and emphasize that you measure the voltage on Pin 30 when the relay is “ON” and “OFF.” When the relay is off the voltage on Pin 30 should be B+. When the relay is energized the voltage on Pin 30 should not drop more than 0.5 V. A decrease in voltage exceeding 1.0 V indicates there is a bad connection in the wiring to Pin 30 causing an excessive voltage drop when the relay contacts close and electron current is flowing through the load, in this case the starter solenoid. Trace the wire back to B+ and repair the bad connection to establish good B+ voltage on Pin 30 when the relay is energized.

In the next segment we will see another control module added to the circuit that increases the complexity of this relatively simple function to crank an engine.

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