Analyzing PCM Data

Once the fault area is identified, the circuit must be checked to determine if the wiring or component is at fault.

When making circuit and component measurements, make sure all accessories, dome, trunk and hood lights are Off. Use any of the following methods to diagnose a suspected PCM wire circuit or device. Some methods are particular to a certain type of PCM device.
- Change Condition to Cause Response by Input
- Change Input and Verify Output Response
- Click Testing/Output Test Mode (Solenoids/Relays)
- Coil Resistance (Solenoids/Relays)
- Harness Opens
- Harness Shorts

Change Conditions to Cause Response by Input
The purpose is to verify the sensor receives and responds to changes.
1. Select, view and record the appropriate sensor PID(s).
2. Create condition or cause condition to change.
3. If reading changes appropriately, then it should be operating OK.

Examples:
- View ECT PID while engine warms up.
- It should change from a higher voltage (2.6v) for a cold engine, to a lower voltage as the engine warms up (0.6v).
- Move TP, observe TP PID change.
- Press brake pedal, watch BOO PID change states.

Change Input and Verify Output Response
The purpose is to verify how the PCM and actuator circuit responds to sensor input.
1. Select, view and record the appropriate sensor PID(s).
2. Create condition to cause input condition to change.
3. Observe change (response) in actuator PID or actuator signal circuit measured by a measuring device.

Example:
- Increase TP under load, observe IAC PID and circuit change.

Click Testing (Solenoids/Relays)
The purpose is to activate a solenoid or relay from the PCM by entering Output Test Mode.
1. Key On.
2. Enter Output Test Mode.
3. Turn outputs On and then Off.
4. Listen for relays to click on and off. If a breakout box is connected to the PCM, measure the control circuit while turning the outputs on and off. A voltage change of greater than 4 volts should occur during the ON and OFF transition.

Example:
- FP relay WAC relay, cooling fan relay and PCM power relay.

Coil Resistance (Solenoids/Relays)
The purpose is to measure the correct resistance value of a device.
1. Key off.
2. Data Link Connector (DLC) disconnected from any diagnostic tools.
3. Disconnect component from vehicle harness.
4. Using an ohmmeter and referencing the Static Resistance Value chart in Graphs and Charts, measure across the component terminals. Graphs and Charts

Harness Opens
The purpose is to check harness for open circuits.
1. Key off.
2. DLC disconnected from any diagnostic tools.
3. Disconnect component from vehicle harness.
4. Install breakout box.
5. Using an ohmmeter, isolate the circuit in question from the BOB to the component connector pin.
6. Reading should be less than 5 ohms.

Harness Shorts
The purpose is to check the harness for short circuits (to ground or power).
1. Key Off only.
2. DLC disconnected from any diagnostic tools.
3. Disconnect component from vehicle harness.
4. Using an ohmmeter, measure between the signal circuit and signal return circuit or power ground circuit or vehicle power.
5. If reading is less than 10K ohms, then the two circuits may be shorted.