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, as well as 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 specific to a certain type of PCM device.

- Click Testing/Output Test Mode (Solenoids/Relays)
- Coil Resistance (Solenoids / Relays)
- Harness Opens
- Harness Shorts

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 in actuator PID or actuator signal circuit measured by use of NGS.

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 should respond this way.

Intermittent Diagnostic Techniques

The purpose is to check harness for open circuits.

1. Key OFF.
2. OBD II DLC disconnected from any diagnostic tools.
3. Disconnect component from vehicle harness.
4. Install breakout box.
5. Use an ohmmeter to isolate the circuit in question from the BOB to the component connector pin.
6. Reading should be less than 5 ohms.

The purpose is to check the harness for short circuits (to ground or power).

1. Key OFF only.
2. OBD II DLC disconnected from any diagnostic tools.
3. Disconnect component from vehicle harness.
4. Use an ohmmeter to measure between the signal circuit and signal return circuit or power ground circuit or vehicle power.
5. It reading is less than 10k ohms, then the two circuits may be shorted.