PCM Output Diagnosis

PCM Outputs Diagnosis





Diagnosis
The following table will diagnose the PCM controlled outputs from the output driver modules only. For diagnosis of the cruise control module circuit, refer to Cruise Control Diagnosis. For specific system components and wiring refer to Electrical Diagnosis or Transmission Diagnosis for transmission related components.

Circuit Description
The Powertrain Control Module (PCM) controls most components with electronic switches which complete a ground circuit when turned ON. These switches are arranged in groups of 4 and 7, and are called either a surface mounted quad driver module, which can independently control up to 4 outputs (PCM) terminals or an Output Driver Module (0DM), which can independently control up to 7 outputs. Not all outputs are always used.
Drivers are fault protected. If a relay or solenoid is shorted, having very low or zero resistance, or if the control side of the circuit is shorted to voltage, it would allow too much current flow into the PCM. The driver senses this and the output is turned OFF or its internal resistance increases to limit current flow and protect the PCM and driver. The result is high output terminal voltage when it should be low. If the circuit from B+ to the component or the component is open, or the control side of the circuit is shorted to ground, terminal voltage will be low. Either of these conditions is considered to be a driver fault. Drivers also have a fault line to indicate the presence of a current fault to the PCM's central processor. A scan tool displays the status of the driver fault lines as 0 = OK, 1 = Fault.





Refer to Schematics for applicable pin-outs.
Diagnostic Aids
- The scan tool has the ability to command certain components and functions ON and OFF. If a component or function does not have this capability, operate the vehicle during its normal function criteria to check for an open or shorted circuit.
An open or short to ground will appear in the open positions on the scan tool only when it is not commanded by the PCM or scan tool, while a short to voltage will appear in the short positions on the scan tool only while the component is being commanded by the PCM or scan tool.

Test Description








The number(s) below refer to the step number(s) on the Diagnostic Table.
1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
2. A 1 in any of the positions indicates that a open or short is present in the corresponding circuit for that position.
3. Refer to the scan tool display art in this section to determine which circuit corresponds to which position of the scan tool display.
4. An open or short to ground will appear in the open positions on the scan tool only when it is not commanded by the PCM or scan tool, while a short to voltage will appear in the short positions on the scan tool only while the component is being commanded by the PCM or scan tool.
5. Proper operation of any component or function with no 1 in any of the positions indicates that system operation is normal at this time.
6. A component or function that failed to operate at this point indicates that the malfunction is not on the PCM side of the circuitry. Go to the appropriate Transmission, 8A or Engine Controls section for further diagnosis.
8. The 1 going away after disconnecting the component electrical connector indicates that the component or component side wiring is malfunctioning. If the scan tool indicates a malfunction after disconnecting the component electrical connector and verifying that no open or short is present in the circuit, then the PCM is malfunctioning.
9. Replacement PCMs must be reprogrammed. Refer to the latest Isuzu Technical Communication System (ITCS) information for programming procedures.
10. If no malfunctions have been found at this point, refer to Diagnostic Aids for additional checks and information.