PCM Controlled A/C Circuit Diagnosis

Circuit Description

When A/C mode is selected, the HVAC control head supplies a battery positive voltage signal to the PCM A/C request input. The A/C compressor clutch relay is controlled by the PCM. The PCM provides, a ground path for the A/C relay coil. When the A/C compressor relay is engaged battery positive voltage is supplied to the compressor clutch coil will enable the A/C compressor clutch.

Whenever the engine is running and the A/c has been requested unless one or more of the following conditions are present:

^ Throttle angle is greater than 90%.
^ A/C head pressure is greater than 414 psi (4.27 volts) or less than 34 psi (0.35 volt) determined by the A/C refrigerant pressure sensor).
^ Ignition voltage is below 10.5 volts.
^ Engine speed is greater than 5440 RPM.
^ ECT is greater than 125°C (257°F).
^ IAT is less than 5°C (41°F).

Any of the conditions described above will Inhibit A/C compressor relay operation.

Diagnostic Aids

Check for the following conditions:

^ Poor connection at the PCM or the HVAC control head. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
^ Damaged harness. Inspect the wiring harness for damage.
^ If the harness appears to be OK, disconnect the PCM, turn the ignition ON and observe a voltmeter connected between the A/C relay control circuit at the PCM harness connector and ground while moving connectors and wiring harnesses related to the A/C relay. A change in voltage will indicate the location of the malfunction.

Check for conditions that will disable A/C operation:

^ Throttle angle greater than 90%.
^ A/C head pressure greater than 414 psi (4.27 volts) or less than 34 psi (0.35 volt) (as determined by the A/C refrigerant pressure sensor).
^ Ignition voltage below 10.5 volts.
^ Engine speed greater than 5440 RPM.
^ ECT greater than 125°C (257°F).
^ AT less than 5°C (41°F).

Test Description

The number(s) below refer to Step number(s) on the diagnostic table:

2. Stored diagnostic trouble codes may disable the A/C compressor. This diagnostic table may lead to improper diagnosis and replacement of good parts if diagnostic trouble codes are present.
3. The A/C compressor clutch should engage if the appropriate conditions have been met.
4. The A/C compressor clutch should not be engaged with the engine running if an A/C mode is not selected at the control head.
6. Checks for an A/C request signal from the control head to the PCM.
12. Checks the ignition positive voltage circuit to the A/C relay.
13. Determines whether the A/C relay or the circuitry is at fault.
21. Determines if the PCM is turning the A/C relay ON due to a false A/C request.
22. Isolates the problem to the A/C control head or the PCM.
27. Determines if the A/C relay is malfunctioning or the A/C relay control circuit is grounded all the time.
35. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.