PCM Controlled Air Conditioning Diagnosis

***THIS ARTICLE HAS BEEN UPDATED BY TSB #71-65-46 DATED 6/97***



















Circuit Description

When an A/C mode is selected, the HVAC System sends a serial data message to the PCM requesting A/C compressor operation. 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 voltage is supplied to the compressor clutch coil. The PCM will enable the A/C compressor clutch whenever the engine is running and 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) (as determined by the A/C refrigerant pressure sensor).

^ Ignition voltage is below 10.5 volts.

^ Engine speed is greater than 4500 RPM for longer than 5 seconds.

^ Engine speed is greater than 5400 RPM for any amount of time.

^ 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, A/C compressor, A/C control head, A/C relay, or A/C refrigerant pressure sensor.

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 to the A/C relay control circuit at the PCM harness connector 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 1 voltage less than 10.5 volts.

^ Engine speed greater than 4500 RPM for longer than 5 seconds.

^ Engine speed greater than 5400 RPM for any amount of time.

^ ECT greater than 125°C (257°F).

^ IAT less than 5°C (41°F).

Test Description

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 not be engaged with the engine running if an A/C mode is not selected.

4. The A/C compressor clutch should engage if the appropriate conditions are met and an A/C mode is selected.

9. Checks the ignition positive voltage circuits to the A/C relay.

10. Determines whether the condition is caused by a malfunctioning A/C relay.

18. Determines if the PCM is turning the A/C relay ON due to a command from the HVAC system.

21. Determines if the A/C relay is malfunctioning or if the A/C relay control circuit is grounded all the time.

29. 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.