PCM Controlled Air Conditioning Diagnosis

Diagnostic Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION

When an A/C mode is selected, the A/C control head or the A/C programmer supplies a 12 volt 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 voltage is supplied to the compressor clutch coil. The PCM 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) (as determined by the A/C refrigerant pressure sensor).

^ Ignition 1 voltage is below 10.5 volts.

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

^ Engine speed is greater then 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. 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 below 10.5 volts.

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

^ Engine speed greater then 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 at the control head.

4. The A/C compressor clutch should engage if the appropriate conditions have been met.

6. Checks for an A/C request signal from the control head to the PCM.

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

13. Determines whether the condition is caused by a malfunctioning A/C relay or circuitry.

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.