PCM Controlled Air Conditioning Diagnosis

Diagnostic Chart:

Wiring Diagram:

Circuit Description

When A/C is selected through the A/C control head a 12 volt signal is supplied to the A/C request input of the PCM. The A/C compressor clutch relay is controlled through the PCM. The PCM monitors the A/C refrigerant pressure. If the A/C refrigerant pressure, and engine operating conditions are within a specific calibrated acceptable ranges the PCM will enable the A/C compressor relay. This is accomplished by providing a ground path for the A/C relay coil within the PCM. When the A/C compressor relay is enabled 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 any of the following conditions are met:

^ Throttle Position (TP) angle greater than 95%.

^ A/C head pressure greater than 440 psi (4.6V) or less than 36 psi (0.4V) (as determined by the A/C refrigerant pressure sensor).

^ Ignition voltage below 10 volts.

^ Engine speed greater than 4300 RPM.

^ ECT greater than 123°C (253°F).

^ IAT less than 5°C (41°F)

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 driver 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 fault.

Check for conditions that will disable A/C operation:

^ Throttle angle greater than 95%.

^ A/C head pressure greater than 440 psi (4.6V) or less than 36 psi (0.4V) (as determined by the A/C refrigerant pressure sensor).

^ Ignition voltage below 10 volts.

^ Engine speed greater than 430 RPM.

^ ECT greater than 123°C (253°F).

^ IAT less than 5°C (41°F).

Test Description

Number(s) below refer to circled number(s) on the diagnostic chart:

The A/C compressor clutch should not be engaged with the engine running if an A1C mode is not selected at the control head.

2. Stored Diagnostic Trouble Codes (That may not illuminate the Malfunction indicator Lamp) may disable the A/C compressor. This Diagnostic Table may lead to improper diagnosis and replacement of good pants if Diagnostic Trouble Codes are present.

3. The A/C compressor clutch should apply if the conditions listed under the Important are met.

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

12. Checks the ignition feed circuit to the A/C relay.

13. Determines whether the A/C relay or the circuitry is at fault.

21. Determines whether or not the PCM is turning the A/C relay ON due to a false A/C request.

22. Isolates problem to the A/C control head or the PCM.

27. Determines whether the A/C relay is faulty or the A/C relay driver circuit is grounded all the time.