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 A/C 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.