A/C Compressor Control Circuit Diagnosis

Automatic Temperature Control With Auxiliary (CJ2):

Manual Temperature Control A/C With Auxiliary (CJ3, C60):

Circuit Description

When A/C is selected through the A/C controller a signal is supplied to the PCM through two different methods depending on the vehicle content. The first is by Class II serial data circuit (CJ2, and the second is by a 12 volt A/C request circuit to the PCM (CJ3, C60). 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 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 positive 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 greater than 96%.
^ A/C Pressure Sensor greater than 440 psi (4.6 volts) or less than 35 psi (0.35 volt) (as determined by the A/C Pressure Sensor).
^ Ignition voltage below 10 volts.
^ Engine speed greater than 4700 RPM.
^ ECT greater than 125°C (257°F).
^ IAT less than 5°C (41°F).
^ Class II communication error between the HVAC controller and the PCM (CJ2).
^ An open or a short to ground in the 12 Volt A/C request circuit (CJ3, C60).

Diagnostic Aids

Inspect for the following:

Important: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection.
Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Diagrams.
^ Damaged harness.
Inspect the wiring harness for damage. If the harness appears to be OK, observe the sensor display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the sensor display may indicate the location of the fault.
^ Inspect the PCM and the engine grounds for clean and secure connections.

If the DTC is determined to be intermittent, reviewing the Fail Records can be useful in determining when the DTC was last set.

Test Description

The numbers below refer to numbers on the Diagnostic Table:

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 parts if Diagnostic Trouble Codes are present.
3. The A/C compressor clutch should apply within 3-6 seconds if the conditions under the Notice have been met.
6. Checks for an A/C request signal from the A/C controller 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.