A/C Clutch Control Circuit Diagnosis

A/C Clutch Control Circuit Diagnosis




Circuit Description
When air conditioning and blower fan are selected, and if the system has a sufficient refrigerant charge, a 12-volt signal is supplied to the A/C request input of the powertrain control module (PCM). The A/C request signal may be temporarily cancelled during system operation by the electronic thermostat in the evaporator case. The electronic thermostat may intermittently remove the control circuit ground for the A/C thermostat relay to prevent the evaporator from forming ice. When the A/C request signal is received by the PCM, the PCM supplies a ground from the compressor clutch relay if the engine operating conditions are within acceptable ranges. With the A/C compressor relay energized, battery voltage is supplied to the compressor clutch coil.

The PCM will enable the compressor clutch to engage whenever A/C has been selected with the engine running, unless any of the following conditions are present:
- The throttle is greater than 90%.
- The ignition voltage is below 10.5 volts.
- The engine speed is greater than 4500 RPM for 5 seconds or 5400 RPM.
- The engine coolant temperature (ECT) is greater than 125 °C (257 °F)
- The intake air temperature (IAT) is less than 5 °C (41 °F).
- The power steering pressure switch signals a cramped position.

Diagnostic Aids
To diagnose an intermittent fault, check for the following conditions:
- Poor connection at the PCM - Inspect harness connections 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; shorts to ground, shorts to battery voltage, and open circuits. If the harness appears to be OK, observe the A/C clutch while moving connectors and wiring harnesses related to the A/C. A sudden clutch malfunction will indicate the source of the intermittent.

Steps 1 - 2:

Steps 3 - 7:

Steps 8 - 9:

A/C Clutch Diagnosis
This chart should be used for diagnosing the electrical portion of the A/C compressor clutch circuit. A Scan Tool will be used in diagnosing the system. The Scan Tool has the ability to read the A/C request input to the PCM. The Scan Tool can display when the PCM has commended the A/C clutch ON. The Scan Tool should have the ability to override the A/C request signal and energize the A/C compressor relay.