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 Engine Control Module (ECM). The A/C request signal may be temporarily canceled during system operation by the electronic thermostat in the evaporator case. When the A/C request signal is received by the ECM, the ECM supplies a ground from the compressor clutch relay if the engine operating conditions are within acceptable ranges. With the A/C compressor relay energized, voltage is supplied to the compressor clutch coil.

The ECM detects air conditioner high-pressure side pressure (load). In response to this pressure, it adjusts engine idling speed and condenser fan operation.

The ECM 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 72% for 1.0 second.
- The engine coolant temperature (ECT) is greater than 118 °C (243 °F).
- The elapsed time after engine start is less than 3 sec.

Diagnostic Aids
To diagnose an the intermittent fault, check for following conditions:
- Poor connection at the ECM - Inspect 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. If the harness appears to 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 fault.

A/C Clutch Diagnosis
This chart should be used for diagnosing the electrical portion of the A/C compressor clutch circuit. A Tech 2 will be used in diagnosing the system. The Tech 2 has the ability to read the A/C request input to the ECM. The Tech 2 can display when the ECM has commanded the A/C clutch "ON". The Tech 2 should have the ability to override the A/C request signal and energize the A/C compressor relay.

Test Description

IMPORTANT: Do not engage the A/C compressor clutch with the engine running if an A/C mode is not selected at the A/C control switch.

The numbers below refer to the step numbers on the Diagnostic Chart:
4. Although the normal complaint will be the A/C clutch failing to engage, it is possible for a short circuit to cause the clutch to run when A/C has not been selected. This step is a test for that condition.
7. There is an extremely low probability that both relays will fail at the same time, so the substitution process is one way to check the A/C Thermostat relay. Use a known good relay to do a substitution check.

Steps 1 - 2:

Steps 3 - 9:

Steps 10 - 17:

Steps 18 - 24:

Steps 25 - 30:

Steps 31 - 38:

Steps 39 - 41: