HVAC Compressor Clutch Circuit Diagnosis

Engine Controls Schematics: A/C Controls:

A/C OPERATION DESCRIPTION
The Engine Control Module (ECM) controls the A/C clutch relay. ECM A/C operation is controlled by two input circuits, A/C request and A/C load, and one output circuit, the A/C compressor relay control. The heater and A/C control sends an A/C request signal, approximately battery voltage, whenever the operator depresses the A/C selector switch on the control panel. When the ECM receives this signal, the ECM will turn the A/C clutch relay ON unless one of the following conditions is observed:
^ The engine is not running.
^ The ignition voltage is less than 9 volts.
^ A Wide Open Throttle (WOT) condition is present.

The ECM will NOT detect the A/C request signal if the A/C system pressure is too high or too low. The A/C request circuit is in series with a portion of the A/C compressor refrigerant pressure switch. The pressure activated switch contacts will open if the A/C pressure is not correct for proper A/C operation. Refer to the electrical schematic diagrams for the specific operating pressures. The heater and A/C control monitors the A/C request circuit for shorts. A shorted condition will cause a Heating, Ventilation And Air Conditioning (HVAC) Diagnostic Trouble Code (DTC) to set.

The ECM also monitors the A/C load switch circuit. This circuit is designed to increase the engine speed by about 50 RPM whenever a high temperature and pressure condition causes the A/C load switch contacts to close.

The ECM controls clutch operation by supplying a ground path for the A/C compressor relay. The A/C compressor relay control circuit is monitored for opens and shorts. An open or shorted condition will cause an ECM DTC to set. ECM control of the A/C clutch relay can be tested using a scan tool.

CIRCUIT DESCRIPTION
The A/C compressor relay switch contact is supplied battery power from fuse 33. When the ECM commands the A/C clutch relay ON, the relay switch contacts close allowing current to flow to the A/C compressor clutch coil and also to the auxiliary coolant fan relay, causing the auxiliary coolant fan to operate. The current flows through the A/C compressor clutch coil to the secondary cooling fan temperature switch. The current continues through the secondary cooling fan temperature switch to ground. The temperature activated contacts of the secondary cooling fan temperature switch will open, turning the A/C clutch OFF, if coolant temperature is too high for proper A/C system operation.

TEST DESCRIPTION

Steps 1 - 11:

Steps 12 - 19:

The numbers below refer to the step numbers on the diagnostic table.
1. You must perform the A/C Request Circuit Diagnosis prior to diagnosing the A/C compressor clutch circuit. This procedure contains important steps on checking the systems for diagnostic trouble codes.
2. The engine coolant must be below the specified temperature in order to properly test the HVAC compressor clutch circuit. When the temperature threshold is exceeded, the secondary cooling fan temperature switch contacts will open the ground path to the A/C clutch coil.