A/C Request 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 B+, 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.
^ 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.

DIAGNOSTIC AIDS
Verify A/C system pressure is sufficient for A/C operation. Refer to HVAC diagnosis, Component Tests and General Diagnostics
An intermittent may be caused by the following:
^ Corrosion
^ A mis-routed harness
^ A rubbed through wire insulation
^ A broken wire inside the insulation

Refer to Testing for Intermittent and Poor Connections for Intermittents.

TEST DESCRIPTION

Steps 1 - 7:

Steps 8 - 11:

The numbers below refer to the step numbers on the diagnostic table.
2. The heater and A/C control will set a DTC if the A/C Request circuit is shorted to ground or to battery voltage.
8. If all circuit and terminal inspections are OK, the fault must be in the heater and A/C control.