A/C Request Circuit Diagnosis

A/C Operation Description
The 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 (approx. 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 an HVAC 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/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 in Heating and Air Conditioning.

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 Symptoms for Intermittents. Symptom Related Diagnostic Procedures

Test Description
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.