A/C Request Circuit Diagnosis

Diagram Chart:

Electrical:

CIRCUIT DESCRIPTION

A/C Request Circuit - When the HVAC controls are in any position that enables the A/C compressor, and the A/C compressor refrigerant switch is closed (low and high), the A/C Request circuit is connected to B+. The ECM energizes the A/C compressor control circuit when a high signal voltage is detected on the A/C Request circuit. The ECM will de-energize the A/C compressor control circuit during the Wide Open Throttle (WOT) maneuvers, when the ECT exceeds a predetermined threshold or when the A/C Request circuit signal voltage goes low (either by a pressure switch opening, or the HVAC system is in a non-A/C mode).

A/C Load Signal - The ECM also receives input on the A/C load signal circuit. This is accomplished through a pressure switch connected to B+ which is normally open. During high ambient temperature conditions or any other conditions which would cause very high refrigerant pressures, the switch closes, sending a B+ signal to the ECM. The ECM will then raise the idle speed by approximately 50 RPM in order to help compensate for these conditions.

A/C Compressor Control Circuit - The ignition voltage is supplied directly to the A/C Compressor Control relay coil. The ECM controls the relay by grounding the control circuit via an internal switch called a driver. The primary function of the driver is to supply the ground for the component being controlled. When the ECM is commanding a component on, the voltage of the control circuit should be low (near 0 volts). When the ECM is commanding the control circuit to a component off, the voltage potential of the circuit should be high (near the battery voltage).

The relay controls the high current flow to the A/C Compressor Clutch. This allows the ECM driver to only have to handle the relatively low current used by the relay.

DIAGNOSTIC AIDS

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. Testing and Inspection

TEST DESCRIPTION

The number(s) below refer to the step number(s) on the diagnostic table.

2. The scan tool should indicate Yes with the HVAC controls in any position that turns on the A/C compressor.