Compressor Clutch Relay: Description and Operation

A/C System Controls Circuit Diagram:

DESCRIPTION
The A/C compressor clutch relay allows the PCM to control compressor clutch operation. The PCM delays the clutch engagement so the IAC valve can increase engine idle speed to compensate for the added load. The PCM also causes the relay to disengage the A/C clutch at wide open throttle, when the engine is overheating, and when the A/C system pressures are not within a specified range.

OPERATION
The relay gets battery power when the ignition is turned "ON" and energizes when the PCM completes the circuit to ground. When the A/C is switched to the "ON" position, and the A/C system has adequate charge, a request signal is sent to the PCM. When the A/C request signal is received, the PCM waits a fraction of a second and then grounds the relay driver circuit, energizing the relay and engaging the compressor clutch. The PCM also checks the actual status of the compressor clutch by monitoring the voltage in the clutch circuit. "High" voltage (at or near battery voltage) indicates the clutch is engaged. The PCM compares the actual status of the compressor clutch with the commanded status for self diagnosis of the relay. The PCM also "watches" the A/C pressure sensor signal immediately after the compressor clutch is engaged. If the system pressure does not increase by a specified amount within a certain time after the clutch commanded "ON", a trouble code will set and the A/C system will be disabled, to prevent system damage.