Compressor HVAC: Description and Operation

The Compressor for the Air Conditioning System is belt driven by the engine through the A/C Compressor Clutch. The Clutch allows the Compressor to be disengaged when Air Conditioning is not required, and also allows the A/C load to be removed from the engine when needed.
Operation of the Compressor depends on the particular mode selected by the driver. When the A/C Mode Selector is in MAX, NORM, BI-LEV, or DEF, battery voltage is applied through the HTR A/C Fuse and A/C Compressor Control Relay to the remaining circuits.
The path to the A/C Compressor Control Relay is through the A/C Low Pressure Cut-Out Switch and the A/C High Pressure Cut-Out Switch, which are both normally closed. The A/C Low Pressure Cut-Out Switch opens if the Refrigerant charge is too low to operate the A/C Compressor without possible damage to it. The A/C High Pressure Cut-Off Switch opens if refrigerant pressure is high enough that overheating of the engine may result.
The A/C Compressor Control Relay is operated by the ECM. The relay contacts are normally open. When A/C mode MAX, NORM, B/L or DEF is selected, an A/C On voltage is applied to terminal 2, connector C2, of the ECM. This signals the ECM to ground terminal 3, connector C1, energizing the relay. The A/C Compressor Control Relay contacts close and provide voltage to the A/C Compressor Clutch.
The ECM will remove the ground at C1/3 to de-energize the relay when the air conditioning load should be removed from the engine, such as during wide open throttle.