Compressor HVAC: Description and Operation

Air Conditioning Compressor:

Compressor

The SATURN compressor is a variable displacement rotary vane type pump. The rotor assembly houses five lightweight vanes that extend outward contacting the cylinder wall creating the compression area. The compressor has an internal control valve that allows it to change its pumping capacity or displacement, reacting to heat loads on the air conditioning system. The range of displacement can vary from 10% (14 cc) to 100% (142 cc). Due to the force of the torsion spring acting on the rotating plate the compressor's natural or resting state is at minimum displacement.

The compressor performs two main functions. It compresses the low pressure refrigerant vapor from the evaporator into a high pressure high temperature vapor, and pumps compressor oil through the A/C system.

Compressor Clutch

The magnetic clutch transmits the engine power to the compressor when the A/C button is depressed. The clutch assembly consists of a drive plate, pulley and field coil. The drive plate is splined to the compressor shaft. The pulley and bearing assembly is pressed on the compressor hub, and the field coil is mounted to the compressor body. When current flows through the field coil, the drive plate is pulled into the pulley by electromagnetic force. When the field coil is not energized the drive plate separates from the pulley and the compressor stops. A diode is used to protect electrical circuits from voltage spikes. For 1991 vehicles, the diode is mounted directly on the compressor. For 1992/1993 vehicles, the diode is located in the wiring harness.

Pressure Relief Valve

Mounted in the front head is a high pressure relief valve. To protect the A/C system from high pressure in the event of an electrical control system failure the valve will begin to open at about 530 psi. As system pressure is reduced the valve will reseat at about 425 psi. If the valve even opens it must be replaced.

Temperature Sensor

Mounted in the front head is a high temperature sensor to protect the compressor from overheating. The sensor will open the clutch circuit at about 302°F (150°C). The temperature sensor can be replaced without discharging the A/C system.

Compressor (PCM) Control Logic

The PCM will monetarily shut off the compressor under the following conditions.

Engine Temperature
A/C off at 244°F (118°C)
A/C on at 234°F (113°C)

Tip-In Launch Logic (Till)

A/C off for 0 3 second in automatic transaxle, 1.3 second in manual transaxle if:

Vehicle speed is less than 10 mph, change in TPS is greater than 0.8% in 12.5 MSEC, manifold air temp is greater than 113°F (45°C) and map is greater than 70 kPa.

Wide Open Throttle (WOT)

A/C off for 15 seconds if TPS is greater than 95% automatic transaxle, 90% manual transaxle back on with TPS less than 95% after 2 seconds.

Compressor Overspeed Protection (COSP)

A/C off if engine rpm is greater than 6500 rpm A/C on if engine rpm is less than 4500 rpm for 2 seconds.

Evaporator Low Ambient Protection (ELAP)

If ambient air temperature is between 40-60°F (14-20°C) and vehicle speed is greater than zero the compressor cycles one every 90 seconds depending on mat temperature and vehicle speed.

Power Steering Pressure (1991 Vehicles Only)

A/C off if power steering pressure is greater than 550 psi (+1-psi) equivalent to a full locked turn.

For 1992 vehicles a power steering switch is not used. The PCM will turn A/C off based on drop in idle speed.

High/Low Pressure Cut-Out Switch

A/C off if discharge pressure is greater than 400 psi +1-20 psi and back on at 250 psi +1-50 psi. Cooling will run for 180 seconds after A/C is turned off if pressure switch is activated. A/C off if discharge pressure is less than 40 psi.

Cooling Fan Logic

Fan on with A/C switch on fan off at speeds greater than 60 mph for 180 seconds. Fan runs for 180 seconds after A/C is turned off.