Compressor Control

The A/C Amplifier is powered by voltage from the HEATER Fuse and permanently grounded at G202.

With the Ignition Switch and A/C Switch in the ON position and the Blower Speed Selector Switch in any position except "OFF," an A/C Request signal is applied to the A/C Amplifier through the Blower Speed Selector Switch and A/C Switch. This prompts the A/C Amplifier to supply the Powertrain Control Module (PCM) with an Idle-Up Signal.

The Dual Pressure Switch closes when the pressure in the A/C system is between 193 kPa (28 psi) and 3137 kPa (455 psi). Voltage is then provided to the A/C Compressor Clutch Relay coil. Whenever the pressure in the A/C system drops below 193 kPa (28 psi) or rises above 3137 kPa (455 psi), the Dual Pressure Switch opens, the A/C Compressor Clutch Relay is de-energized, and voltage to the A/C Compressor Clutch is interrupted. These conditions arise only under extreme circumstances or when a fault is present, such as a low refrigerant charge or improper Evaporator temperature.

The Engine Coolant Temperature (ECT) Switch opens when the engine coolant temperature rises above 113 °C (235 °F). This signal warns the A/C Amplifier that the engine is overheating so that the A/C Amplifier will cut out the A/C Compressor Clutch in order to reduce engine load.

The A/C Compressor Clutch Relay coil is grounded through the A/C Amplifier at G202, provided that the A/C Amplifier has not received a low temperature signal from the Evaporator Thermistor or a high temperature signal from the ECT Switch and the Dual Pressure Switch is closed.

When the relay is energized through the Dual Pressure Switch, Battery voltage is applied to the A/C Compressor Clutch through the A/C Fuse and the contacts of the A/C Compressor Clutch Relay. Since the A/C Compressor Clutch is permanently grounded, the clutch engages and the compressor operates.

The A/C Amplifier monitors the Evaporator Thermistor mounted inside the evaporator housing. The Evaporator Thermistor converts evaporator core temperature into a resistance value. Using this resistance value, the A/C Amplifier can sense when evaporator temperature drops below 0 °C (32 °F). When this occurs, the A/C Amplifier removes the A/C Compressor Clutch Relay coil ground. This prevents evaporator frost and ice build-up which reduces air flow and reduces the unit's cooling capacity.

The A/C Amplifier also provides a signal to the PCM. The PCM uses this signal in order to increase the engine idle whenever the A/C system is active. If the A/C system is off, then the PCM controls the engine idle according to other inputs.