With Air Conditioning

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 ENGINE CONTROL MODULE (ECM) 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 ENGINE COOLANT TEMPERATURE (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 ENGINE CONTROL MODULE (ECM). The ECM 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 ECM controls the engine idle according to other inputs.