Heating and Air Conditioning: Description and Operation

CIRCUIT OPERATION
The A/C amplifier is powered by voltage from the heater Fuse and permanently grounded at G201.

With the ignition switch and blower speed selector switch in the "ON" position, an "A/C REQUEST" signal is applied to the A/C amplifier through the blower speed selector switch, A/C switch fuse 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 207 kPa (30 psi) and 2641 kPa (384 psi). Voltage is then provided to the A/C compressor clutch relay coil. Whenever the pressure in the A/C system drops below 207 kPa (30 psi) or rises above 2641 kPa (384 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 and when a fault is present, such as a low refrigerant charge.

The Engine Coolant Temperature (ECT) switch closes 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 G201, provided that the A/C amplifier has not received a low temperature signal from the evaporator thermistor or a high temperature signal from the coolant high temperature 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 case 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 condenser fan relay coil ground as well as 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.