Circuit Description

When air conditioning and blower fan are selected, and if the system has a sufficient refrigerant charge, a 12-volt signal is supplied to the A/C request input of the Engine Control Module (ECM). The A/C request signal may be temporarily canceled during system operation by the electronic thermostat in the evaporator case. When the A/C request signal is received by the ECM, the ECM supplies a ground from the compressor clutch relay if the engine operating conditions are within acceptable ranges. With the A/C compressor relay energized, voltage is supplied to the compressor clutch coil.

The ECM detects air conditioner high-pressure side pressure (load). In response to this pressure, it adjusts engine idling speed and condenser fan operation.

The ECM will enable the compressor clutch to engage whenever A/C has been selected with the engine running, unless any of the following conditions are present:

• The throttle is greater than 72% for 1.0 second.
• The engine coolant temperature (ECT) is greater than 118°C (243°F).
• The elapsed time after engine start is less than 3 sec.