HVAC: Compressor Controls

A/C System operation is monitored and controlled by various relays, a Dual Pressure Switch, an Evaporator Thermistor and the A/C Amplifier. The A/C Compressor Clutch will engage when the following conditions are met: engine running with A/C Switch depressed, Blower Speed Selector Switch in any position except "OFF," refrigerant pressure and evaporator core temperature within specifications, and a relatively light engine load.

With the Ignition Switch in the "ON" position, system voltage is applied through the GAUGE Fuse, the Audio Alarm Module and Junction Block 3 to the coil of the Heater Relay. With the Blower Speed Selector Switch in any position except "OFF," the Heater Relay Coil is provided a ground through the Blower Speed Selector Switch to G204. With a ground provided, the Heater Relay is energized, thus applying system voltage from the Heater Fusible Link through the closed contacts and through the A/C Fuse to the A/C Switch, the A/C MG Relay, and to the Dual Pressure Switch (incorporated into the A/C Dual pressure Switch). System voltage is applied to the A/C Amplifier when the A/C Switch is depressed. With system voltage applied, the A/C Amplifier can begin monitoring and controlling various inputs and signals. This is indicated by a Light Emitting Diode (LED) within the A/C Switch that lights when the A/C Switch is depressed. The A/C Amplifier provides a ground path for the A/C MG Relay if a number of conditions are met. With both power and ground provided to the coil of the relay, its contacts close permitting voltage to be applied to the A/C Compressor Clutch, thus causing the clutch to engage. The A/C Compressor Clutch engages provided that the refrigerant pressure and evaporator core temperature are within specified ranges and the engine load is relatively light. The A/C Amplifier will disengage the A/C Compressor Clutch if a problem is detected with refrigerant pressure, evaporator core temperature or a high engine load. Once the problem is no longer present, the A/C Compressor Clutch will re-engage.

The A/C Amplifier determines if the A/C System has proper pressure to operate by receiving a signal from the Dual Pressure Switch. The Dual Pressure Switch opens with extremely low or high refrigerant pressure. The Dual Pressure Switch remains closed when refrigerant pressure is between 206 kPa (30 psi) and 2648 kPa (384 psi). The evaporator core temperature is monitored via the Evaporator Thermistor. The Evaporator Thermistor's resistance changes with evaporator core temperature. The A/C Amplifier applies a voltage across the Evaporator Thermistor. As evaporator core temperature increases the resistance of the Evaporator Thermistor decreases, thus reducing the voltage being monitored at the A/C Amplifier. As evaporator core temperature decreases the resistance of the Evaporator Thermistor increases, thus increasing the voltage being monitored at the A/C Amplifier. The A/C Amplifier converts this voltage value into a temperature value and uses it to prevent the evaporator core temperature getting too low. If the A/C Amplifier has determined that either the refrigerant pressure is too low or too high, or if the evaporator core temperature is too low, it will remove the ground for the A/C MG Relay, thus disengaging the A/C Compressor Clutch. Once the pressure and temperature return to normal the A/C Compressor Clutch will re-engage.

The Engine Control Module (ECM)/Powertrain Control Module (PCM) can also interrupt A/C Compressor Clutch operation. During high engine loads, it sends a voltage signal to the A/C Amplifier, causing the ground for the A/C MG Relay to be removed. The A/C Compressor will re-engage when the ECM/PCM has determined that the increased load from the compressor will not affect engine performance.