Heating and Air Conditioning: Description and Operation

CIRCUIT OPERATION
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 simultaneously: 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 with 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 the A/C Fuse to the A/C Switch, the coil and the switch portion of the A/C MG Relay and to the Dual Pressure Switch (incorporated into the A/C Triple 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 within the A/C Switch that lights when the A/C Switch is depressed. The A/C Amplifier will provide 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 allowing for the clutch to engage.

The A/C Compressor Clutch engages provided that the refrigerant pressure and evaporator core temperature are within specified ranges with a relatively light engine load. The A/C Amplifier will interrupt A/C Compressor Clutch operation if a problem is detected with refrigerant pressure, evaporator core temperature or 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 enough 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 15 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 frost and ice formation on the evaporator when evaporator core temperatures get 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. It does this by means of a voltage signal at the A/C Amplifier. If the A/C Amplifier receives this signal, the ground for the A/C MG Relay will 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 the engine performance.