Compressor Controls

With the Ignition Switch in RUN, voltage is applied from Fuse 7 to the Heater and A/C Control Assembly. If the driver has selected MAX, NORM, BI-LEVEL or DEF, the Control Assembly will provide voltage through various switches and relays (which are explained below) to the Compressor Clutch.

Pressure Cycling Switch

The Pressure Cycling Switch prevents the formation of ice on the coil of the evaporator. Because the coils are cooler than the surrounding air, condensation forms on their surfaces. This condensed moisture can freeze if the temperature of the evaporator goes too low. Since pressure and temperature of refrigerant vary in proportion, the Pressure Cycling Switch opens when too low pressure indicates an equivalent too low temperature.

When the switch opens, it interrupts voltage to the Compressor Clutch. The clutch and the compressor stay off until refrigerant pressure and temperature rise enough to prevent icing.

A/C Cut-Out Relay

The A/C Cut-Out Relay interrupts voltage to the Compressor Clutch when engine conditions require a reduced load. With the Ignition Switch in RUN, BULB TEST, or START, voltage from Fuse 5 is applied through the coil of the relay to the Electronic Control Module (ECM). The ECM provides a path to ground if engine conditions permit compressor operation. When the path to ground is available, the coil energizes, closing the contact.

Voltage is then available at the compressor.

Compressor Clutch Diode

when current through the Compressor Clutch is turned off the magnetic field surrounding the coil collapses. As the field collapses, energy from the magnetic field is converted to voltage. The diode provides a path for the current.

A/C High Pressure Cut-Out Switch

When the pressure in the system is in excess of 2760kpa (400 PSI), the A/C High Pressure Cut-Out Switch opens, turning off the compressor to prevent excessive compressor head pressures which reduces the chance of refrigerant escape through a safety relief valve.