Air Conditioning System Circuits

The air conditioning system controls the radiator fan, the condenser fan, and the A/C compressor clutch.


Radiator Fan


Voltage is provided at all times to the radiator fan relay and the radiator fan control module. For the module, voltage is also provided in RUN and START The module provides power to the radiator fan relay when the ignition switch is in RUN or START and for 2 minutes after the ignition is switched off. The radiator fan relay is energized whenever the BLU wire is grounded. The BLU wire can be grounded by the engine coolant temperature switch, the A/C switch and fan switch, the engine control module (ECM), or the radiator fan control module.


Condenser Fan


With the ignition switch in RUN and START, and for 2 minutes after the ignition is switched off, the module also provides power to the condenser fan relay coil. The condenser fan runs under the same conditions as the radiator fan. The module can ground the relay coils only when the engine coolant temperature switch is closed. The condenser fan can run after the ignition is switched OFF until the engine coolant temperature switches open, or the module removes power.


Compressor Clutch


When the A/C switch is turned on, and the fan switch is in position 1,2,3, or 4, the RED/BLU input to the engine control module (ECM) is grounded. This signals the ECM to ground the compressor clutch relay by grounding the RED/BLU wire. This energizes the relay, applying voltage to the A/C compressor clutch, and the clutch engages. If the ECM receives a signal from the distributor assembly that engine rpm is above 4500, a signal from the vehicle speed sensor that vehicle speed is above 79 mph, or a signal from the engine coolant temperature sensor that coolant temperature is above 221 F ground will not be provided to the compressor clutch relay and the clutch will not engage.


A/C Pressure Switch (dual)


A/C system pressure below 38 psi indicates a loss of refrigerant. The A/C pressure switch opens with system pressure below 38 psi to prevent compressor damage. The high pressure switch will disable the compressor clutch at 242 psi.

A/C Thermoswitch


The A/C thermoswitch senses evaporator temperature. With evaporator temperature above 41 degrees F the A/C thermoswitch is closed. With evaporator temperature below 33 degrees F, the A/C thermo switch opens interrupting power to the A/C switch which shuts off the compressor. In this way, the evaporator temperature stays above 32 degrees F and moisture on the evaporator fins will not freeze and block air flow.


A/C Diode


The diode prevents the engine coolant temperature (ECT) switch from grounding the A/C ON input to the engine control module (ECM).