Compressor HVAC: Description and Operation

For MFI

With the Ignition Switch (X134) in position II, battery voltage is applied to the Ignition Load Relay (K127). The relay is energised, applying battery voltage to the Compressor Clutch Relay (K108) and the A/C Logic Relay (K170). When the Front Fan Speed Switch (X247) is turned to positions 1,11, III, or IV and the Front A/C Switch (X225) is turned on, ground is provided to terminal 86 of the A/C Logic Relay (K170) through the closed contacts of the A/C Dual Pressure Switch (X102) and the Front A/C Evaporator Temperature Switch (X101).

The relay is energised, applying battery voltage to the Engine Control Module (ECM) (Z132), which signals the ECM that compressor operation has been requested. The ECM then provides ground to the coil of the Compressor Clutch Relay (K1 08). The relay is energised, applying battery voltage to the Compressor Clutch (K107) Compressor Clutch operation is now controlled by the status of the A/C Dual Pressure Switch (X102) and the Front A/C Evaporator Temperature Switch (X101).

The Front A/C Evaporator Temperature Switch (X101) is fitted in the airflow out of the front evaporator to sense the temperature of the exterior fins. Should ice begin to form due to low temperature, the switch will open. The A/C Dual Pressure Switch (X102) monitors refrigerant pressure in the high pressure line. Should pressure become too high or too low, the switch will open. When one or both of these switches open, the ground supply to terminal 86 of the A/C Logic Relay (K170) will be interrupted. The relay de-energises, removing battery voltage from the Engine Control Module (ECM) (Z132), which signals the ECM that compressor operation should be turned off. The ECM removes the ground supply to the coil of the Compressor Clutch Relay (K108). The relay then de-energises, removing battery voltage from the Compressor Clutch (K107), thereby disengaging the Compressor.


For NAS SFI

With the Ignition Switch (X134) in position II, battery voltage is applied to the Ignition Load Relay (K127). The relay is energised, applying battery voltage to the Compressor Clutch Relay (K108). When the Front Fan Speed Switch (X247) is turned to positions I, II, III, or IV and the Front A/C Switch (X225) is turned on, ground is provided to terminal 28 of the Engine Control Module (ECM) (Z132) through the closed contacts of the A/C Dual Pressure Switch (X102) and the Front A/C Evaporator Temperature Switch (X101). The signal informs the ECM that compressor operation has been requested. The ECM then provides ground to the coil of the Compressor Clutch Relay (K108). The relay is energised, applying battery voltage to the Compressor Clutch (K107). Compressor Clutch operation is now controlled by the status of the A/C Dual Pressure Switch (X102) and the Front A/C Evaporator Temperature Switch (X101).