Heating and Air Conditioning: Description and Operation

Circuit Description
All electrical power for the air conditioning (A/C) system is supplied via fuse #17 (S17) on the main fuse/relay panel. When the A/C is turned on (A/C select switch) it:

- Energizes the A/C relay to power the ventilation blower (low speed)
- Energizes the radiator cooling fan low speed relay to power the radiator cooling fan (low speed)
- Powers the A/C control unit
- Provides an A/C-on input signal to the fuel injection (or Motronic) control unit

The A/C control unit supplies power to the A/C compressor clutch and, on 4-cylinder cars, the idle speed boost valve.

The A/C refrigerant low pressure switch cycles the A/C compressor on and off. As pressure drops (lower temperature) the switch opens and interrupts power to the compressor; as pressure increases (higher temperature) the switch closes and switches the compressor clutch back on.

The A/C refrigerant high pressure switch operates the second speed of the radiator cooling fan. The A/C refrigerant high pressure safety switch protects the compressor from excessive pressure.
The outside temperature switch opens and interrupts power to the A/C system and prevents the system from operating at low temperatures.

The A/C control unit interrupts power to the compressor:

- When engine coolant temperature is excessive
- When the kickdown switch is closed (automatic transmission cars)