Compressor Control (735i/iL)

IHKA E32 COMPRESSOR CONTROL (735i/iL)







Compressor control on six cylinder E32 vehicles is handled by the IHKA control module as follows:
- Pressing the "snowflake" or recirculating air button on the IHKA control panel sends an "A/C ON" request to the IHKA control module.


NOTE: On vehicles equipped with Version "01" IHKA control modules, selecting maximum defrost mode also activates the compressor.


- The control module then energizes the compressor control unit (early 1988) or compressor relay (late 1988 through 1992), which directs power to the A/C compressor electro-magnetic clutch.
- With the high pressure, low pressure, and compressor overheat switches closed (see chart below), power reaches the A/C compressor clutch and the compressor operates.
- At the same time that the A/C clutch receives power, the engine control module (DME) gets a "compressor running" voltage signal which it uses to stabilize engine idle speed.
- The IHKA control module cycles the compressor on and off as necessary to maintain evaporator temperature at about 35 °F (3 °C), using the signal from the evaporator temperature sensor.
- The engine control module has no control over A/C compressor operation; the compressor will not "cut out" during wide open throttle acceleration.

Compressor Protection Pressure/Temperature Switch Values

HIGH PRESSURE SWITCH OPENS @ 441 psi (30 bar), PRESSURE INCREASING
CLOSES @ 338 psi (23 bar), PRESSURE DECREASING

LOW PRESSURE SWITCH * OPENS @ 17 psi (1.2 bar), PRESSURE DECREASING
CLOSES @ 32 psi (2.2 bar), PRESSURE INCREASING

COMPRESSOR HOUSING OPENS @ 266 OF (130 °C), TEMP INCREASING
TEMPERATURE SWITCH

* The Low Pressure Switch is used to sense low pressure on the HIGH side of the refrigerant system.


NOTE: As of 3/92, the refrigerant system on 735i/iL vehicles was filled with R-134a.