Compressor Control (850Ci/CSi)
IHKA E31 COMPRESSOR CONTROL (850Ci/CSi)
As introduced in 1991, compressor control on V-12 E31 vehicles has an additional protection device, compared to the system on the V-12 E32.
A compressor cut-off relay is placed between the pressure switch and the lock-sensor control module. When energized by the ECM, the cut-off relay interrupts the compressor request signal from the compressor control relay. The ECM energizes the cut-off relay during wide open throttle acceleration at low vehicle speeds (less than 5 mph (8 km/h)).
As on E32 vehicles, the lock-sensor control module monitors the following sensors:
- A/C compressor RPM (speed sensor inside compressor)
- Engine crankshaft RPM (TD signal from DME engine control module)
- Engine coolant temperature
If coolant temperature rises too high, or if the compressor and crankshaft speeds differ by more than 30%, the lock-sensor control module will interrupt power to the A/C clutch.
Mother E31 difference is that the control module may run the A/C compressor at engine start-up regardless of "snowflake" button status, depending on ambient temperature and evaporator temperature.
At start-up the control module compares these two temperatures and, if evaporator temperature is lower than ambient temperature, the compressor is cycled on and off (for increasingly shorter intervals) for up to 15 minutes. This allows the evaporator to gradually warm to ambient temperature, slowly removing condensation which may be present inside the housing assembly If evaporator temperature is higher than ambient temperature at engine start-up, the control module does not automatically cycle the compressor.
Similarly, if the A/C compressor is switched "Off" while the engine is running, the compressor continues to run, cycling on and off for about 15 minutes. This allows the evaporator to gradually warm up to ambient temperature, thereby reducing the chances of window fogging.
NOTE: As of 3/92, the refrigerant system on V-12 E31 vehicles was filled with R-134a.