First of all: this is for a 928S4 and later. Early models/years may be different.
The system uses both a switch and a sensor element. Two different "Sensors"
each with its own function.
|3 bar||43 PSI||27 ohm|
|3.5 bar||51 PSI||31 ohm||At about 35 Ohm the fans stop as the system is considered not running (compressor should remain off)|
|4 bar||60 PSI||40 ohm||This is the normal system pressure when all is shut down, and system properly filled|
|12 bar||174 PSI||99 ohm|
|15 bar||218 PSI||125 ohm||This is the high value where the fans go to maximum|
|19 bar||276 PSI||143 ohm|
note: be aware that sensors are not too precise. 5 ohm deviation is considered normal.
The analogue pressure Sensor at the AC dryer assembly controls the Fan activity. Pressure in the AC when not running should be about 4 bar (60 PSI) measured at the pressure port, and this relates to a sensor value of 40 ohms (values vary based on temperature, filling of the system, type of refrigerant). Fans run at intermediate power when the engine is not running but ignition and AC is turned on. When the AC is turned on and the compressor runs, interior fans to max cooling, pressure rises a bit and the sensor reads 80 ohm. Ąt full heat the pressure may rise up to 15 bar, the sensor then reads about 120 ohm, and the fans go to max power to cool down the refrigerant and case pressure drop.
When the sensor signal is not connected to the controller (wire disconnected at the sensor) the controller thinks the pressure is way up and turns the fans to their maximum power to cool down the AC and make pressure drop.
1992 928 GTS midnight blue
With the ignition switch on, the fans are triggered on at low speed at
approximately 78 deg C coolant temp, and reach full speed at approximately 95
deg C. They are triggered on at low speed at approximately 110 deg C
transmission fluid temp. They are triggered on at low speed at approximately 3
bar Freon pressure, and reach full speed at approximately 20 bar.
With the ignition switch off, the fans are triggered on at low speed at approximately 85 deg C coolant temperature, and off at approximately 83 deg C. They are triggered on at approximately 87.5 deg C air temperature in the intake plenum, and off at approximately 82.5 deg C.
ACM A/C Pressure Switch
Part number 959 613 137 00
Drier Mounted. 2 bar low pressure and 30 bar high pressure binary switch. This switch is designed to be compatible with R-12 and R134a refrigerants. Located on the lower part of the receiver-drier with 2 spade terminals. Used for compressor cut-out.
The Analog pressure sensor for the fan controller is located on top of the assembly.