Hi,
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.
 

  1. Switch:  Located  the bottom of the drier assembly. This is a high pressure and low pressure switch that is normally closed and opens when pressure is either below a certain point or above a certain pressure level. When it "activates"  it turns off the 12 connection to the compressor coupler, thus stopping the compressor. It is an on-off switch, that is normally "on" when pressure is over minimum 2.2 bar. It switches off at 25 bar (360PSI). Part number is 959 613 137 00.
  2. Sensor: Located on top of the drier assembly. This is a analog pressure sensor. It measures the Freon pressure and sends this to the climate controller. This controller evaluates and modulates the PWM signal to the fan power stage to make the fans run at a requested speed. Failure to measure a normal resistance (shortcut or wire broken) will trigger the fans to run at full speed as a safety precaution. First stage running at 78C, full speed at 95C. (for the record: the fans also activate fully when automatic transmission fluid gets to 110C in order to cool down the ATF or partially when the intake overheating is detected)  
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.  

regards

Theo

1992 928 GTS midnight blue

http://928gts.jenniskens.eu

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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.

Wally Plumley

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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.