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First, fan operation triggered by the radiator temp sensor (thermofab switch.) From Clarks: "Cooling Fan Operation On Late Model 944s The cooling fan operation on late model 944s is similar to that of early 944s. However, there are some distinct differences.
The late model 944s use a "slow speed" resistor similar to the early cars. However, on later cars, there are two resistors, one for each of the fan motors.
As with the early cars, the thermofan switch (at 92 °C) will complete the fan circuit, even if the ignition is not on. However, unlike the early cars when the fan circuit is completed, both fans run instead of just the driver's side fan. Also, on the later cars the thermofan switch is a dual element temperature switch with one contact closing at 92 °C for slow speed operation and one closing at 102 °C for high speed operation (slow speed resistor bypassed). The fans will operate is slow speed regardless of ignition switch position. However, fast speed operation is only available with the ignition switch on."
What the AC does to the fan:
Turning on the AC switch triggers the AC relay to turn on both fans at low speed (late cars 86 on). If the AC temp sensor is active, the fans go to high. This can happen very quickly since the Freon pressure/temp increase quickly on a warm day. They started out using a 50C temp sensor but in mid 86 switched to a 14 bar pressure sensor (see parts list, item 16). Since the temp goes up when the pressure goes up, they basically did the same thing, switched the fans to high as the AC heated up. The output of the AC temp sensor is run in parallel to the high temp (105C) radiator sensor (see diagram). When they changed the AC temp switch in 86, they also changed the metal AC high pressure line so the sensor are NOT interchangeable.
There is also low AC pressure switch that is in-line with the AC compressor clutch circuit (see diagram). Pressure below 2.2bar will not allow the AC clutch to engage.so no AC operation.
03-02-2019, 07:44 PM
