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2000 Audi TT Question: AC Compressor cut-out due to high pressure/fan not operating

 

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Visitor, 1.8L 4 Cylinder Turbo, Columbia, MD, May 24, 2010, 08:57

AC works fine as long as I am driving over 20-30MPH, or if outside temperature is not very hot. AC Compressor cuts out if I am stopped.

I tested and the pressure sensor on the high side is causing the compressor to stop due to high pressure. Manual says fan should be running at least at speed 1 when AC compressor is activated, however, it does not, although at times it will run at speed 2 if engine gets hot enough.

Should I replace Fan Control Module? My theory is that not enough air is flowing over condenser coil, causing high pressure. Fans should be running, but are not, but since they work on high speed (speed 2) I suspect a problem with Fan Control Module.

Thanks.

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  • Answer #1

    May 26, 2010, 17:38
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    I would try to verify that the module is getting the proper signal to operate the fan and that the module is sending the signal to the fan motor. But I think you're on the right track, it's probably the module.

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    Visitor, June 03, 2010, 02:18

    Thank you for your response QuattroMan.

    I ordered the fan control module before you responded. I should have done more testing/diagnosis because it turns out the FCM was not bad. I knew I was being lazy but thought it was likely the culprit.

    After extensive testing I determined that both radiator fans were bad as well as the high/low AC pressure switch (this is the same sensor that switched off the compressor due to high pressure).

    The high/low switch worked correctly for the excessive pressure condition, however, it did not work for the elevated pressure condition (it should trigger fans to operate at speed 2).

    Now that I have replaced both fans as well as the high/low switch all is well. Both fans now operate at speed 1 when compressor is engaged, and they operate at speed 2 when refrigerant pressure is higher (for example, on a hot day idling for extended periods).

    When I picked up the new fans from the VW/Audi parts place the guy said they sell a lot of fans. He said on the older cars they (for the most part) only sell fans when a car is in an accident, but for the newer ones they go bad. My guess is that the resistor for speed 1 fails due to heat over time.

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