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Trying to figure out A/C

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  • Trying to figure out A/C

    I've been browsing through the archives and have yet to find a solution to my a/c problem. When i start up my compressor and a/c clutch kick on and i get cold air. Once the car gets to operating temp (a tick below half on the gauge), the a/c system shuts down. My auxiliary fan never comes on. I've recharged the system with 134 so it's not a problem with the system's pressure. I think i've narrowed it down to the temperature sensors for the system, because from what i've read the relays come into play only when running past 90*C.
    I have an early '88 model, so the sensors are in the radiator not on the thermostat. Are there any other sensors i should worry about..or any other possible culprits to the problem? Also, does anyone know what temp ratings for each sensor i'm looking at?

  • #2
    I have seen expansion valves do very odd things. I commonly replace them on these cars, and they are usually always very corroded. I have gotten to the point where I like changing them along with a reciever/drier preventively to insure quality A/C operation.

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    • #3
      the system was retrofitted old-school style (new compressor/the whole thing) about 4 years ago...seems like the expansion valve would have been replaced with it, but i'll check it out. the a/c started acting up after i replaced my radiator a couple months ago, so that's what is drawing my attention to those sensors.

      also, i hooked up the aux fan wires to the the 2-prong sensor closest the wheel-well, and the fan kicked on before i even turned the engine over...engine was still warm. At least i know the aux fan is good, but it still won't switch on when the a/c is turned on.

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      • #4
        I have a question for you guys. The auxillary fan is the one on the front of the raidiator, just behind the bumper right? When is it supposed to operate?

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        • #5
          yeah, that's the auxiliary fan. it's my understanding that it's supposed to kick on when the temp goes above 98*C or whenever the a/c is turned on. mine doesn't come on when i turn the a/c on, and i haven't run hot enough to see if the sensor will kick it on, but whenever i hooked up the aux. fan to the lower temp sensor it came on right away, which is making me think that sensor is bad because there's no way the engine is above 90*C before i even turn it on.

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          • #6
            another question...anyone know what temperature shuts down the a/c?

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            • #7
              Hey M3ubbs,
              The aux fan gets it's power to run through that large resister clamped next to the fan when the a/c is switched on. The fan is supposed to run in low speed with the a/c selected on. This has nothing to do with any coolant temp sensors. The only time a coolant sensor comes into play is when the engine is running hot, then the fan runs at high speed. Your resister must be bad, a very common problem
              Mark Jon Calabrese
              1988 M3
              2008 535i
              2005 K1200S
              2011 135i

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              • #8
                thanks mark.. that must be the problem with the fan not coming on. I'll get that resister replaced and hopefully that will cure that problem. My temp sensors come tomorrow and hopefully replacing those will take care of the compressor shutting down once I get to operating temperature.
                Thanks again, Matt.

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                • #9
                  I have the Magna style aux fan resistor...i was wondering where this large resistor was -- this one is a long thin .5 ohm resistor that was just hanging next to the fan and had seemingly been spliced in with the fan wiring. I've been surfing around for the part...Autohausaz has them for $25. Anyone know a better price out there, or a suitable replacement that could be used??
                  Thanks, matt.

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                  • #10
                    Matt

                    I was wondering why you think the a/c compresser will shut on or off with any temp sensors. It does not. I would check for the ground wire that is attached to the compresser clutch and the compresser frame. This short wire has caused problems many times. Look at your compresser from under the car and you should see this ground wire, make sure it's good. If it is see if you have power at the clutch. Let me know and we'll go from there.
                    Mark Jon Calabrese
                    1988 M3
                    2008 535i
                    2005 K1200S
                    2011 135i

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                    • #11
                      update...
                      well replacing the temp sensor didn't fix the problem. Seemed like that had to be the reason...it's like clockwork: a/c works until operating temp is reached (a tick below half on the gauge), then after 30 sec. or so the compressor kicks off and so does the a/c until the temp cools off.
                      I will try getting the new resistor. Any other suggestions on the culprit??

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                      • #12
                        Mark,

                        Ground wires are good. Clutch and compressor both work until i've been at operating temperature for a couple minutes. I found no fault in the wires anywhere else in the system (compressor wires or aux fan wiring). The reason I thought it was the temp sensor was the fact that the a/c seems to shut down at the same temperature each time, and when I unplugged the wiring to that low temp sensor the a/c would not work at all. Apparently this was coincidental because replacing the sensor did not work. I also looked at k1 relay. I was unsure how to check it. I felt a click in the relay when i turned the car on, but nada whenever i turned on/off the a/c. I swapped it with the horn relay as I knew it was ok and same result (click on start-up). So I guess I'll see if replacing the aux fan resistor will cure it. Until then i'll live with my five minutes of 60 degree air blowing at me before the compressor decides to shut down.

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                        • #13
                          have you tried putting a jumper wire into the temp sensor connector? this is how my car is rigged up until i get a new temp sensor. if the compressor STILL shuts off with the sensor jumpered, your problem lies elsewhere
                          James
                          sigpic

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                          • #14
                            Matt

                            There is a low pressure switch, a high pressure switch and in some cars an a/c temp switch that are in between the evaporator temp regulator and the compressor. If one of these are open the compressor will quit. Look for where the 12v stops when the compressor quits. Compressor operation has nothing to do with coolant temp.
                            Mark
                            Mark Jon Calabrese
                            1988 M3
                            2008 535i
                            2005 K1200S
                            2011 135i

                            Comment


                            • #15
                              Welcom MJC....hardly a novice. Glad you are here....I enjoy your columns.

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