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Thread: AC clutch not engaging

  1. #1
    Join Date
    Feb 2010
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    '92 345iC, '11 335D

    AC clutch not engaging

    Having trouble getting the AC compressor clutch to come on. I'm converting the system to R134a and so far have done the following:


    1. Replaced receiver/drier and o-rings
    2. R/R'd compressor, drained all oil, replaced o-rings
    3. Vacuumed to 30" Hg
    4. Verified no leaks
    5. Added 6oz PAG oil
    6. Added 24oz R134a
    7. Turned on engine, AC, and HVAC fan


    The clutch isn't kicking on. First things that come to mind are the pressure switch on the R/D and maybe the temp switch on the compressor housing. Is there a way to bench test these switches before replacing them?

  2. #2
    Join Date
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    1992 750il / 1991 560 SE
    Did you flush the system? Mineral will not vacuum out, needs a proper flush otherwise it remains in the evaporator. PAG oil and mineral are not comparable.

    What are your pressure readings? You can jump the switches for testing if needed.
    ______________________________
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  3. #3
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    Quote Originally Posted by unity View Post
    Did you flush the system? Mineral will not vacuum out, needs a proper flush otherwise it remains in the evaporator. PAG oil and mineral are not comparable.
    Didn't flush, no. I took the extra step of fully draining the compressor (most just replace the R/D) which is the lowest point in the system. All the oil should have collected there, and I poured out right about 6oz which was the factory fill amount. I've never had any oil collect in the evaporators I've pulled before?

    Quote Originally Posted by unity View Post
    What are your pressure readings? You can jump the switches for testing if needed.
    High and low sides were equalized at 70psi after the 6z of oil and 24oz of refrigerant. Not sure why I didn't think about jumping the switch, d'oh! Maybe I need more coffee... It's a 3-wire so I need to find the wiring diagram somewhere.
    Last edited by DarinS; 05-20-2019 at 05:46 PM.

  4. #4
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    Jumped the aux fan and discovered only high speed is functional, so I'll have to replace the resistor. Jumped the pressure switch as well (high, low, and both circuits) and still no engagement of the clutch.

  5. #5
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    I'm able to get the clutch to engage by running 12V to the black/grey wire on X129 (low pressure contacts on the pressure switch). Thinking maybe the switch was bad, I replaced it. Still no AC clutch engagement. Any ideas out there?

  6. #6
    Join Date
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    1990 735I (E32) M30
    I'm wondering since everything else is checked, do you think the compressor is just shot?
    1990 735I (Auto) E32 Feb 1990 to Present
    1996 Z3 Roadster (5 Speed) E36/7 Jan 1999 to Dec 1999
    1983 733I (5 Speed) E23 April 1983 to Feb 1990
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  7. #7
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    Quote Originally Posted by CroughtonE32 View Post
    I'm wondering since everything else is checked, do you think the compressor is just shot?
    Quote Originally Posted by DarinS View Post
    I'm able to get the clutch to engage by running 12V to the black/grey wire on X129 (low pressure contacts on the pressure switch). Thinking maybe the switch was bad, I replaced it. Still no AC clutch engagement. Any ideas out there?
    Nope. Compressor works just fine and the AC blows ice cold if I jump X129 as described.

  8. #8
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    1992 750il / 1991 560 SE
    Then it had to be electric related. Did you check the high pressure switch? My only though is its sending a false high pressure "reading", meaning its stuck. I have to assume you have gone over fuses and relays, so next might be a IHKA module that has failed/needs a rebuild. It is the brain of the entire system.
    ______________________________
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  9. #9
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    Given that it's a new switch I would hope that it's not a false high signal. I'm not familiar with the IHKA module though, I'll have to research it.

  10. #10
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    11/88 E32 750iL+98 E36M3
    IHKA and sword pinout http://www.bimmerboard.com/forums/posts/426626/
    the IHKA can with a fault code tool be diagnosed says the German language training manual http://www.e38.org/e32/klimaautomatik.pdf
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