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Thread: A/C Compressor Intermittently Locks Out

  1. #1
    E500E Guru emerydc8's Avatar
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    A/C Compressor Intermittently Locks Out [RESOLVED]

    I have been chasing this annoying problem for several years now with no luck. Most of the time the compressor will engage and the a/c will work on the first start of the day; but occasionally it locks itself out where the compressor will not engage on initial startup. Restarting the car usually fixes it, but sometimes it takes several attempts. What is strange is that when the compressor does engage on the first start of the day, it always seems to disengage about ten minutes into the drive. When I restart the car it usually goes back to engaging, but sometimes it takes a few attempts. I have always been able to get it to work, but on rare occasions it has taken quite a few restarts.

    The codes I'm repeatedly getting on pin 8 are DTC 6 (a/c clutch jammed or v-belt broken) and DTC 7 (poly-v-belt slips). I have replaced the compressor (including speed switch), receiver/dryer, expansion valve, interior temp sensor, poly-v-belt and just installed a new factory tensioner (3rd version). Even tried some belt dressing with no improvement.

    I thought the new tensioner would finally solve the problem but it didn't. The system is properly charged and the pressures are within spec. When the compressor operates it cools well -- It either works well or it doesn't work at all. I always run with the temperature selector in the max cool position here in Arizona, so I think this would bypass any issues with the tempersature control head.

    So, what's left to replace on this? Any thoughts? I'm thinking of swapping base modules next to see if that's the problem. Thanks.
    Last edited by emerydc8; 08-12-2017 at 12:28 AM. Reason: Resolved with Base Module replacement
    Jon D.
    1994 E420
    1995 E420

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    Re: A/C Compressor Intermittently Locks Out

    Jon, it sounds like you have covered most of the basics here. Some random thoughts:

    1) Try turning the compressor by hand, when cool. It should rotate quite easily. Compare to your other car if possible. This would rule out any unusual resistance inside the compressor.

    2) It MIGHT be the speed sensor at the rear of the compressor... or, the wiring. Easy to check the wiring, harder to check the sensor, and I'm not sure if you can easily replace the sensor? (EDIT - ooops, I totally missed that you already have new compressor + sensor. My bad!)

    3) Unlikely, but the Basic Module (BM/GM) which controls the compressor could be flaky. Swap with a spare for testing, if possible. Easy and cheap (or free, if you have another M119 car).

    Dave M.
    1997 E420 (Bugeyes)
    1994 E420 (Blondie)
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    E500E Guru emerydc8's Avatar
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    Re: A/C Compressor Intermittently Locks Out

    Thanks, Dave. The compressor and speed switch are both new (Denso). I turned the compressor by hand to prime it with oil before installing it last month. The wiring to the speed sensor looks good. I turned all the other components (generator, water pump, air pump) by hand yesterday when I replaced the tensioner and they also turn freely.

    I think you may be right about a flaky basic module. Are there any adaption issues when swapping basic modules that I need to know about?
    Jon D.
    1994 E420
    1995 E420

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    .036 Hoonigan™
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    Re: A/C Compressor Intermittently Locks Out

    Jon, I missed the part about you having a new compressor+sensor. D'oh!

    The basic module is a direct swap... no adaptations of any kind, just want to check/clear codes after installing to make sure it's not carrying over a code from a previous car.


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    E500E Guru emerydc8's Avatar
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    Re: A/C Compressor Intermittently Locks Out

    Thanks, Dave. Can you confirm that if the wheel for the temp control inside the cockpit is clicked all the way into the full cold detent then it should bypass the control head?
    Jon D.
    1994 E420
    1995 E420

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    Re: A/C Compressor Intermittently Locks Out

    Quote Originally Posted by emerydc8 View Post
    Thanks, Dave. Can you confirm that if the wheel for the temp control inside the cockpit is clicked all the way into the full cold detent then it should bypass the control head?
    Jon, I'm not certain, but I believe when set at max-cold detent the HVAC PBU will ignore the interior cabin air temp sensor, and run the compressor all the time... only cycling off based on the evaporator temp sensor, if it gets too cold (to prevent icing). There are safeties in the system to turn off the compressor if refrigerant pressure is too high or too low, but I think that's about it besides the compressor speed sensor signal which is compare to engine RPM, to detect a slipping belt or seized compressor.


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    Re: A/C Compressor Intermittently Locks Out

    Thanks, Dave. The only thing I didn't do was flush out the system when I changed the compressor and expansion valve. In retrospect I should have done it just as a precaution, because my receiver/dryer filter was covered with black powder. I don't know if this is possible, but if there is debris or contaminant floating around the system that causes the pressure to surge and the compressor to lock itself out, I'm wondering if that event would show up as a slipped/broken belt DTC or is that DTC only related to an actual slipped or broken belt.
    Jon D.
    1994 E420
    1995 E420

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    Re: A/C Compressor Intermittently Locks Out


    After a month of flight simulator training, I was finally able to get back to town and try a new (used) Base Module. That seems to have done the trick. I hate to jinx myself, but I haven't been able to get the a/c to stop working all day; so, it must have been the Base Module that was causing the false poly-v-belt slip DTC and locking out the compressor. Well, at least I have an extra compressor now for the other car when it goes out. Thanks to M104 (Poland) for the PM explaining that this is exactly what happened to his car too, and the Base Module solved it. No one knows why.
    Jon D.
    1994 E420
    1995 E420

  10. The Following 4 Users Say Thank You to emerydc8 For This Useful Post:

    gsxr (08-12-2017), JC220 (08-12-2017), Stevester 500E (08-11-2017), TerryA (08-12-2017)

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