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    88 750iL A/C cuts out

    I've got an A/C problem in my '88 750 that hopefully someone can help with. It blows cold, but after 10 minutes or so, the compressor seems to disengage. The only way to get the compressor to come back on is to turn the car off and immediately restart it. But if the car's been siiting in a parking lot, then on a warm restart, the compressor usually won't come on until I've driven the car a few yards, turned it off, and then immediately restarted it. Odd. Again, in all cases, the compressor won't stay engaged for more than 10 minutes or so.

    Also, there seems to be some heat intermittently blowing from the windshield and floor vents whether I asked for it or not.

    Thanks in advance for any advice!

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    might be the pressure switch

    which sits on top of the dryer on the right hand side under the airbox in engine room. Depending on the system, you either have a switch with 3 switch points, low-medium-high, or the newer version low-high.
    Have you checked the refridgerant? Enough? or too much filled?
    Here you can see pics of the switch

    http://bmwe32.masscom.net/johan/airc...con_R134a.html

    "Also, there seems to be some heat intermittently blowing from the windshield and floor vents whether I asked for it or not."

    test the temperature sensors. Test data here
    http://bmwe32.masscom.net/johan/ihka/ihka.html

    or check out for more info on my website


    Shogun E32 Tech Tips: http://twrite.org/shogunnew/topmenu.html

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    Re: might be the pressure switch

    Thanks for the info and links to even more great info. I'll start with checking the freon level. I hadn't suspected that initially because the A/C seems pretty cold when working. Is there a way (unless it is unwise) to jump the switches so the compressor is always on? I live in a climate where A/C at some level is generally called-for during most of the year.

    Also, would disconnecting the wires to the aux water pump at the heater valves disable the heat? That would be appreciated until I get the A/C sorted.

    Your pictures were very helpful. Thank you for the advice.


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    Inner Circle ©157 since: Aug 14, 2003 cheung1 is an unknown quantity at this point cheung1's Avatar
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    disabling the heat

    there are a couple of ways to disable heated coolant entering the heater core. If the heater valves and solenoids are sound you can use a wire to ground the outer pin (both sides, two wires) at the connector top of the heater valve assembly by the firewall. Some other posts below:

    http://forums.roadfly.com/forums/bmw/bmw-7-series-e32/8024930-1.html

    http://forums.roadfly.com/forums/bmw/bmw-7-series-e32/7394847-1.html

    I also fit a garden brass valve to the hose between engine and valve assembly inlet (and removed the aux water pump permanently). This way guarantee no coolant entering heater even if the valve core is broken. Patrick C 88 750 159K

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    Patrick, I still need some pics from you

    and a brief description how to separate the heater valves and the aux water pump from the heater core with a brass valve to the hose between engine room and valve assy. Many people do not understand the flow of the coolant which comes from the radiator and from the back of the engine near firewall. Want to add that on my website, send the comment and pics to my email known to you or directly to hairywithit to the address shown on the website. Thanks.

    @willistwillis:
    the aux water pump's only purpose is to feed a constant flow of coolant/heat to the heater core, no matter what the ROM of the engine is. When aux water pump is eliminated, the volume of the flow depends only on the RPM of the engine, that means less heat comes to the heater core. You find that out immy when a aux water pump is out of operation at a red signal light or in traffic jam when car is in idle.
    Shogun E32 Tech Tips: http://twrite.org/shogunnew/topmenu.html

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    Re: Patrick, I still need some pics from you

    Thanks alot for the help, guys. Unplugging the aux pump and installing the on/off valve sounds like a very sensible fix that can also easily be undone in the event of a need for heat. If possible, I'd like to see the diagram as well.

    Is it possible that a bad IHKA module could cause un-asked-for heat in addition to the A/C problem I'm experiencing? The other day on the way home from work I had cold A/C on the driver's side and full-on heat on the passenger's side, even though both dials were on max cold. Of course, the A/C lasted about 10 minutes and then the compressor disengaged. The passenger's side returned to normal when I restarted the car after dinner.

    Could a bad IHKA control module be causing this?

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    Shogun, my post can be searched in BB by

    "patrick heater plug" and check "Picture". Now that I reread that post I realize there should be a photo of the three hoses with the plugs at the ends. When I can get to work on my 750 I'll take that photo. At this moment my car has the heater core and evaporator removed. I am pissed at myself that I left the A/C pipes by the side of house in the rain and now they have developed some rust inside the pipes. ... Hope some C.L.R. cleaner can remove the rust.Patrick C 88 750 159K

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    Re: 88 750iL A/C cuts out

    It's 20 below wind chill here, so I'm not having any cooling problems:) What's up guys?

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