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  1. #1
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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


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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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    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: 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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    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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    could be the IHKA module too

    but also the temperature sensors, and also a hraircrack in the temperature sensor behind the IHKA control module.
    here are all temp sensors shown
    Temperature sensors:

    There are five temperature sensors in the system.

    1. Passengerside heatercore temperature sensor
    2. Driverside heatercore temperature sensor
    3. Controlpannel temperature sensor
    4. Evaporator temperature sensor
    5. Outside temperature sensor

    check the IHKA control module
    and resolder it, if necessary. It sits behind the console at the firewall behind the sword and the filter (if your car has a microfilter). You have to disconnect from driver and passenger side each 2 plugs. So remove the 2 carpet covers on each side of the console. 1 screw each side. It is clipped in from one side (as far as I remember from the passenger side). Just pull it out.

    Here a copy:

    Subject: Climate Control Misbehaving Fix
    Author: shogun Posted on: 2008-02-13

    winpenny: symptoms are:

    1.output low through all vents

    2.on starting when set to medium heat on dial car will get warm inside but once on the move a medium heat isnt possible. Either full heat or nothing.

    3.fan seems fine but air not getting out well except on max setting. windsreen
    vents seem ok though.

    Car has FSH and the microfilter was changed 25k ago
    I have removed IHKA front control panel and cleaned temp sensor which improved things slightly.

    I am thinking maybe IHKA control module is at fault since I cannot see that all
    the flaps and stepper motors all to be at fault

    would like some advice on what to try in order of economy first !

    I know there is plenty on the sites on climate control but I'm not sure what to check next. I'd like to do some checks in order of economy without major dismantling !
    -------------------------------------
    A: Shogun
    Change the microfilter, that is dirty and plugged again, much too long installed. After that you will have a good flow.
    Recommendation is 15 k km (not miles) or once a year, whatever comes first.

    Clean once more the temperature sensor behind the control panel in the console.
    If that is dirty or not working, temperature cannot be controlled.
    http://www.[Oops!].com/forums/posts/425554/

    It can also be damaged,
    according to some people in Germany it is called

    TNC-Widerstand mit einem R25 von 10k Ohm

    = TNC resistor with R25 of 10 Kohm.
    article number at Conrad electronics:

    Artikel Nr. 500538 HEISSLEITER; TEMPERATURFÜHLER; M89

    Probably Timm can tell you what you need then. Some people described the same symptoms your car has with changing temperatures.
    It was solved by changing this feeler/thermistor.
    -----------------------------------------------------------
    A: Timm:
    Erich is right (as usual) on both counts. Start by changing the Microfilter, this will mak a HUGE difference. The climate control will not work correctly until this is done. The problem is that unless you have a very small friend you will be hanging half out of the car during the process:
    http://www.the7seriesregister.co.uk/forums/index.php?showtopic=1191
    Give it a while after changing the Microfilter and let us know how it goes. If you still have problems we wil take it from there......
    ---------------------------------------------------
    Winpenny:
    i've changed filter. you're right about needing to be small. i found myself hugging the sill at times ! i also put the filter in the wrong way round first off which of course means the sword cover doesn't go on properly.
    nb for info for others the torx head screwdriver you need is 20mm.
    there is some improvement in output. from your comments timm is it correct that i need to leave it some miles to adjust ? thanks Simon
    ----------------------------------------------
    Timm:
    No, it should be better straight away. The E32 never puffs out air in an uncouth manner, it is just too refined. It should regulate the cabin temperature perfectly though, has that got any better?
    ----------------------------------------------------------------
    Winpenny: cabin temp still a problem. its either really hot when past the click on the drivers temp wheel or nice and cold on any setting below. however soon after starting it will give a warm temperature at a middle setting but soon goes cold.
    I do have a spare control unit bought off ebay which i could clean and try but your advice on next steps would be appreciated.
    -------------------------------------------------------------
    Timm: Yes, clean the sensor on the new one and try fitting that.....let us know what happens........
    ---------------------------------------------------
    Winnpenny: Unfortunately the new one has only 2 prongs to accept the smaller of the 2 plugs from wiring loom ( the black one not the big blue one ). It appears it was manufactured like this. The unit in the car at the moment has 3 prongs. The plug fits but when I connected it I just got air from the screen vents when the button was on for floor only. So I suspect it wont talk properly to the IHKA unit. BMW lists lots of varients for the control unit.

    What now Timm ? Should I try to source another control unit ( anyone got the right one ? ) ? Or is their something else I can try in the meantime.
    ---------------------------------------------------
    Timm: sounds as if the control unit is working as is the IHKA, but the thermister sounds like it is faulty. Using Erich's specification of Negative Temperature Coifficient and 10k resistance this one will do nicely:
    click
    Try replacing that, also check that the little sucker-motor is working, this is the little fan that samples the air from the cabin. If the fan does not run then it will not sample the temperature and will do exactly what you describe.....
    -------------------------------------------------------------
    Winpenny: I got the thermistor from RS components and a friend soldered it on and tested it. Seems to work fine now. Flaps sound a bit "sticky" when opening but I guess that's no big deal.
    Thanks for your help Timm and Shogun.
    -----------------------------------------------
    Shogun: Great! Just for the records I copied the spec here too
    Glass bead NTC G540 thermistor,10k
    RS Stock No. 528-8558
    Manufacturer EPCOS
    Manufacturers Part No. B57540G0103J
    Catalogue page 2 - 3587
    1 £3.99
    spec
    pic
    ------------------------------
    Timm: Hooray! Another satisfied customer. And now we have a source for the components that have been tested in real life..........I'll remember that for later...

    original posts here: http://www.the7seriesregister.co.uk/forums/index.php?showtopic=9145


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

  9. #9
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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

  10. #10
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    I got it despite the oopsing, thanks

    If you have more, send the to Hairywithit, he will include it into the website. (email addy is on website)
    Often people do not understand how the flow it behind the engine, under and in front of valves. I am not so good with the PC to make drawings, so if you have time and like it, make it and send to him.
    Thanks

    PS: your post is here
    com/forums/posts/459398/

    with the additional stuff in front to complete the link.

    here another good link:
    ZKE and EDC explained, in German, but good pics if you do not understand German
    http://pok.atw.hu/index_elemei/home_elemei/letoltes_elemei/EDC%20-%20Central%20Chassis%20Electronics.pdfShogun E32 Tech Tips: http://twrite.org/shogunnew/topmenu.html

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