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Thread: AC, Where to start

  1. #1
    Join Date
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    AC, Where to start

    01' 740iL, 60k
    Multiple intermittent AC issues, just looking for a place to start. Might be related, might not.

    All these scenarios are with HVAC on manual, center vents, fan at max, recirc off. Essentially just toggling AC switch.

    • Aux fan runs with AC button most of the time, but not always.
    • Blower fan runs most of the time, but occasionally it will only run with AC button OFF.
    • Sometimes the blower fan will blow at a higher velocity when the AC button is OFF.
    • Compressor engages with AC button most of the time, but not always. Then it will cycle on/off, periodically. This might be normal.
    • AC temp is very cold when compressor is running, even if blower is not running and it's just air drifting through.
    • All this happens more often when ambient is above 90 F, but it's happened in the 70s too.


    Other seemingly unrelated issues I'll get to after AC:
    • Loose about 8oz. of coolant every week or two.
    • Wipers come to rest slightly off their stop occasionally. Not really an issue, more an annoyance.
    • Unlocked my doors while approaching the car and both front windows opened about an inch or two. Only happened once.


    I was going to start with fuses and relays, then move to aux fan and hvac control unit last. Well, last before the dash is calling my name. Only one local shop can diagnose the pulsed width aux fan but they are booked solid for weeks. I've got a week of travel before I can really dig in but not opposed to replacing any usual suspects (sensor/switch/relays) if it's simple/reasonable cost.
    Thanks all, S.

  2. #2
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    Start with getting it re-gassed using one of the automatic machines that are prevalent these days. It will evacuate all the Pag oil, refrigerant and moisture - measure it - and then pull a good vacuum and hold it to make sure you don't have any leaks. Once all that is done it will replace the gas and Pag oil. That is always the best place to start as it lets you know if the system had initial problems - and you can then be sure that you have oil and refrigerant at the correct levels.

    All the things you have described can be due to low refrigerant level - including rapid cycling, low air-flow, intermittent operation etc. The A/C system is quite electronically complicated and it behaves strangely if the system cannot reduce the evaporator temperature sufficiently. Depending on temperature, the compressor may fail to run at all....
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  3. #3
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    As far as two of your three unrelated A/C issues:
    A valley pan leak of 8oz in a week or two (depending on how much you drive) wouldn't be out of reach. You wouldn't ever see any leak on the ground as the radiator fluid leak from a valley pan would evaporate while sitting in the valley of the "V" of your V8.

    One thought of the front windows dropping an inch or two once - could you have accidentally held the remote key unlock button down to long? If you hold that unlock button down past when the doors unlock, the windows will start to roll down & then the sunroof will open.

    Just some thoughts.
    <== Steptronic Sealbeach740
    2000 740i sport: 74k Green/Tan chrome MPars, clear corners, quad brake lights, AIC hi-beams, Hoen fogs, 16x9 screen, MKIV, TFT LCD screen in back, license plate backup camera with "on demand" switch, iPod audio/video (CDC/iPod audio switching, iPod video on 16x9 screen), Basslink, gauge rings, ///M pedals, switched steptronic +/- shifting mode, E46 paddle shifter steering wheel, Dinan engine & tranny software upgrade, DDEs controlled via Euro fog light switch, painted calipers with "BMW" lettering, windows up/sunroof close via remote.

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  4. #4
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    Quote Originally Posted by Timm View Post
    Start with getting it re-gassed using one of the automatic machines that are prevalent these days. It will evacuate all the Pag oil, refrigerant and moisture - measure it - and then pull a good vacuum and hold it to make sure you don't have any leaks. Once all that is done it will replace the gas and Pag oil. That is always the best place to start as it lets you know if the system had initial problems - and you can then be sure that you have oil and refrigerant at the correct levels.

    All the things you have described can be due to low refrigerant level - including rapid cycling, low air-flow, intermittent operation etc. The A/C system is quite electronically complicated and it behaves strangely if the system cannot reduce the evaporator temperature sufficiently. Depending on temperature, the compressor may fail to run at all....
    Second Timm's suggestion, correct charge is critical to proper function.

    AND...check the interior hvac filters, one left side one right. These filters, if clogged, have a diverse effect on air distribution and blower speed.
    Right side is easy to change, left side (right and above the throttle) can be a pain if you're not limber. Order replacements, or at LEAST pull the fabric
    foam out and thoroughly clean it with soap and water, let air dry.

    Once you're over the back spasms, check the under hood filters, one each side. Easy, can even have a beer while doing it.
    Cheers!

  5. #5
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    Yes, as a matter of fact I did hold the unlock button down too long, just tested it. I do have a whine/groan in the ac compressor so I will start with recharge. I’ll follow up after that is done. As for the coolant I do expect a valley or other leak that evaporates before visible, that’ll come next. Interior filters? I replaced the inlet filters in the hood, and don’t even like that idea. But interior for recirc? Man, who thinks this stuff up. Great help and thank you all.

  6. #6
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    Update.
    Full evacuate and recharge and it was down quite a bit. No change except the compressor is now whisper quiet. At the moment no blower operation at all. Aux fan does not come on reliably with ac button but will run at times. Week at beach so for now it’s research and thought till I’m home next week. Want to find a diffinitive means to test aux fan, could have used that on the 530 too. Thanks all.

  7. #7
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    The auxiliary fan is controlled via the DME - and the only decent test is to use INPA diagnostics to command the fan to run. On the pre-facelift models with the two-speed fan it would run immediately the A/C was switched on - not so with the facelift models.
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  8. #8
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    Right, shopping for INPA cable/software now. Any first hand experiences/recommendations greatly appreciated. Thanks, s.

  9. #9
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    While waiting for INPA and back probes I've gone after the glove box and recirc filters (which I might just discard actually) but back to the point.

    I couldn't understand what was happening until now, with eyes on the lower flapper valves. The unit switches to Recirc whenever the AC button is ON regardless of which state the actual Recirc button is in, the flapper valve closes of outside air from entering the cabin.

    At highway speeds I swore the AC button was turning OFF the blower motor........but now it looks like the motor was never running, just vented air coming through. I've thrown out all assumptions of "normal behavior" on this car so I ask: Is this the intended operation of the AC unit? To shut the fresh air vents when AC is switched on, regardless of Recirc button.

  10. #10
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    If using the 'max' button, yes. In that mode your fan will be full, recirc will be engaged, and temp readout will be
    60. If using the snowflake button, you should be able to switch from normal, AUC sensor, and recirc, and adjust blower to your desired
    flow. Does the blower respond to speed request in other modes?

  11. #11
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    All these scenarios were with HVAC on manual, center vents, fan at max, recirc off. Essentially just toggling AC (snowflake) switch.

    HVAC on manual, center vents, fan at max, recirc off, AC (snowflake) switch OFF.
    Recirc flaps actuate open/close with Max AC button on/off, as expected and confirmed.

    HVAC on manual, center vents, fan at max, recirc off, AC (snowflake) switch ON.
    Recirc flaps actuate open/close with Recirc button but only if temperature setting is above 68 or 70. Ramping temp below this range flaps move to Recirc ON regardless. Recirc button no longer has any impact.

    I'm beginning to suspect the hvac brain, assuming there is one, as this behavior is just to bizarre and random to be simple electro-mechanical devices failing. And the blower still never runs despite indicating at Full on for both driver and passenger sides. Thanks for all the help thus far, I should have back probes by the weekend and meter the blower control wires. If it's getting the proper signals then I have to assume at the very least the blower motor is shot.

  12. #12
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    That sounds as simple as the blower fan (or the final stage unit which is strapped to it) has failed.
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  13. #13
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    +1 you have to pull the dash and check the blower motor. It's not as bad as it sounds just do a google search and you will find plenty of info.
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  14. #14
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    So test drive confirms it. Temperature setting controls Recirc on/off. I get AC at speed from fresh air input as long as temp setting is set fairly high say 78-80. I’ve wondered why this car has always felt stuffy and stale. Any temp below 80 turns off the fresh air, it’s like living in a ziplock bag. I’ll go after the fan first but I’ve got to sort out the lack of outside air.

  15. #15
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    This is called the 'Y' factor operation - it is where you set a cabin temperature far below what the cabin temperature is - the 'Y' factor is the difference between the demanded temperature and the cabin temperature.

    With a large 'Y' factor the IHKA will try to rapidly reduce the cabin temperature by going into re circulation mode. In a working system the cabin temperature will drop rapidly and the 'Y' factor will reduce to a point where the IHKA will allow fresh air back in again.

    This is normal operation - but your problem is that your system doesn't work because the fan doesn't move air around!

    Although this is for the E31, the basic operation of the E38 IHKA is just the same:

    https://www.meeknet.co.uk/e31/IHKA.pdf#page=16
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  16. #16
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    Thank you Timm, glad to know it’s inteneded, one less thing to chase. In the mountains here the quickest means to close a large delta between commanded and actual cabin temp is fresh air venting. The solar gain sitting in a parking lot can be substantial even in mild temps. Otherwise the AC is working to cool cabin air that is easily 20-30 warmer than ambient. In practice Ive rarely used recirc in any car I owned, just not much call for it here. Thanks again, the time I save not chasing ghosts is invaluable. Unfortunately ive got some trips coming up so digging deeper will have to wait. Cheers!

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