'89 735IL A/C questions - BMW Forum - BimmerWerkz.com
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#1 (permalink) Old 06-13-2008, 08:35 PM
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'89 735IL A/C questions

Good evening all,
Looking for help as I am my neighbors mechanic on his '89 735 IL!! We are having issues where the a/c seems to be working fine, but the heater is also on at the same time. This car has dual climate control. In the off setting, eveything is off, during the winter in the heat setting everything appeared to work fine. Now that we have charged the a/c and attempting to make it cool (it is 98 here in Texas), the a/c is running, cooling, but we then are getting hot air overriding the a/c system. When engine is cold--first start up--a/c works fine, but as engine heats up, the heater evidently is also. We think it is only the passenger side, but now not sure. Does this sound like a servo problem, a circuit board issue, or what? I am scratching my head as I have lots of experience with cars, but not BMW. Any suggestions? Thanks, Steve
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#2 (permalink) Old 06-13-2008, 08:43 PM
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#3 (permalink) Old 06-13-2008, 09:35 PM
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Couple things to look at:

-First make sure you don't have the dial in the center of the dash set to the "red" heat settings; it controls a little stepper motor that allows you to fine tune the air temp. I really doubt this is the issue, but ... you never know; often it's the simple things being overlooked.

Just a FYI, there is a dozen or more of those little stepper motors throughout the car that fine tune the air temp and all sorts of crap; sadly it's all getting very old so it's prone to failure. Just hope it isn't one of those ... first I honestly (even after about 5 years of owning these cars) have no idea where they are, and second it means ripping out the whole interior to test and replace them ... and they're expensive too ...

That said, I'd start by checking the actual A/C controls; if you have the fancy controls ... you know, the automatic climate control with the horizontal dials, then it's possible that's bad. The sheer amount of "stuff" that does is amazing ... bad part is it's hard to find them to swap and see if that fixes it. If it's the regular one with the 2 temp knobs and 1 fan speed knob, then I'd swap a good unit in and see if that solves anything.

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#4 (permalink) Old 06-13-2008, 09:39 PM
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btw, air conditioned air will only be blowing out the dash vents; so make sure you have it switched to just that. The car is not designed to blow cold air through other vents (and guess what allows it to do this: blow heat everywhere, yet only blow cold air through the dash? yup you guess right, those stepper motors.) It's a hugely complicated system in the e32/e34 ... a real nightmare sadly.

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#5 (permalink) Old 06-14-2008, 09:38 PM
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given what you said, and i assume that you stated you case accurately, i would start looking at the AUTOMATIC HEATING & A/C CONTROL UNIT.

it's located between the front footwells ahead of the IKHA plenum

the "blow heat" is the default setting when the control unit malfunctions ;-)

...and yes, may i humbly say: "this is the wrong forum to postthis query"

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#6 (permalink) Old 09-18-2008, 11:54 AM
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Take a look at Johan and Sean's site:
http://www.bmwe32.masscom.net for a good description of the stepper motors and repairs.
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#7 (permalink) Old 09-18-2008, 11:55 AM
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