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Old 02-03-2010, 11:39 AM   #1
Dr.Thunder
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Location: Springfield, MO
Default Climate Control

[1991 740T]

I'm having problems with the climate controls. I imagine there's a vacuum leak under the dash, which I'm not looking forward to, since I get a hissing noise from the control unit on any setting except for defrost & panel/floor. On panel, floor and off, I get the hissing & air comes out of the panel vents no matter what setting it's on.

Also, the temp is stuck on heat. If I turn it all the way down, it still lets out hot air.

Also, the fan is not working. So the only time there is hot air actually "blowing" out, is if the recirc button is off and the car is going.

Very strange, I'm hoping it's just the control unit. At first I suspected the blower motor, but after I noticed that the problem was more than just not blowing air, I'm now suspecting the control unit.

Would I be correct in my assumption?

Thank you.
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Old 02-03-2010, 02:06 PM   #2
Three Fat Tigers
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Location: ↓ Lynchburg, VA →| '89 740Ti | '90 760Ti | '92 240 | '94 940T | '96 960 | '96 M1
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What type of control do you have, MCC, or ECC? I'm not sure you could get the ECC on that year and model.

CC=climate controls
M=manual
E=electronic
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Old 03-05-2010, 10:38 PM   #3
Sanfam
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I guess I'll chime in here, as my problem appear to be the same.
I'm running an '88 740 Turbo with the Manual Climate Control module. The specific leak appears to be centered around the vacuum hose bundle on the right rear corner of the unit, though I can't verify whether or not this is limited to one of the hoses or more.

In my case, the face vent valves are effectively inoperable in everything except for Air Conditioner modes, where the other vacuum leak(s) appear to be cut off and suction maintained.

(It's mildly upsetting that it only occurred to me *after* re-assembling the entire dash post-heater core swap to test the vacuum actuators....*doh*)

Junkyard run, or fix?
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