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Old 05-13-2012, 04:36 PM
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Join Date: Dec 2011
Posts: 2
W126 heat/AC blowing at same time

I have a 1987 560 SEL and am having issues with the climate control (surprise!)

The heat and A/C blow at the same time
Makes a loud "clank" noise when 'switching' from heat to A/C (even though both blow at the same time)
Will not stay off, even when "0" is selected it comes off/on intermittently

Any suggestions are appreciated, I've tried switching climate control units and that doesn't make a difference so it's not the unit itself.

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Old 05-14-2012, 02:00 AM
JamesDean's Avatar
Electrical Engineer
Join Date: Apr 2005
Location: NE Ohio
Posts: 5,034
I think you've got two symptoms here.

1) Heat+AC
-This might be caused by a torn/bad mono-valve. When the monovalve gets torn, coolant will flow through the heater-core giving the car heat all the time. If you've got your car in A/C mode, you might get cold air at some vents. Alternatively you mono-valve might be partially bad or partially functioning, giving you heat sometimes and cold air another? Maybe.

-Pull the monovalve to be sure. Its right next to the battery, 4 small screws and the top comes off.

2) On/Off Intermittent-
-Since you changed out climate control unit, assuming one of them was known to be good, you might have an issue with your blower control box? Just a thought.


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