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Old 12-05-2000, 06:02 PM
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Join Date: May 1999
Location: Charlotte, NC, USA
Posts: 688
Been having an ongoing problem for a few years which I'd like to clear up. My heater control valve works OK if vacuum is put to it at the end of the line at the control switch unit (under right vent). Everything else appears to switch OK the rest of the year. To keep heat off when unwanted I put a manual shutoff on the heater hose under the hood. That is a pain to control in winter, either too little or too much the hood and adjust!

So, is there a way to diagnose if the vacuum switching unit that controls the heat flow (#11) without using dealer test boxes etc.? Have the switch out and all looks to intact, no cracks etc, or could the problem be elsewhere? How would one check the operation of the unit?

Tobias MB
4 MBs

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Old 12-05-2000, 09:23 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
More than likely you have one of two problems.

1) You could have a vacuum leak somewhere in the car. The most common problem is the vacuum leak at the line that goes under the battery tray to the vacuum reservoir.

2) You may have a problem with the blend flap potentiometer. If it gives a false reading, the heat and or A/C modes will not work correctly.

Donnie Drummonds
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