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Old 10-30-2007, 05:59 PM
ramjensen ramjensen is offline
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Join Date: Jul 2006
Posts: 79
Climate control on and off rapidly

The climate control on my 87 300D has been cycling on and off rapidly under certain conditions. When it is warm out and I put on the a/c everything is fine, but if it is cool out the system powers on and off sometimes as fast as 2 or 3 times per second. At first I thought only the blower and recirc function were going on and off, but looking under the hood it appears that the auxiliary water pump under the washer fluid reservoir iscycling on and off in the same pattern. The a/c compressor seems to be doing it also. To summarize the symptoms:
1.system powers off and on rapidly when temps are cool outside
2.this occurs regardless of which pushbutton is selected (e/c, defrost, etc.)
3.This occurs regardless of the fan speed setting.
4.The rapid cycling occurs less once the car is up to operating temperature.
What I have done so far:
1.Put piece of paper over the inside temperature sensor to insure aspirator motor and tube are ok. Paper sticks, so no problem there.
2.Checked draw of aux. Water pump, pump draws .8 to .9 amps which I believe is ok.
3.Removed pushbutton unit and resoldered pushbutton connections and rheostat connections and fan pushbutton connections. Nothing.
4.Removed and inspected all 3 boards from PBU, inspected and found no evidence of burned circuits or cracked solder joints. Checked under magnification. All appears ok.
5.Tried jiggling PBU and its buttons and connectors: no effect on cycling of climate control.
6.Tried turning ignition key counterclockwise slightly and also tried jiggling it. Nothing.
7. Tried contact cleaner sprayed into rheostats. No effect.
What do I do next? I have scoured the posts and don't know where to go from here. Can the blower motor regulator being faulty cause the whole system to shut off and on, including the aux water pump and the a/c compressor? The blower motor is new and runs great with direct power. I have bypassed the low pressure switch near my a/c accumulator because it is faulty, but I don't see how that should affect anything. Could the problem be my outside temp sensor? I did rear end someone last winter and damaged the front of the car, could I have busted the sensor or shorted a wire?
87 300D 162,000 miles: It's not just a car, it's an adventure!

Last edited by ramjensen; 10-30-2007 at 06:19 PM.
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