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Old 07-12-2000, 11:38 AM
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Join Date: Mar 2000
Location: Dallas, Texas
Posts: 218
I'm going to buy a new control panel this afternoon. Have replaced the monovalve insert and coil... With test light, I have constant power to one wire, none to next unless plug is attached to coil. If I turn the temp wheel to heat, the vacuum lever switch the vents over, but the air gets about 10-15 degrees COOLER than it is with the wheel on cold. On cold it is SO hot that it has cooked part of the vent tubing. So, I'm driving with the heat dial on max, switch off. The Texas heat is getting to me...gonna have to start driving the Cadillac (12MPG).

Will the new temp control panel cure this, or is it something much more drastic? The control unit itself seems to be really hot after driving (this could be the CD player) I understand there are two boxes behind the glove compartment--one black one aluminum. Should I try to get new ones from a donor car, or is this problem purely the control panel?


R Talley
84 300D-T
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Old 07-12-2000, 07:59 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Basicly the heater (mono) valve's ground is controlled by the pushbutton. With the control on a/c the p/button should send ground signal to shut off the valve. The p/button receives signals from in-car temp sensor & ambient sensor to determine when to send signal. Also temp wheel must send correct signal as well.
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Old 07-13-2000, 01:01 AM
Registered User
Join Date: Mar 2000
Location: Dallas, Texas
Posts: 218
It WAS the control. Got a used one with warranty for $40. A/C now works well as it ever did...think it may be OVERcharged.
One last question--The aux fan works sometimes, sometimes not. Is this normally the sensor, and where is it located? If not, where is the relay?
Sometimes I hear it running, and the car cools fine--then sometimes like today, it ran real hot--like in the 110 range.
Thanks for the help M.B.Doc.

R Talley
84 300D-T
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Old 07-13-2000, 07:41 AM
Posts: n/a

Glad you got it fixed. The heat is getting unbearable, especially after I was out of it for two weeks, came back and it really hit me hard.

I think your aux fan is okay. I'm not positive about your model, but I think that you will find two pressure switches at the filter drier in the front passenger side of the engine compartment. One of them is low pressure to cut out compressor when there is an inadequate charge and the other is for the aux fan, which runs at high pressure. When the fan is turned on at a high pressure level, the extra air flow across the condensor will reduce the high side pressure.

Good luck, and we'll have to have lunch again soon,

Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles
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