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Old 11-27-2001, 12:35 PM
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Join Date: Jun 2001
Location: San Diego, Ca
Posts: 186
Angry New AC Servo and Amp, Still Full Heat

I posted this in the Diesel section but figured I should try here as well.

This little climate control issue is really getting under my skin. I have a 79 300D.

Basically, I'm on my 3rd servo and one new AC amp and I can't get the dam heat to shut off right. The original servo leaked but worked.

On a side note, the front panel adjustment knob is adjusted as far to the cold side as I can get it. The pneumatics in the climate control don't work, I've zip tied the front vents open and also the re-circ part. As for the rest of the car, it shuts off fine and will operated the door locks for about 3 hours or so after you shut it off.

The 1st new one never worked at all. The second worked for about a month then intermittent full heat for about a week. After that it stayed on full heat. The 3rd I installed Tuesday the 20th. It didn't work at first, then worked fine for a trip from San Diego to San Fransisco and back over the holiday. I get back Saturday and it works all day Sunday until Sunday night it goes to full heat. Now I can't get it to shut off.

I replace the Amp because I read that it can also cause this problem and the original is 22 years old.

Now in my driveway, it first doesn't work at all. I shut off the car to see if I can hear the servo doing anything, it is doing it usual noises.

I restart the car and hold the thermistor in front of the passenger side vent. It shuts the fan down and the heat goes off. Now I try to turn the heat back on, it won't go on all the while holding the thermistor in front of the vent.

I restart the car again and repeat the process and am able to get the heat to cycle on and off along with the AC as is should, again holding the thermistor in front of the vent.

I re-assemble the dash, and locate the thermistor in its spot just above the passenger side foot well. I relocated it here a long time ago when the defrost vents stoped closing. If it's up on the dash, the defrost vents just blow cold/warm air right on it and it shuts off the AC long before the car is warm/cool. Down there it has been working quite well as the car will get good and cold in the summer and warm in the winter. The heat comes on full force again and won't shut off. What a pisser eh?

So, could it be a bad thermistor, maybe front panel? Is there some way I can test this as it's gotten quite expensive between the servo and amp.

Your help is greatly appreciated.


'82 300SD
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Old 11-27-2001, 01:14 PM
MikeTangas's Avatar
Senior Member
Join Date: Apr 2000
Location: So. Cal
Posts: 4,430
You may find your answers, or even the cure in by clicking here. Similar topic was recently covered.

Hope it helps.
Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

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Old 11-27-2001, 10:35 PM
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Join Date: Jun 2001
Location: San Diego, Ca
Posts: 186
That post talked a lot about the sensor and making sure air was going past it. I tried to play with that line of thought and was holding it in front of the vent. While it, after a few minutes in blazing hot air, slow down the heat, as soon as it cooled a little the heat came on full force again. All the while, the car was way, and I mean way toasty. It's almost like the sensor isn't working well. Like it takes a really strong temp change to get it to do it's thing.

Is there a way to set the thermistor? How about that potentiometer in the dash control?


'82 300SD
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