1983 380 SEL
108 k miles
Cincinnati OH Weather mild lately 40 to 60
Only driven this car a few hundred miles, bought it a few months ago.

The symptoms:

When it was warmer and I ran the ac, it would freeze me out. Seems no matter where I set the temp wheel, the air just kept blowing cold. I would have to turn it off and back on to keep from freezing.

Now that it is getting colder I don't get any heat. The blower runs but no heat.

What I have done so far:
1. I replaced the control module (with the wheel and buttons etc) I had an extra one so I decided to try that. No change.
2. I checked the voltage at the monovalve. It has 12 volts steady on the hot and the ground cycles about every one or two seconds. I left the monovalve unplugged so no current would get to it and I get heat in the car, but he blower does not seem to kick down once the heat is up.
3. I checked the vacuum operated door behind the glove box and does open and close when the air is turned on and off. Seems slow but it does work.

What I think: Seems to me that the control module is not getting any feedback on the cabin temperature. I don't know how to test this.

Any help is appreciated.
51

Sounds like two problems - Interior temp sensor in the dome light assembly is either bad or the fan that draws air across it has quit.

It also sounds like the monovalve diaphragm is torn, which is very easy to check. 4 screws.

Good luck.

__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition

"Interior temp sensor in the dome light assembly is either bad or the fan that draws air across it has quit."


Is the fan in the area of the dome light also? Should it run all the time?

51

No, the fan is located in the fender, in the passenger's footwell, by the door. A foam hose runs up the A-pillar through the roof to the dome light assembly. The air is drawn across the sensor (in the dome light assy.) through the hose and exhausted in the fender/footwell area.

The fan should be on whenever the climate control is on.

Standard fan test is to put a 1cm square piece of paper over the air slots in the dome light assembly, and see if it sticks there. No stick = no fan.

__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition

Well, I put a small piece of paper on there, turned on the heat and the paper sticks. Let it stay there for 2 min, turned off the car and the paper dropped. Guess that part is working.

I'll pull out the monovalve in the am.
Thanks!
51

Monovalve rubber is torn.

OK, I think I understand now, the heat is what was supposed to be controlong the cooling, and since I had no heat the ac was freezing me. Then when I needed heat due to the winter, I had none but didn't realize it yet.

Thanks!
51

PS /// Thanks a lot, my week just got better!

The problem is, if your monovalve is torn, it should be letting hot coolant through your heater core, so you should have heat. Check your aux pump - it's what pumps the coolant through the heater core.

__________________
A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.

Will have monovavle here to install tomorrow am. I'm going to do it and see what happens. I'll go to the aux pump if I still have the symptoms.

Thansk!
51

Monovalve did it. The system works like a charm.
Thanks so much!
51