heat problem - PeachParts Mercedes-Benz Forum

#1 06-13-2002, 02:18 PM

I replaced my mono valve this past winter, and now the heat works awesome. (I was loosing heat on highway...) BUT now the heat is always on. If I turn the temp. all the way down to cold, it just changes the vents that blow and it still blows hot.

Any suggestions?

-Erik
'85 300TD 220k

Ali Al-Chalabi · #2 06-15-2002, 05:06 PM

It may be that the monovalve has stuck in the open position or the monovalve is good and just recieving the wrong signal from the climate control unit. Check to see that the valve is working properly first.

tstarr · #3 06-18-2002, 08:49 AM

Ali,

Were you able to check on this? I have exactly the same issue - I was getting no heat after I started rolling this winter, changed the monovalve and everything worked great. Now, during the summer, I get heat constantly. I'm not even really sure how to check everything that might go wrong. I saw the wiring diagram with the resistance on the heater core temp sensor expected at given temps, so that should be pretty straightforward. But I'm not sure how to check the heater control unit (I believe that's what it's called). The silver box that sits behind the glove box.
Anyway, I'd appreciate any clues on how to diagnose and your results. Thanks!

tstarr · #4 06-23-2002, 08:06 AM

I went through the climate control testing procedures and found that my heat exchanger temp. sensor was not changing at all with respect to the increased temp of the heater core. Put a volt/ohmmeter on the sensor, drove the car and it read a consistent, very high resistance. I have ordered a new part and will install on Monday to see if that cures the problem. I hope it does - it's a relatively cheap part and easy install! Certainly MY preference in a fix to a problem. I'll let you know how it goes...

AndyM86 · #5 06-23-2002, 08:34 PM

Hi,I'm new here and just happen to read about your heater problem. Make sure you reconnected the 2 prong plug on the mono valve and that the prongs are not spread to far apart.
Hope this helps.
AndyM86

tstarr · #6 06-27-2002, 01:53 PM

Well, I finally found the replacement temperature sensor that monitors the heater core temperature. Bingo! That solved the heat problem. It took a while to realize it, since the heater core had already filled with hot water when I plugged the new one in. But this morning I got into the car, drove to work, saw the car was at operating temperature and everything was cool. In multiple ways.

Now I need to install the new A/C Compressor and get some R-12 shot in and everything will be copacetic.

#7 06-28-2002, 08:17 AM

where is the temperature sensor and what is involved with changing it?

tstarr · #8 06-28-2002, 09:03 AM

It's behind the radio and ashtray, plugged into the heater housing. I had already pulled the radio because I was having some shorts in it. Once you have those openings available in the console, reach back to the heater housing (plastic, relatively flat surface) and feel around. You'll find a wire attached to a square plastic plug which plugs into the housing. Just unplug the wire, pull on the plug to pull it out of the housing.

If you want to test - what I did was clip the leads for the volt/ohmeter onto the two prongs coming out of it (leaving it in the housing for this). I set the VO on resistance and watched the needle as I drove around and the heater core heated up. The resistance should have changed, indicating that the sensor was sending information out that the housing was heating up. It never changed. The climate control part of the CD gives the ambient temps and expected resistance off of it, but mine never changed at all so I knew it was bad.

The part only cost $14 at the dealer, so if you want to bypass the test and just replace it, it's not going to cost an arm and a leg.