Can't get any heat, please help - PeachParts Mercedes-Benz Forum

BigBenzWV · #1 12-02-2004, 08:14 AM

Car: 1991 560 SEL 104100 miles
Problem: Can't get heat from heater
Setting: temperature wheel with 75 F in the middle, outside T reading 32 F, coolant T reading 82C, fan set at auto, control set middle one.

Observations. Fan will run at max all the time, however, very cold air will coming for about 3 minutes to the foot area. After that, fan keeps running at max but nothing seems to come out of anywhere. Same if I use the setting next to the defrost.

Please give me some tips as what and where to check first. Many thanks in advance.

benzboy87 · #2 12-02-2004, 08:44 AM

I would check the monovalve first. There is a repair kit which you can find on this site or on ebay. Don't buy the expensive "whole unit" that costs around $150. The repair kit is the "sleeve" contained inside with a screen and rubber diaphram and a gasket. This is an easy DIY. Search for "monovalve" and you should get tons of info. Good luck.

rschleicher · #3 12-02-2004, 05:27 PM

Just a quick note on diagnosing the monovalve:

If your monovalve is still basically working, then it will go to "full-open" when the electrical connector is pulled off of the top, resulting in full-heat. If you have no heat, but then you get heat by removing the elecrical connector to the monovalve, it tends to implicate the control unit (or maybe something else, like a sensor of some kind) rather than the monovalve.

If, on the other hand, you are still getting cold air, even after the monovalve electrical connector is pulled, that tends to implicate a stuck monovalve (fixable with the repair kit.

Bob Schleicher, 1991 420SEL, 174,000 miles.
bullswin5@aol.com

BigBenzWV · #4 12-02-2004, 07:53 PM

thanks for all your kind helps.

Coming home this PM, I turn the dial to 85 F and I did get "warm" air at foot area. And I also realized that I did not open the side vent.

However, the air is just not "hot". Is there a inside temperature sensor on this car (91 560 SEL)? If I put a therometer in the, how "hot" should be temp get up to? I did put the recycle button on and it does seems to help. BTW, I check coolant level and it is not low.

Gilly · #5 12-02-2004, 08:08 PM

CHECK the monovalve, the rubber diaphragm gets a tear or hole in it and the coolant is resent back to the engine instead of the heater core. MOST time in a case like this, it's a torn diaphragm and a repair kit will fix it. Don't go chasing electrical problems yet, i am really sure it's the mono water valve.

Gilly

BigBenzWV · #6 12-02-2004, 09:41 PM

Quote:

Originally Posted by Gilly

CHECK the monovalve, the rubber diaphragm gets a tear or hole in it and the coolant is resent back to the engine instead of the heater core. MOST time in a case like this, it's a torn diaphragm and a repair kit will fix it. Don't go chasing electrical problems yet, i am really sure it's the mono water valve.

Gilly

Thanks Gilly.

Looking at the valve, I found the first thing is to remove the diagnostic connector. I saw a detailed post on this forum by Philip Fox. But on my car, one of the screws is completed blocked by the transmission fluid level check sticker. Does my 560 SEL has a different screw to secure the diagnostic connector?

psfred · #7 12-03-2004, 10:23 PM

Monovalve should be the same on all W126 bodies, it's held in place by four screws on a small plastic cover. No need to remove a diagnostic cover or anything else unless I'm spacing out again. It's up behind the bulkhead just in front of the firewall near the air intake for the climate control.

Peter

fz500sel · #8 12-03-2004, 11:13 PM

I bought a W126 vehicle and drove it from Florida to Michigan in January. Once I got up in Michigan then I noticed that my heater wasnt working. I got a replacement monovalve sleeve assm for $20 and at 1am in a lonely self serve carwash I replaced the monovalve sleeve. Next morning I noticed all these scratches on my passenger fender. After cursing and swearing to myself at "how could these vandals do such a thing! this is an outrage!, blah, blah, blah!" I then realized I was such an idiot that when I was fixing my monovalve, it was 1am and so friggin cold that I had my jacket on and the zipper on my jacket did this horrendous job on my fender. DOH!!! Luckily for me the scratches werent too deep and I was able to rub them out.

Anyways the monovalve fix was easy. 4 screws to take out, pull the guts of the monovalve out. install new. 4 screws back in.