Plenty of heat...little heat....slow down...heat comes back - PeachParts Mercedes-Benz Forum

#1 01-09-2003, 11:15 PM

My 81 300SD has all the heat you would want...for the first ten minutes or so, then it cycles down....not to really cold...but not warm. If I go to defrost there is still no heat (and it should default to full heat) and I figured out that if I slow down to 45mph...the heat comes back for a short time and back to the same thing. What the heck is going on?

jbaj007 · #2 01-09-2003, 11:29 PM

monovalve most likely.

lrg · #3 01-10-2003, 12:47 AM

I would second that. Sounds like a typical failed monovalve. You should be able to replace just the insert. An easy job.

lorenztl · #4 01-10-2003, 09:58 PM

My '84 300D had the same symptoms last week. Heat was great at idle however with increased speed it cooled off. Replaced monovalve and now it works great!

#5 01-10-2003, 10:37 PM

I double posted this problem, sorry but the first one had a time delay built into it somehow, anyway...thanks for the input, general consensus points to the mono valve. The other post has one hint that it may just be short on water. I'll fill it up and go from there...thanks for the help...I'll report back.
Ernie

jjalex · #6 01-11-2003, 01:39 PM

lorenztl,

I have the same problem with my '84 300D at highway speeds, but when I slow down the heat comes back strong.

Some guys on my posting for the above problem suggested sheilding a portion of the radiator because this usually only happens in a cold driving wind at highway speeds....

I have to wonder about the valve though.....

michakaveli · #7 01-12-2003, 10:06 AM

Don't know if my problem is the same as everyone elses, because it's not related to engine speed. Our '82 300D has sporatic heat. It's been a while now that if she does blow heat, it's luke warm and usually quickly changes to cold. It doesn't matter what position the climate control is in, whether it's defroster or regular blower. Last week I was lucky enough to leave the defrostor on while uncovering the car of snow and heat was blowing out the whole time, until I was almost done wiht the trip it was turning to luke warm. Is this a monovalve problem too? If so, what and where is the monovalve. My dad took the climate control panel apart and she looked brand new inside. Good contacts and all. What you guys think?

jjalex · #8 01-12-2003, 10:14 PM

I just ordered a monovalve repair kit. If not maybe I'll try the thermostat.

Tim51 · #9 01-13-2003, 10:47 AM

Another vote for the monovalve.
My 84 300D had the same symptoms. Lots of heat at idle, but not heat above 40 mph. It took me about 10 minutes to install the monovalve repair unit and the problem was solved. Here is a site that explains it pretty well:
http://www.kite.com/kite/w126/

Tim

jjalex · #10 01-13-2003, 05:17 PM

Tim,

Nice Web page and great pictures!

Thanks

#11 01-13-2003, 10:24 PM

Yesterday, between the "49ers" and the "Raiders", I slipped out into the garage and took the valve apart. The diaphram was torn. I went out to my "Parts Betty" (a 83 300D) and removed the element from the mono valve. The two valves are different, but the element is the same. So.....this morning I had heat for the full hour plus on the way to work.
Thanks for all the help, I tried adding a little more water, hoping for a quick and easy fix. But then this fix probably really didn't take much more time. Really an easy job. The 300D valve was tucked slightly under a lip at the top of the fire wall. So the back two screws were a little tougher to loosen. I still used a stright screw driver (at an angle). The 300SD valve was right there in the open-- nothing over the top of it-- just sitting there waiting to be fixed. Thanks again....for anyone that may have a question about checking their valve, I would recommend....open the valve and take a look. If it's torn you'll know in a second. If not ..button it back up. A word of safety...I don't remember anyone saying anything about waiting for the pressure in the cooling system to get down to a safe condition.

jjalex · #12 01-18-2003, 11:07 PM

I change my monovalve today and I had more heat in a couple of miles than all last week!

Thanks!

artpb · #13 01-19-2003, 11:52 AM

Replaced my monovalve insert this week. All the strange symptoms gone. Everthing works fine now. Took abt 20 minutes on my 1983 300SD. Make sure the coolant is not to hot as you will lose some which you'll have to replace.

I would recommend doing this first rather than letting the problem rent space in your head. 1 - Because at some time you will have to replace it anyway - and 2 because it's inexpensive and easier than trouble-shooting the climate control system.