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1983 300CD former, 1988 300CE former, R107 future
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Discussion Starter · #1 ·
Hi all, pretty new here. I purchased a 1983 300CD a few months ago and for whatever reason the heat is stuck on full hot. Not such a problem in January; but now it's getting annoying. I'd like to be able to adjust the air temperature. Any ideas what may be broken and where to start at? I tried searching but kept coming up empty; and half of the links in the tech articles are dead; so yes I did try to look before adding another post.

Thanks in advance.
 

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Hi all, pretty new here. I purchased a 1983 300CD a few months ago and for whatever reason the heat is stuck on full hot. Not such a problem in January; but now it's getting annoying. I'd like to be able to adjust the air temperature. Any ideas what may be broken and where to start at? I tried searching but kept coming up empty; and half of the links in the tech articles are dead; so yes I did try to look before adding another post.

Thanks in advance.
You most likely need a Monovavle rebuild kit. I would bet your monovalve rubber boot is torn or just plain old and it is leaving the heat on all the time!
 

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1983 300CD former, 1988 300CE former, R107 future
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Discussion Starter · #3 ·
Is there somewhere I can get more information? Search really isn't my friend on here so far; but I'll get used to it so please continue to bear with me.

What is a Monovalve; where is it located at? How can I tell if it is indeed the culprit and where might I source a rebuild kit?
 

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W-1-2-3 Go!
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The monovalve is at the firewall, near the battery, right behind the air cleaner. It's the small round cylinder jutting out of the brown monovalve assembly. The solenoid is all you'll need, the kit costs around $30+. It's an easy fix but can get confusing with the washer orientation, and symptoms are shown by max heat no matter what temperature. Also now is the time to check the plug, sometimes the contacts get loose giving a false reading.

Look up "monovalve" or "wavy washer"
 

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1983 300CD former, 1988 300CE former, R107 future
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Discussion Starter · #5 ·
Thanks for the reply. I'm learning all kinds of new "technical" terms: Wavy Washer, Pool Noodle, Cigarette Butt trick (did that this morning; works perfect)
 

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When the rubber diaphragm goes out or is worn out, your heat is on all the time. A good place to get a monovalve rebuild kit is Auto Hauz Arizona.

The valve kit you see on the bottom of this page for $36.92 is the part you need. https://www.autohausaz.com/search/p...te Control&[email protected] & Heater Control Valve

Do not break it getting it apart! These are not being made anymore (the monovalve body) and you will be hunting one down is wrecking yards if you do!
 

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1983 300CD former, 1988 300CE former, R107 future
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Discussion Starter · #7 ·
Thanks for the link! I drove my Miata to work today so I'll have to check the Panzer Tank when I get home just to physically see the condition of the valve and then go from there. So far this site rocks!
 

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SLK 230K auto
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Just before you look further, check that the side vents are closed by rotating the central small knob.

My two (cheap) cents
 

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1983 300CD former, 1988 300CE former, R107 future
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Discussion Starter · #9 ·
What difference does having the side vents closed make? Do those always pump out max heat no matter what?
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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^^No, they don't always pump heat under normal circumstances.
Check the mono valve, four screws, five minutes, little to no loss of anti-freeze. A new wavy washer comes with the valve, even if not, any hardware store should have 'em.
 

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Discussion Starter · #11 ·
I'm starting to get the impression these are overly complex cars that are fairly easy to fix. :)
 

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I'm starting to get the impression these are overly complex cars that are fairly easy to fix. :)
Give it time.

Some problems are easier to correct than others.
 

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Discussion Starter · #13 ·
Give it time.

Some problems are easier to correct than others.
I sold a Porsche 944 to get an MGB-GT; so far this Merc is a blessing:D

Is there any sort of temporary solution or fix-it for a bad valve? Granted; spending $40 isn't a lot of money; but if I can "rig" it just to be able to tell if that's $40 spent in the right place I'd be more comfortable before I order parts. That MG taugt me to hate just throwing parts at a car.
 

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Good for you on being hesitant to throw parts/money at something before understanding the where the culprit is. Yet in this case you can take it out and see if the diaphragm is torn.
The one in the pic is a good one that I removed before sending that car to the crusher, hence the rust. I tried to hold the plunger to give you an idea of the operation.
If it is good, you'll need to check if it's getting a signal, not sure how to do that.
Again, a very easy quick inspection.
My first car (Mercedes) was bought from someone who buys BMW's to fix and turn a profit, he doesn't understand MB's.
 

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