1984 300D heat problem - PeachParts Mercedes-Benz Forum

Hotdogseller · #1 11-21-2005, 08:48 AM

I am now the proud owner of a 1984 300D!

Of course there are some issues to address due to its age and former owners lack of repairs.
I can not get the heat to work all the time.
I leave the temp setting on Max. I have pushed all the directional buttons in the middle and pushed the fan buttons to the right.
Only time I did get heat was after startup with the second middle button pressed and the top fan button pushed in. Then after warming up I released the middle button and later when I pushed it in no heat. And I cannot get it to do that again.

I have searched the froum and could not find a case similar to mine.
I did learn from reading about the "monovalve"?
What is everyones considered opinion as to what may be the problem?
BTW I also have no problem attempting to repair anything.
They don't call me 'scrapy' for nothing! LOL
Also can someone advise me to the best repair manual to seek out?
All replies appreciated. Thanks!

tangofox007 · #2 11-21-2005, 09:36 AM

Could you be more specific about what does and does not work? When you say that you have no heat, does that mean that the blower is moving cold air? Does it mean that the blower is not working? Which button is "the second middle button?"

boneheaddoctor · #3 11-21-2005, 09:43 AM

Also do a search on Monovalve.

Hotdogseller · #4 11-21-2005, 09:52 AM

Quote:

Originally Posted by tangofox007

Could you be more specific about what does and does not work? When you say that you have no heat, does that mean that the blower is moving cold air? Does it mean that the blower is not working? Which button is "the second middle button?"

Sorry.
Yes blower works.
Yes it is only moving cold air.
As to the buttons.
On the climate control panel,
There is a temp dial to the left
five buttons across the middle
second button has the arrow at top and 1 pointed to bottom both colored full white.
To the left is 3 buttons from top to bottom smaller one in middle with the word "automatic'.

All seem to function correct, as when you push them the air is directed as to the pattern described in the owners manual.

tangofox007 · #5 11-21-2005, 10:01 AM

Disconnect the electrical plug on the monovalve. You should get full heat with the plug disconnected. See if that happens. Push the "EC" button on the CCU.

kerry · #6 11-21-2005, 10:03 AM

Try pressing far left (defrost) button, this should give you full blower and full heat. If no heat, you should check the voltage at the monovalve. If at 0 with far left button pushed, you climate control unit is working, at least on defrost. The monovalve is likely the culprit at that point, although there is an auxilliary coolant circulation pump used to move coolant at low rpm's. With no heat at all, it's possible there is a thermostat problem. What is the temperature gauge showing?

Hotdogseller · #7 11-21-2005, 10:04 AM

Yes, boneheaddoctor.
I had read the thread by Tulsa Benz about his heat problem.
Four things were brought up:
1. auxilliary pump
2. Servo and servo amp
3.monovalve
4.loss of vacuum

Loss of vacuum does not seem to be my problem because I think the breaks work good.
It was stated the 79 didn't have a monovalve but I assumed my 84 did. All though I have not done enough reasearch to know.
BTW that was the only thread that I saw that had symptons similar to mine. ie intemittent heat.

Hotdogseller · #8 11-21-2005, 10:12 AM

Quote:

Originally Posted by kerry edwards

Try pressing far left (defrost) button, this should give you full blower and full heat. If no heat, you should check the voltage at the monovalve. If at 0 with far left button pushed, you climate control unit is working, at least on defrost. The monovalve is likely the culprit at that point, although there is an auxilliary coolant circulation pump used to move coolant at low rpm's. With no heat at all, it's possible there is a thermostat problem. What is the temperature gauge showing?

I get heat at start up by pressing the far left button(defrost) temp set max, fan button on top right pushed in.
after letting the defrost button out and latter pushing it in I only get cold air.

Tempeature after warm up or driving is just below the 80 mark.

Is the "monvalve" on the fire wall towards the passenger side next to the battery?

Is the auxillary pump towards the front by the radiator towards the passenger side fender?

tangofox007 · #9 11-21-2005, 10:12 AM

The '84 has a monovalve, but no servo/amp. The monovalve is electrically controlled by the ACC system. The heat is "turned off" when the monovalve is closed electrically. When electrical power to the monovalve is removed, it opens to a "full heat" condition. The monovalve is located on the firewall in the engine compartment, just inboard of the battery. Usually brown in color, has an electrical plug on the top.

Hotdogseller · #10 11-21-2005, 10:17 AM

Quote:

Originally Posted by tangofox007

The '84 has a monovalve, but no servo/amp. The monovalve is electrically controlled by the ACC system. The heat is "turned off" when the monovalve is closed electrically. When electrical power to the monovalve is removed, it opens to a "full heat" condition. The monovalve is located on the firewall in the engine compartment, just inboard of the battery. Usually brown in color, has an electrical plug on the top.

Thank You .

Hotdogseller · #11 11-21-2005, 10:36 AM

Quote:

Originally Posted by tangofox007

The '84 has a monovalve, but no servo/amp. The monovalve is electrically controlled by the ACC system. The heat is "turned off" when the monovalve is closed electrically. When electrical power to the monovalve is removed, it opens to a "full heat" condition. The monovalve is located on the firewall in the engine compartment, just inboard of the battery. Usually brown in color, has an electrical plug on the top.

Located the "monovalve" unpluged it and still no heat.
Looked for my multitester and couldn't find it so i don't know about the voltage to it( darn kids!)
Think I located the aux. pump.
If it is not working will it cause no fluid to flow through the monovalve?

boneheaddoctor · #12 11-21-2005, 10:52 AM

Quote:

Originally Posted by Hotdogseller

Located the "monovalve" unpluged it and still no heat.
Looked for my multitester and couldn't find it so i don't know about the voltage to it( darn kids!)
Think I located the aux. pump.
If it is not working will it cause no fluid to flow through the monovalve?

Ake the top off the monovalve housing....its four screws...look at the insert..looks like a shotgun shell wearing a ring like on Saturn....if that rubber skirt it torn or perferated it needs replaced....I'll wager that it is.

Hotdogseller · #13 11-21-2005, 11:08 AM

Thanks BHD ! I'll check it out.

tangofox007 · #14 11-21-2005, 01:49 PM

Quote:

Originally Posted by boneheaddoctor

if that rubber skirt it torn or perferated it needs replaced....I'll wager that it is.

Concur. There is a repair kit for the monovalve that is much cheaper than buying the entire assembly.
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1MP10UP0M1O00THIB8&year=1985&make=MB&model=300-DT-001&category=R&part=Mono+Valve+Repair+Kit

rg2098 · #15 11-21-2005, 02:27 PM

If the Aux. Pump isn't working, it can restrict coolant flow to the monovalve so no noticeable heat. You can try bypassing the aux pump with copper pipe connecting the 2 hoses and you should get heat while driving if the pump has failed and indeed restricting flow.