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Discussion Starter #1
Hello,
My heater is alweays on, I understand that there is a vaulve in the Engine Compartment near where the heater hose goes into the firewall. I cannot locate it. Is there a diagram?

Any help would be appreciated
 

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A bunch o of 'em - see signautre
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mr -

The "monovalve" you're referring to does indeed fail and causes constant heat. Easy to find and easier to fix (screwdriver & 5 minutes).

Valve assy. is attached to the firewall, passenger side of the engine compartment. It will have a couple of hoses (carrying the coolant) connected to it and a tdwo wire connector - the little valve element that needs to be replaced ($25+/- at parts store, $60+ at dealer) has a stainless steel cylinder protruding out the top of the assy. and is about the size of a dime. You'll see four screws holding it in. Pull that out and you'll most likely see the rubber is torn. Only caution is to make note of the position of the washers on the element - put them on your new one in the same order.

It really is just a 5 minute fix.

Kevin
 

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Administratoris Emeritus
2021 SL770
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Except that this car is not equipped with a mono valve. It has a servo, which may be stuck open. This is a fairly large black part with connections for 4 water hoses and lots of vacuum hoses. Servos fail frequently and are quite costly to replace.

http://www.germanstar.net/107h.htm
 

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A bunch o of 'em - see signautre
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what HE said (I stand corrected).

I looked at the price of the servo as I might be needing one for my '77 280SE -

OUCH!

Kevin
 

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88 560 SL
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I had the same problem on my 88 560sl. I needed a quick fix for this hot air inside the car in the summer. I jimmy rigged mine. I just pinched off the hose going into the firewall with some locking pliers.
 

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Discussion Starter #6
There is a large Black Plastic Part # P52-102-35.

I guess this is the Servo you are reffering to - German Star Part # 0008300384?

Which of the four hoses needs to be clamped to stop the Heater Coil from getting heat?
 

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SanDiego - 3/25/2005 11:19 AM

I had the same problem on my 88 560sl. I needed a quick fix for this hot air inside the car in the summer. I jimmy rigged mine. I just pinched off the hose going into the firewall with some locking pliers.
You're car is equipped with a mono valve, which is an easy, inexpensive fix. Read halman2228's post.
 

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1976 450sl
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Any ideas how to tell if the car is equipped with a servo or monovalve? Summer is only a few months off and even with the top down the heater blasting hot air in 90 degree weather is brutal! Thanks. '76 450sl
 

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1980 450SL
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Thanks for the info...I have the same problem (1980 450sl). I just posted a question: "Help with Heater/AC" about my heater/ac not working, my question to you is, is this a connected problem or are we talking about 2 different issues with heater/ac?

Thanks, LPS
 

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1980 450SL
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Hi Germanstar...I went to your Mercedes R107 Climate Control Parts parts list but not sure which part you are talking about as being the "servo" in this discussion...The ac servo and heater control panel are the same part number and looks nothing like what you descibed as "fairly large black part with connections for 4 water hoses and lots of vacuum hoses".

Thanks, please clarify.
 

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1976 450sl
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Not sure if the 2 problems are connected. Issue with mine, is that the blower is constantly blowing hot air through the vents in the top of the dash. Despite the fan being off and all levers in the closed position. I have read elsewhere that by law at the time, the blower constantly runs because of the carbon monoxide threat. I recall reading that this was the case if the AC was on but the but the blower was switched to off. I thought maybe that the AC on my SL was not completely switching off.
 

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Always Remembered RIP
1973 450 SLC AMG, 1995 Chevy Tahoe
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I had the same problem on my 88 560sl. I needed a quick fix for this hot air inside the car in the summer. I jimmy rigged mine. I just pinched off the hose going into the firewall with some locking pliers.
You know I had that problem to and couldn't figure where the heat was coming from till I finally found it wasn't hearer heat at all it was engine heat coming though the fire wall though rust holes from the battery tray and a missing rubber guarmet (sp) near the driver side. I plug them all and no more heat in the cabin.
It's worth checking into.
 

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1976 450sl
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Thanks, my '76 450sl has the battery in the trunk but I will look for other places in firewall where air might be coming through. If this were indeed the case, would I only feel the hot forced air when actually driving? I can feel forced heated air when at a stop.
 

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1973 450 SL
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Your engine fan produces quite a lot of air flow, so you would feel it at a stop as well.

Loren
 

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Always Remembered RIP
1973 450 SLC AMG, 1995 Chevy Tahoe
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Your engine fan produces quite a lot of air flow, so you would feel it at a stop as well.

Loren
^^^^^^^^^^^^^What Loren said
Yup it was so hot I thought my feet were burning
 

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'80 450SL
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Thanks for the info...I have the same problem (1980 450sl). I just posted a question: "Help with Heater/AC" about my heater/ac not working, my question to you is, is this a connected problem or are we talking about 2 different issues with heater/ac?

Thanks, LPS
I have the same issue (also posted in Help w/ Heater/AC), is there a way to test if a servo has failed (and not something else)? My climate options are currently off & defrost w/ full heat. ALTHOUGH I do see that the #9 fuse in my car is missing, does the servo get its power via that fuse? If so, I may be in good shape. Yes, I'm aware that a trip to the auto parts store would quickly answer my question but hoping a reply would give me more knowledge, haha.

Edit: installing fuse #9 gave me zero change before/after. Then again it was about 35 degrees so probably under the temp that it will run. Next step, going to have my other half turn the key to accessory so I can listen for a whirring noise above the servo. Beaverton have you tried that yet?
 

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2021 SL770
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Hi Germanstar...I went to your Mercedes R107 Climate Control Parts parts list but not sure which part you are talking about as being the "servo" in this discussion...The ac servo and heater control panel are the same part number and looks nothing like what you descibed as "fairly large black part with connections for 4 water hoses and lots of vacuum hoses".

Thanks, please clarify.
We no longer carry remanufactured Servos, opting for the Servo elimination kit instead. We did carry them in 2005, when this thread originated.
 
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