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Discussion Starter #1
I have an 88 560SL with about 118K miles. The heater is always on. When the climate control off button is set there is still heat entering through the defroster vents. Air cond. works but is fighting the heat. Read about a "Monovalve" or "servo" located in the engine compartment. This was described for an older model. Could anyone point me in the right direction for my 88. Could this be a vacuum problem?

Thanks
 

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One of the BW Old Guard/R129, W204 Moderator
1997 SL500- 40th Anniversary
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Vaccuum problem? Possibly, but it sure sounds like the heater control valve (monovalve). It is located on the firewall on the passenger side in the same vicinity of the auxillary water pump. It fails open, allowing heat (water flow).

I am far from an expert; however, am well read on the subject. I also have a 1988 560Sl (105Kmiles)so I am always looking for the next snake to bite me on the a**, if you know what I mean.
 

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Discussion Starter #3
Thanks for the advice. Is there a way to test the valve or do you just install a new kit?

Bud
 

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Depends on the day!
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Just install a new kit.. Your CC being out is a pretty good test;-) If you are lucky this will do the job..CC vacuum can be a REAL hassle.

Jonathan
 

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Depends on the day!
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the 25$ jobbies are sketchy aftermarket..

What you install in your's is entirely up to you though[:p]

Jonathan
 

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'89 560SL
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The monovalve is a solenoid that when open (normal position) admits engine coolant into the heater core. When energized, it closes off the flow and allows the core to cool down. When the diaphragm ruptures, the heat is therefore constantly on. They can fail electrically but usually rupture the diaphragm instead. It's an easy replace to do.
 

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Discussion Starter #9
You were right. The rubber washer at the base of the Mono Valve failed. Just install a repair kit and all works like new.

Thanks
 
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