Image- Location of Monovalve on 1985 300D Turbo
 

For those who don't know what and where is Mono valve, I am attaching a photo of its location in 1985 300D Turbo(123)

Monovalve is electrically operated selenoid valve which opens and shuts based on the signals received from ACC( Automatic climate control), This valve control the amount of hot coolant that enters the heating core and thus regulated the heating of the interior.
If this valve fails, its usually a rubber diaphragm, that tears. Its very easy to repair and repair kits are available at many places including FastLane and the dealers.

Thanks

MVK

Is lack of heat the only sympom of a busted monovalve?

Not really!
 

Too much heat is the symptom of the failed monovalve! The failure of the piston's rubber diaphram, the most likely mode, allows continuous flow of coolant to the heater core.

It is heck getting old - I do not remember which it was but when the monovalve went bad in my 300D I seems to remember that I did not have heat while driving down the road but I did have heat while sitting at a redlight (idling) or it may have been the other way around. They are easy to renew - remove the four phillips head screws on top and pull the unit out and swap it. Make sure the engine is cold when you change it out. ALWAYS be careful disconnecting those plastic electrical connectors on these old cars - they get brittle and will break off one of the tabs when you disconnect it.

Hi speedy300Dturbo


Yes on mine it was otherway round. It took too long to heat and that too a little heat. I had a broken diaphragm and also my Auxillary water pump did not work. MB calls this pump as recirculating pump. Now both are fixed and I have nice heat.
MVK

When my monovalve failed it would only get warm but not hot except at idle. After the repair it now gets as hot as I want. The only odd point now is that I can hear it sometimes going "thunk" on and off. I'm wondering if I put the spring washer in the wrong way. It's one of those things that isn't quite annoying enough to take the time to do.

Hi LRG:
I am impressed that you could hear your monovalve while the deisel engine is running. Either youe engine is extra quite or your monovalve is too loud. I think its later. I would just open it up and reassemble. Check the DIY tripod pages just to make sure you installed all parts in right sequence. Just my suggestion.

MVK

MVK,
I think you're right. My guess is that the spring (wavy) washer was meant to cushion the solenoid action. I must have installed it improperly. I'm usually best at screwing up the easy jobs because I'm not really paying attention. My monovalve is a PIA to get to because of the heat shield around the turbo. I'll get to it one of these days. Thanks for the input.

i'm having a problem with the monovalve not receiving it's 12volts when it's supposed to resulting in hot air. I replaced the mono valve earlier thinking that it was the problem but on further investigation it doesn't receive 12 volts when the AC is enaged resulting in warm-cold mix of air coming from the center vents. I found a temporary fix to the problem is to bypass the wiring so that it continously receives 12volts and thus blows cold air only (of course, I would need to disconnect this for heat in the winter).

Seems as though my ACC switch control unit is the problem. Turning it on and off repeatedly I can get it to intermittently deliver the 12 volt current to the mono-valve and other times not.

Are the two other control modules under the glove compartmetn responsible in regulating the heating/cooling (i.e. supplying the current to the mono-valve?) I would like to restore the function of the ACC system (but at least for now, the 12-volt bypass seems to work). I want to make sure I'm replacing the right part, before spending the money on it.

any others with similar experiences?