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Discussion Starter #1
my heat has been malfunctioning for about 3 months. it would go cold and if played around with the temperature control knob, it would usually come back. now it goes cold and stays cold.

i disconnected the monovalve and same thing...no heat. the operating temp is normal...right in the middle.

i was leaning towards the push button control unit, but now the only thing that i can think of is that the monovalve is stuck closed, because when it is unplugged it should be open and giving me full heat, regardless of anything else.

any thoughts?
 

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'84 300D/ dakota benz trk. Sold... miss it. "88 260E '91 190E 2.6 '89 Volvo 240DL "85 300D
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wrong... unplugged is no heat. unless this is some kind of fantastic primo car, take the valve out and simply put in a gate valve. no more problems.
enroute to florida last jan mine did the same thing. i would have to get out and tap on it with a knife to get it to open. took it out put in a manual valve, good to go. just turn it off when you start running the AC.
 

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1983 Mercedes-Benz 300D
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56 Posts
Buy a new monovalve, it's around 50$ or so. Easy job to do. That will fix it up great.

In the meantime, take the old monovalve out and look for a tear in the rubber diaphram. You can confirm it for sure.
 

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1985 300CD
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2,137 Posts
i thought the valve was normally closed? when power is applied, it opens. is this not correct?
Correct. The valve is normally closed. When the monovalve is activated it pushes the valve open and allows hot coolant to enter the heater core. If the valve is closed, no coolant enters the heater core and you have no heat.

Unplugging the monovalve just guarantees there is no heat.

I bought my monovalve repair kit for $35. The first one I replaced had a torn diaphragm. The second one had an internal issue that caused the plunger to get "stuck".
 

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Discussion Starter #7
sorry i'm getting twisted up here...lets start over.

i thought that the valve default was to let coolant through...meaning that it is normally OPEN. when activated, the valve closes. so 0 volts = open and 12 volts = closed.

if the temp dial is set to cold, you should have 12 volts at the monovalve so that it closes. if the temp dial is set to hot, you should have 0 volts at the monovalve so that it is open.

am i wrong?
 

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1984 Mercedes-Benz Turbo Diesel
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3 Posts
Can someone point me to the exact location of the monovalve, and perhaps a link where to buy a replacement?
 

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81 300D US & 81 300D EURO & 83 300CD & 83 300D
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376 Posts
I'm pretty sure most everyone here has the monovalve backwards.

12V shuts the hot water off, unplugged is heat on.
 

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If the monovalve's rubber seal gets a hole in it it will cause the valve to close no matter where you have the temp wheel set at. I have taken the plunger out of mine for the winter, this will allow flow all the time. Come spring and hopefully a job I will replace this part.
Good luck, Brent
 

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1967 300
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Before we go crazy unplugging stuff,

What happens if you put the car in defrost? Last I checked that should put the car in a fail safe and you will get blasted with heat.

On my 85 I was having awesome heat for quite a while and then all of a sudden my heat was wonky on the highway. It would get cold and then hot then cold.

I discovered it was this hose in the glovebox that had rotted apart. I replaced it with a fat rubber pipe and all is well. It doesn't respond as quickly as it did to heat changes, but it works fine.

See if that is the case too.
 

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'83 300CD
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Ok on the topic of heat or the lack thereof. My heat runs at luke warm reguardless of where i set it.

Im new to all of this benz/diesel stuff so any pointers as to what direction to start trouble shooting would be amazing.
 

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Ok on the topic of heat or the lack thereof. My heat runs at luke warm reguardless of where i set it.

Im new to all of this benz/diesel stuff so any pointers as to what direction to start trouble shooting would be amazing.
Welcome, The first thing I think to do is unplug the electrical connection on the monovalve which will let hot water from the radiator flow freely to the heater core. However there is a rubber boot inside the monovalve has a slight tear in it, it will restrict the flow at rpms above idle. There is also a auxilery pump in-line which if it is locked up will also restrict water flow. I removed mine. It was put there to assist with flow at idle speed. Start there and post again and I will help you through this. Brent:thumbsup:
 

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'83 300CD
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53 Posts
There doesn't seem to be a temp difference between idle and at speed. I'll give that a shot tomorrow after class and post back up with the results.
 

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'83 300CD
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hokay, So

alright, when I got home from class i unplugged the electric to the monovalve and took it for a spin The heat was a little warmer than before... untill i got out to the main road and got to speed. At that point it went back luke warm. by this point the engine temp was reading around 45-55ish so i know it was warmed up. When i get back to the house I let it idle in the drive for a few minutes to see if the heat would come back, no luck.

from what Im reading it means that the problem is either in A) the little rubber plunger valve thing or B) someone was mentioning there is an aux pump in the system.....


Anyway, i took a quick pic so if anyone can confirm that I did unplug the monovalve that would be amazing (its not unplugged in the pic).

I really dont care about controlling the heat as much as just having some. I've only owned a motorcycle for the past 5 years so its not that much of an inconvience to turn the heat off if i get too warm.
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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Yes, that's the mono valve, I understand if the diaphragm is torn, you'll get heat all the time.
There is an aux pump, in front of the turbo, you'll see heater hoses attached.
 

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'83 300CD
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What would be the best way to determine if that is the issue?

Thanks to everyone for walking me through everything!
 

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What would be the best way to determine if that is the issue?

Thanks to everyone for walking me through everything!
I removed the auxilery pump on my 300Td and everything is fine. I put a half inch street 90 degree copper elbow it in its place. Take the monovalve apart and remove the plunger and inspect it. You can run the car lke this for now until spring. If this doesn't solve the problem, you have a clogged hose/coil or other flow problem.
 
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