monovalve

[84300DT]

i think my monovalve is failing. symptom is full heat on all the time at any setting.

to test i would need to set key switch to 'on' , CCU on EC setting, temp wheel to 'off' and see if there is voltage at the wire that connects to the monovalve, correct?? or??

[84300DT]

edit - i am getting 12v from the wire that attaches to the top of the monovalve with they key + CCU on.
what would be the next step for diagnosis..?

[romansek]

Just open the monovelve and check the rubber seals. Most of the time the top seal is damaged and coolent flow thru heating coil. The voltage indicates that signal to close the monovelve is sent from a/c controler.

Roman
-----------
87 300 SDL 155K
86 420 SEL 225K
86 300 SDL 211K
83 300D 226K
81 300D 139K

[Fisherman]

Very simple process and you can actually look at the monovalve to see if the gaskets/rubber are damaged or if there are any foreign particles that may have lodged around the seal at the bottom of the valve.
If I remember I think it will also react when it is out of the body so you can see it function.
You are right in that failure equals open all the time or constant heat. When 12 volt is applied it will actually move down in the cap to cause the seal to close off water supply at the bottom.
If you are getting constant 12 volt to the unit then this would indicate that it is trying to close to stop the flow of water into the heater core.

[autozen]

As has already been stated, the default position of MB heater systems has always been full heat. If you lose vacuum, voltage, or signal the systems are designed to go to full heat to keep passengers from freezing to death in extreme cold conditions. You should have 12 volts there to keep the heater off. I concur with prior recommendations to remove the connector and the 4 screws and pull the slug out to check the diaphragm. If you don't find a problem there, remove the plastic cover from the pin connectors and squeeze down on the springy parts of the connectors with a pair of needle nose pliers. I have found on several occasions that these springy things lose tension and cotact. The result is heat always, intermittent heat, or piss poor A/C because the heat is on.

Peter

[84300DT]

thanks for the replies. i will try to open it up and check out what's going on inside of the valve + also under the plastic of the wire connector.

[84300DT]

i messed a bit with the no.14 fuse which was ok. i cleaned off the terminals on top of the monovalve as well as the wire connector, both of which were corroded. eventually i think the whole valve needs to be taken apart and checked, but for now at least it's working.
i had previously replaced the in dash temp sensor and foam tube behind the glovebox.

fwiw i am loathe to touch these 20yr old parts unless really necessary!!

[BrierS]

"fwiw i am loathe to touch these 20yr old parts unless really necessary!!"

Oh come on . . . having just bought an '84 I was hoping to learn from what you ran across.

Actually, I am searching for anything I can find for common problems on the '84 300DT (wonder if there is also a non-turbo carrying the 300D w/o the T).

[84300DT]

steve,
hehe, not touching stuff has actually helped me in the past...
i am certain the thing will have to come apart eventually but using the wise
words of PEH, ''if it ain't broke don't fix it".
i will try to post some pics when and if i need to disect it

as you are in NH you can appreciate what the salt does to our cars here in
new england. wires, heck everything gets corroded in no time.

again fwiw i recently replaced the down pipe on my car and can tell you was very very careful when removing the stud nuts on the engine side.
turned out MB in their wisdom made them out of bronze? so i luckily was able to remove them without a problem.

good luck with your '84.

[autozen]

Congrats. I had a sneaking suspicion the problem was with the connector. I still recommend retensioning the little springy things.

Peter