Mono Valve Housing leaking! - PeachParts Mercedes-Benz Forum

BenzBid · #1 10-04-2010, 08:46 AM

Greetings,

I "rebuilt" the mono valve last night. Very straightforward, however I know have a coolant leak from the mono valve housing, that brown plastic casing. It's dark out this am and I can't quote tell if this is an overflow function or if I have cracked it. Anyone ever done this or know of the part number for the plastic housing? Unfortunately it will be dark by the time I get home and not sure I will be able to tell, but would like to get something on order so I don't have to be without my beloved benz for too long

Thanks in advance.

Jeremy5848 · #2 10-04-2010, 11:47 AM

There is no "overflow" function in the monovalve. If the housing is cracked and you cannot seal it up with glue or whatever, you must replace the entire monovalve. The casing is not and never was available as a separate part. If you are on a budget, try to find a used one at a junkyard or from someone on this forum.

BenzBid · #3 10-04-2010, 12:58 PM

Thanks for the info. I know it was wishful thinking, but I figured it was worth a shot. Have a great day!

BenzBid · #4 10-04-2010, 09:50 PM

Below our the photos of the housing. It looks like it is leaking out of designed hole in the bottom. In the 2nd picture the housing is upside down and you can see the square holes.

After carefully making sure the new valve was seated I re-assembled and installed. Started her up and took her for a spin. Up to operating temp and no leak, yahoo. I then decide to test the heater and it worked! With the engine still running I turned off the heater and it began to leak out of one of these holes. A steady drip. Turning the heat back on seems to take away the drip. Any ideas as to what may be causing this?

3rd pic is of the old mono valve with the torn diaphragm.

Jeremy5848 · #5 10-04-2010, 11:08 PM

I haven't worked on a 300D monovalve for several years, so maybe someone with more recent experience will post a better answer. Those holes are there for the purpose of allowing small leaks to get out of the valve rather than getting into the electrical coil and ruining it.

My guess would be that the new diaphragm isn't seating correctly. When the heater is "ON," water is flowing through the valve; heater "OFF" means the valve is closed by application of 12 volts to the solenoid. However, with the engine running, there is still pressure in the cooling system due to the water pump and maybe even the aux pump. (I think the aux pump runs whenever the ACC is turned on, whether in heat or cool mode.) This pressure is causing small amounts of coolant to get past the new diaphragm and leak out of those holes.

Try taking the monovalve apart again. Check for small cracks in the inside of the housing. Make sure everything is clean and that the new diaphragm wasn't damaged before you got it. Some plumber's grease might help the diaphragm seat in its groove.

Jeremy

BenzBid · #6 10-05-2010, 11:03 AM

Drove it into work this AM and popped the hood. No leaks, but I didn't use the heat at all. Thanks for your help, I will keep you posted.

Have a great day!

Scott

neverether · #7 12-10-2014, 12:33 AM

So I know this thread is really old, but it's the top result on Google when you search for this problem, so I figured I'd update it with my solution.

For me, the circumstances were the same as BenzBid's. I replaced the mono valve (not the housing, just the valve), and I found that the following day, I was leaking coolant everywhere. I removed the housing and found the square holes that are pictured above. Turns out, that when I seated the mono valve into its wire coil, I actually pushed it down way too hard. The wavy washer was almost flat, and the rubber was not seating correctly to the plastic of the housing. I resolved the issue by seating the monovalve 3/4 of the way, then putting the whole assembly together in the housing. This ensured that the rubber was seating properly, and there was no space for antifreeze to leak out.

Jeremy5848 · #8 12-10-2014, 01:52 AM

Excellent blow-up diagram. Helps to see the problem in pieces. Thanks for posting.