Mono valve home made diaphragm replacement DIY
 

Just discovered the mystery's of the mono valve and heating problems.
Just unplugging the connector from the mono valve will not check its operation, if the internal rubber membrane is torn or has a hole in it because the membrane actually builds pressure on it when the valve is in the closed position, and needs this pressure to help open the valve when the valve is De-energized (turned off).

But the good news is that you can fix the membrane very cheaply if you want to.

I just fixed mine by cutting a new rubber disc from a bicycle inner tube and used it to replace the membrane.
It works GREAT!
I now have heat again.

Monovalve repair
 

#1. You have to "Un-stake" the bottom washer of the valve.
#2. Remove the bottom washer of the valve.
#3. Cut a new diaphragm (if possible use the old one as a template).
#4. Install your new sheet of rubber.
#5. "Re-stake" the washer back on with a small punch.
#6. Then glue the old rubber membrane perimeter lip seal to the back of your new diaphragm.
#7. After it dries, you can carefully trim the edge with Scissors and reassemble the valve.

I am trying to add pictures because it is hard to explain without them.

An alternative repair
 

for members.


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Just did this since I'm impatient when it comes to waiting on parts. How long did it last? Seems pretty solid but will take it out and look when the actual part arrives and it's seen some use. Thanks ALOT for taking the time to put this together, it helped alot :)

Tagging this for later.

I simply found a recently replaced one at my local salvage yard on a 5xx gasser. The counter guy had no idea what the hell it was, nor was I going to educate him, so I was allowed to leave with it. :D You simply mention Mercedes and every price goes up by $50.

Side question so I can prepare: How much coolant do you lose when you take the valve off?

-Rog

Answer
 

If the system is cold, and you only replace the cartridge = zero coolant loss..


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Did this today and I can confirm that it completely fixed my heat issue (no heat at speed, fan blowing like mad, no doubt in attempt to scavenge something from the tepid coolant). The temp wheel actually functions now, or rather setting it to a temperature triggers strong warm air until it reaches that temperature and it backs off, rather than before where it was just going nuts and accomplishing nothing. Much recommended as an easy thing to fix before mistakenly diving into something more complicated.

The middle of the rubber piece on mine was completely gone. No need to carefully inspect it for tiny tears in that case. ;)

The only hard part was getting the washer off without totally ruining the flare that holds it on. I pretty much mangled the flared bit, but it went back together and stayed together. Hopefully it'll get through the winter until I can add a new one to my spring fix-it list.

-Rog

what's a 5xx gasser? Anyone know what other makes and models use this mono valve ? It would be handy to know when scouring junk yards.

bump