Glad someone on the site put a heads up about bad aftermarket mono valves. Finally got fed up and cut the bottom cone shaped seal off. Heater works fine now. Is there some way to buy/find a valve that actually works? I can't find a Bosch valve anywhere. Or is there a permanent fix other than what I did?

I've had a couple of 123's in lately with heating issues and came up on the bad monovalves. Tell me more about removing the bottom seal to make em work.

__________________
Jim

There is a tiny c clip that holds the bottom seal and mesh piece on the spring loaded plunger shaft. Pull that off and there'll be guaranteed MAX HEAT!!

__________________
Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"

Thanks, I will check it out tomorrow.

__________________
Jim

Still a work in progress. you can get max heat with the above but the mono valve won't function to regulate heat. I am going to modify the rubber seal. I will let folks know how it works.

The bottom cone shaped seal is the stopper that the solenoid acts on. Cutting it off allows constant flow, same as replacing the monovalve with a pipe.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Apparently there is something I'm still missing here. Am I correct that with no current the valve is open (max heat). Why doesn't this valve work? Does the valve get stuck in the hole or does the negative pressure created by water flow suck it shut? Trying to see how to fix this so the valve works correctly. Is this company a division of URO? It is hard enough figuring how to fix these cars when the parts work!

Something like that. The spring is too week to resist the flow.

__________________
Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"

Yes, no current, valve is held open (by spring tension)= max heat. If you had a leak in the large diameter diaphragm, corrosion that developed could cause the solenoid to hang. If you are rebuilding it, you have to take it to bits and clean it so the iron core plunger slides freely in the coil. There is also a float valve that could jam due to debris. If you shake the monovalve, you should hear it rattle. If you blow into the bottom port (with monovalve in orientation as mounted in car), you should hear the float rise up and flow stops.

I find it hard to believe a weak spring can stop flow due to water current. Until I inspect one that actually does that, I am not buying it. I suspect it is due to other causes.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Ya, I find it hard to believe that also. Wish I had not thrown the old on out. I could take a micrometer and check dimensions. I have replaced several of these with discontinued Bosch part and never had a problem. Wrote a nasty email to MTC but will probably receive the standard reply: No one had this problem but you or we make the finest quality parts and everybody loves them but you.

After you go through all that and yo azz is still freezing then you'll be buying that.

I just went through my own mono valve saga and I am certain that
weak spring was the cause and removing the lower seal from the plunger shaft was a definite fix for the short term. New MTC valve kit restored normal operation and is working as it should so far.

__________________
Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"

Where would I find an MTC valve kit?

I mean one that works, have tried two already that don't.

That may be so. Thing is, there are so many reports of new MTC kits that do not work, that they have weak springs, they fit tight etc that I don't know what to believe. Did the spring in your MTC feel stronger than the old one? By how much?

The monovalve can be tested on the be bench. With no power, connect horizontal inlet port via hose to faucet. It should flow out the bottom outlet port with faucet on low. If you turn up the water pressure, it should continue to flow. If not, then it may be due to a weak spring. Did you/ can you run such a test? While water is flowing, applying power should stop flow.

If you run the faucet into the outlet port, the check valve should prevent flow out of the inlet port if working properly.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Idid the diagnostic work and ordered the part. My ole Dad drives the car so when the part came he put it in and reported that normal heat operation had been restored.

Did i? NO. All the old parts are still there so i could do this for the benefit of forum of knowledge.

I worked through all the online mono valve info and trouble shooting threads. I'm sure that i arrived at the spring conclusion by process of elimination and logic with out actually doing that test.

It would have burst of lukewarm heat when driving, usually at lower RPMs. It would also sit and idle with the heat on blast and get hot. The temperature would also function normally. When i wanted the heat off turning the temp selector to blue would do that. I had the classic symptom of no heat at highway speed.

The electrical signal to the valve is good and i can verify that by the fact that it will shut off the heat as it should. So i'm thinking that the valve was getting physically stuck in the closed position by corroded plunger internals or crudded up seal seat area but no. If took the valve apart to look at the operation i could see that it was not sticky or stuck down, i moved freely and i could see the solenoid operate by electrical signal both from the ccu and from a 12v jumper. Once i took the mest seal part off the shaft and started getting 100% max heat i am convinced that it is that spring is worn out. Either that or my water pump is too strong.



I did do this when i was trying to make sure the heater core was not air bound. i connected the flow to the hose on the inlet side of the aux pump, flowing towards the valve and got a quick shower. I connected my flow to the hose on the other side of the valve and flowed freely all the way back the the expansion tank.

__________________
Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"