Its not just the seal that causes a problem, the electromagnet can get weak or fail and then it does not operate properly either. I had that happen to the valve on my 420SEL, swapped it with a known good electromagnet, (same seal assembly) and it works great now.

They're listed in the pelican as MTC with only one thumb up user rating. I actually got one somewhere else also listed as MTC brand.

Internet said they can be 50\50 new defective. Guess i got lucky.

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.

I'd rather not tear the car apart. I have an old (78) Dodge D-100. A cable runs out to a valve in the heater hose.Push the lever to open. The further it opens the more hot water you get. Sometimes I long for the old days. May be putting the diesel out of my 83 300CD parts car in it if I live long enough. Probably leave the heater the way it is though.

I have a NOS Bosch monovalve in my grey 300SD that I installed 2 years ago. I never have any problems with it. Everything in that ACC system works perfectly. My silver 300SD has an MTC monovalve and before my Sanden kit fell apart, its ACC system worked flawlessly except for reliable heat. I swapped in the rubber parts from an MTC kit onto the solenoid from a Bosch monovalve and the heat works a lot better now. I have not been able to drive the car a lot lately but for a week's time, it seemed to behave normally with regards to heat. I have had very mixed results from the MTC kits. Make sure you install the shims and spring washer correctly. If you don't, you will have issues guaranteed. Another note is that the spring washer on a new MTC kit failed shortly after installation. It came out in 3 pieces after inspecting it due to no heat issues and leaking around the monovalve area.

There's still a lot of unknowns with regards to how the monovalve operate. I think the answer lies behind that mesh and what's connected to it. I may take my spare monovalve and run some bench tests and see if I can learn anything from it.

Phillytwotank. have you removed the spring when you had it apart?

i think that was what was going on with my 126, put a new mono-valve insert in,no heat i then put in a new magnet(from a parted out 126) and the heat works great, but when i would try to unplug my mono for full heat(it failed in February very cold here) no change, still blew cold air

I don't think it is that complicated. The system works like this: heated water comes from the outlet at the driver's side rear of the block. It goes to the center port on the heater core. The heater core splits that input into two outputs and both run to the upper half of the monovalve assembly. The valve defaults to the upper position so that water flows downward to the boost pump and back to the return side of the engine-block-mounted water pump. When the monovalve plunger is in the closed or downward position, it stops the flow of water and has the boost pump pulling it in the downward position and positive flow from the heater core pushing it down as well. With the solenoid disconnected, the monovalve never gets a chance to be sucked into or stuck in the closed position. I have seen several monovalves with broken seals and corroded solenoids that are very stiff to move - this condition may cause constant heat as the monovalve does not fully extend to the closed position.

My theory behind why the MTC part does not have as stiff a spring as the Bosch part is that the engineers that designed the MTC monovalve did not account for or under-compensated for vehicles with a boost pump pulling the monovalve down. The part is supposed to be interchangeable with BMW, MB and various other European model vehicles. Some of which may not use any boost pump. Increasing load on the spring will increase the amperage needed to push the monovalve plunger down and may cause electrical problems in some cars.

Just an aside but vehicles with the windshield heater have it routed in a bypass configuration so that any time there is water pressure, there is water running through the windshield heater. It completely bypasses the monovalve and enters the circuit right before the boost pump. You may feel the windshield heater hoses are hot even of there is no heat in the cabin - this situation partially invalidates the common troubleshooting claim that one hose may be cooler than the other with the heat on; both hoses will be hot due to hot water flowing though them at all times.

Nice write up eatont9999! I didn't know the same monovalve was used in other cars. Do you think the presence or absence of an aux elec coolant pump affects how the monovalve operate? Some model year W123's did not come with an aux coolant pump. Did they use a different monovalve because of that?

Have you had an MTC kit with a known weak spring causing no or weak heat?

I'd like to get a hold of one of those MTC kit, measure the spring and run some tests. If anyone has one to donate, please pm me.

The only way I see the aux pump affecting the operation of the monovalve is by adding suction in the closed position. I do know some earlier model cars did not have an aux pump but that would not necessitate a monovalve with a weaker spring if the electrical components are rhobust enough. What I don't know anything about is the other cars the MTC monovalve is used in. Perhaps they are more sensitive to electrical load or designed in a different way. We could also be looking at a simple case of the weaker spring being .001 cents cheaper than the stronger spring.

I have a collection of used Bosch monovavles with broken diaphragms and a new, unused, MTC monovalve that I could test. I'm not sure how I can test them to get accurate and measurable results, though.

I installed an MTC monovalve as-is into my later 300SD and I had the typical no heat at driving speed and hot heat at idle problems. I purchased another MTC monovalve and swapped the rubber parts onto a Bosch solenoid body. The heat in that car seems to be working like normal again but I have not had a lot of opportunities to test it with the mild weather we are having this winter. If I lock the ACC in full heat mode, I do get hot air.

I've got another in with no heat. 85 300CD that does not seem to matter whether the heater valve is plugged in or not. Will be removing it to confirm flow thru heater core here over next week or so. Already tried two different monovalve inserts.

by heater valve plugged in , do you mean that 2 prong on top of the mono valve? when mine went out(126 380se) i would unplug that and never get full blast heat, i fixed it when i replaced the magnet after i replaced the insert which was only a year old so I am not sure if the insert failed again or if it was the magnet all along

I am referring to the two prong connector.