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  #1  
Old 10-23-2016, 12:25 AM
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MTC Monovalve No Heat Issue and Solution

My 300SD was having problems with not getting any heat. The original monovalve was a Bosch 1 147 213 007-711 that last about 10 years. I bought a replacement MTC Mono Valve Repair Kit 0008350644/64118390132 and installed it. I still had the same problem. In the other threads, I discovered that the MTC mono valves have a weak return spring. The return spring is not strong enough to overcome the pressure of the water coming through the monovalve and the suction of the recirculating pump. Unfortunately, the Bosch monovalve seems to be out of production and now very expensive. MTC is the only choice. My do not buy list of brands now includes MTC and uro. What we are going to do for monovalve repair kits?

My solution is to add an o-ring to the base of the monovalve and reinstall. Get an o-ring a little small than the base of the monovalve. Just place the o-ring centered into monovalve seat, and put everything back together. The o-ring increases the open distance between monovalve stopper and the seat. In turn, this makes it more difficult for the water pressure to close the monovalve.

I do not think this was necessary, but I disconnected the power (near the battery) for the recirculating pump to get rid of the suction. I did not like the recirculating pump because it was always running. There was probably a stuck relay in the climate control circuit board. I also put silicone adhesive around the screen of the monovalve thinking that it would keep the water from pushing down on the stopper.

I have been testing the o-ring solution for 2 weeks. My results are: No heat with air conditioner running. No heat on EC mode and max cold. 145 F degree heat from the heater at max. At 22 C degrees setting on climate control, moderated heat. I hope this keeps working through the winter.

While I do not think that MTC will ever fix this issue with their monovalve, this o-ring modification should fix the problem for nearly the price of nothing if you have extra o-rings laying around. Hope this helps those who have no heat in their W126.

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  #2  
Old 10-23-2016, 04:02 PM
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Location: N.W. In.
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Thanks for sharing Mercedes Man I had the same problem with an MTC valve not working put the original back in and it worked I will remember the o-ring trick in the future. I think I still have the MTC valve if I did'nt toss it yet.
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  #3  
Old 10-23-2016, 04:52 PM
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Any chance you could post pictures please? Having a hard time imagining this.
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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild
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Old 10-23-2016, 09:00 PM
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Pictures

The pictures are fuzzy. I could not get the camera to focus.

The o-ring size I used is metric: ID 22.1, OD 29.1, Section 3.5. You can try to use anything that fits.
Attached Thumbnails
MTC Monovalve No Heat Issue and Solution-img_2151.jpg   MTC Monovalve No Heat Issue and Solution-img_2152.jpg   MTC Monovalve No Heat Issue and Solution-img_2153.jpg  
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  #5  
Old 10-23-2016, 10:33 PM
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Ah. Interesting. Thanks! Worth a shot
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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild
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  #6  
Old 10-24-2016, 10:57 AM
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Location: Carlsbad, CA
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O-ring Spacer for MTV

Thanks for the tip. I have two mtc valves that have weak springs. It's fall season so I'm thinking about "the heater" again.

Just wondering...does the addition of the O-ring keep the valve from closing / sealing. Seems the result is water always reaching the heater core.

Also wondering if the "fix" holds at higher rpms i.e. highway speeds? That's when the valve seems to have a problem i.e. heater fine putting around town, then no heat on the freeway.

Thanks

P.S. I've also disconnected the pump. My understanding is that it is to help the heat at low rpms.
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  #7  
Old 10-25-2016, 10:14 PM
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86-300sdl,

I have observed that the monovalve has a very long 1/2 inch stroke length when I tested it with the battery and monovalve coil outside the housing. I believe that the monovalve with the spacer o-ring still closes as normal since I do not see the temperature of the center vent rising when the climate control is set to cool without air conditioning.

I tested the heat at 65 mph and saw 145 degree heat. I think the pump does help at lower speeds. I noticed that the heat increases when the engine rpms rise. This condition has not happened in the last week and temperature has been comfortable.

I would like to do more observations of the solution such as long high speed drives, but this is much better than having no heat at all.
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  #8  
Old 10-26-2016, 02:50 PM
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Thank You Mercedes Man

Quote:
Originally Posted by Mercedes Man View Post
86-300sdl,

I have observed that the monovalve has a very long 1/2 inch stroke length when I tested it with the battery and monovalve coil outside the housing. I believe that the monovalve with the spacer o-ring still closes as normal since I do not see the temperature of the center vent rising when the climate control is set to cool without air conditioning.

I tested the heat at 65 mph and saw 145 degree heat. I think the pump does help at lower speeds. I noticed that the heat increases when the engine rpms rise. This condition has not happened in the last week and temperature has been comfortable.

I would like to do more observations of the solution such as long high speed drives, but this is much better than having no heat at all.
I'm going to give it a try tonight. I was ready to bite the bullet and by a whole new mono-valve unit from MB for a cost in the neighborhood of $250.

Thank you for adding the "Fix" to the forum. These days I'm not one to "jerry-rig" fixes. Although at one point in time, I always had a spool of bailing wire in the trunk (other vehicles). LOL

In this case it certainly fits into the "improved engineering" fix category
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  #9  
Old 10-27-2016, 11:41 AM
KCM KCM is offline
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I agree that the return spring force is the problem. I believe there was a fine line even with the Mercedes/Bosch version with spring force versus water pressure. I've taken the new seals off of the MTC and put onto the old Mercedes/Bosch plunger/spring assembly and it still would not work. The theory that the MTC spring is too weak does not totally hold water as using the old Mercedes spring should have worked. Of course the Mercedes spring probably had gotten weaker over time, and the MTC rubber could have been slightly stiffer than the original. Overall I think it was a marginally workable design. Too bad it is not easy to take apart the plunger/spring assembly without destroying it, as all that is really needed is a slightly stronger spring.

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