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Old 11-14-2014, 07:57 PM
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Join Date: Aug 2006
Location: Seattle
Posts: 24
126 Mono-valves again

A year ago the mono-valve on my 84 300SD failed. It was the usual torn diaphram. I took a shot at the MTC repair kit despite the warnings that they don't work and sure enough, it didn't work. Instead of no heat at road speed I now had no heat ever.

I believe the problem with the MTC repair kit is that the screen is a coarser mesh than the factory one. That's the only difference I can see. This might not seem like a big deal but I believe that the screen acts to balance the pressure of the water against the spring in the "magnetic lifter." The upshot is that the water pressure overcomes the spring and forces the valve closed. The spring is not powerful enough when using the coarse screen.

I followed the DIY on repairing the existing valve, including the fine-mesh factory screen, with a bicycle inner tube and it worked, until this week. The e-clip that holds the valve-rubber/screen assembly on had gone missing. So I replaced the e-clip and the spacer and put it back together. But now no more heat at road speed.

After a good bit of experimenting and re-soldering with still no heat, I said screw it and went to the wrecking yard after a good used one. But the counter man refused to disassemble to inspect the diaphram and said that if I did, I owned it for $85. So, no deal.

So back to the shop. I took the MTC unit that I had not previously destroyed to cannibalize and put the valve assembly onto the factory "magnetic lifter" as they call it. I tested the unit (valve body and hoses still on the car) off the car with jumper wires and it worked fine.

I rummaged through my spring box and came up with a compression spring the same diameter as the valve piston (it fit just inside the rubber lip) and about 3/4 inch long. This spring rests on the bottom of the valve housing and supports the valve such that it's open even when there's water pressure. Now as the control unit signals the valve to close it has to overcome its own spring as well as the "auxillary" spring I installed.

The result is that I now have head at idle and at road speed using the MTC valve assembly and the factory "magnetic lifter."
98 E320 4matic Wagon
84 300SD (2 ea.)
51 Chev 1-ton PU
Assorted old motorcycles
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Old 11-16-2014, 12:06 AM
Registered User
Join Date: Dec 2007
Posts: 4,958
Great creative problem solving! Too bad the morons at MTC couldn't have figured that out. Mercedes has a handful of complete monovalve assemblies for sale at (are you aready?) $500 each (ouch!!).
1991 560SEL, 1984 300DT
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