I have an 85 300Sd that I have been going over a year with no heat. Finally cut the bottom rubber seal off just to get through this tough winter.
I have replaced virtually every component in the system including w/pump. aux pump, temp control unit, pushbutton set, in car temp sensor with aspirator pump, water pump, t/stat, cap. Had tried several used mono valve magnets, replaced valve with MTC unit. Still no heat at highway speed only at idle.
With everything leading to mono valve I bit the bullet and bought a new mono valve assy from the dealer and used only the magnet and plunger and guess what the heat works like a charm now.
I am suspecting that the combo of the MTC plunger and the magnet coil loosing effectiveness because of exposure to heat, moisture and age they are going bad. I tried 3 different used coils with the same effect.
I also compared the MTC plunger to the one in the new mono valve assy and talk about a difference in construction. The Bosch new has a stronger spring and thicker rubber.
Now I have heat at the temperature I want not either on or off. There are alot of good aftermarket parts out there but these MTC plungers are junk

I've been going through this no-heat circus for the past week. Fortunately it's almost spring here in Seattle. Car is an 84 300SD.

So far, I've replaced the monovalve plunger with the dreaded MTC unit which looks to be nearly identical to the original (spring tension, travel, etc). No heat; sigh.

So next day I took the Monovalve and pump assembly out of the car and checked their respective functions with a jumper wire and a battery. Both work fine on the bench. Monovalve plunger moves to closed position as seen though the port on the valve. I ran the garden hose into the heater core outlet to check for flow there. Plenty of water coming out.

So since it had it all apart, I installed a fuse in the pump circuit and put it all back together. I started it up and let it warm up while parked nose down on the steep hill that is my driveway. That makes the heater the high point. At first no heat; then after gunning the engine a few times wa la! heat coming out all the vents. Maybe I burped the air out of the heater. Parked overnight nose down on steep hill. Today I drove the car and again no heat. Repeated the presumed burping procedure (gunning motor, nose down) and no heat.

Presuming these things do fail open, at it appears they do from the spring action and direction of motion when energized with the jumper, I should have heat even if the monovalve is totally non functional, right? But I don't. I makes me think that there is still an air lock in the system somewhere. But, when I checked the hoses for temperature, the hose returning to the engine water pump is hot as is the windshield heat return hose. So water is flowing through the system via the WS heater but not through the cabin-heater core for some reason.

I noted that the valve and pump are on the suction side of the heater core rather than the pressure side for what it's worth, so the auxillary pump would not be able to suck air out of the heater core. How do I get air out of the heater core should that prove to be the problem?

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98 E320 4matic Wagon
84 300SD (2 ea.)
51 Chev 1-ton PU
Assorted old motorcycles

To update my last post: I discarded the MTC replacement part and repaired the old diaphram, or more accurately replaced the diaphram with a bit of bicycle inner tube. This involves using a die grinder to remove the brass lip that retains the upper disc holding the diaphram. After making a new diaphram (see next paragraph) and installing it in place of the old one, I soldered the brass retaining-disc into its original position, capturing the diaphram.

Note there are tiny holes in the two brass discs AND in the diaphram (sorry no pics) that must be lined up. I believe these are to equalize pressure on both sides of the diaphram and modulate the speed the valve engages/disengages. Also note that there is a fine brass screen between the valve rubber and the diaphram. This screen is the only difference I can see between the MTC valve and the original. I think this screen is an add-on to the original design and serves to create or mitigate some turbulence in the water flow. The MTC screen is coarser than the original and I suspect this is why the MTC valve closes under higher water pressure at highway speeds.

I used a leather punch to make neat holes in the tube, replicating the original. Then I carefully cut the original diaphram to salvage the lip that grips the metal flange and glued it on to the inner tube. But first I assembled the unit so I could get the lip glued on concentric with the locating flange. Then I carefully trimmed off the excess. After the Gorilla Glue set up I reinstalled the valve assembly into the electromagnet and put the unit back into the monovalve body still on the car. I now have heat both at idle and on the road. Since the inner tube stretches each time the valve closes, it may not last as long but when it fails, I have two more diaprhams from the MTC units I bought. I'll try those.

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98 E320 4matic Wagon
84 300SD (2 ea.)
51 Chev 1-ton PU
Assorted old motorcycles

I put in two new MTC monovalve inserts. Neither worked. I ended up buying the whole valve. I don't think they make the spring inside strong enough to stand up to the water flow. Total garbage.

When I previously replaced one with a Bosch (no longer available except from the dealer at stealer prices), it worked fine. One of the MTC inserts was too big to even fit into the opening. Even when replaced neither of the two worked.

In my case, the magnet must have been fine because I took the old Bosch insert out of another used valve and it worked. I mostly blame the MTC inserts. As an aside, I actually spoke to a person at MTC. He told me that he only had 2 returns in over 10,000 sales. Either he is miserably misinformed or they are not being returned because they are cheap. The problem is you get cheap.

Old but updating. With last winter coming, I did not want to do anything that would kill the heat outright.

But FINALLY a whole valve came up on eBay for $15 (read: cheap). After testing, I concluded it had to be the driver coil itself. So when the part came today, that is what I swapped right away. Everything works just fine now.

Even though the old coil bench tested okay, I dont think it work in actual use. Very happy that I finally have this one solved.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car