My 420 has the same issue, higher speeds I get weak heat, low speeds ultra heat. I did confirm my monovalve is weak/dead. I replaced the torn diaphragm unit with an MTC and that got me my low speed heat back (used to have almost nothing). I pulled my valve and magnet and used a 12 power supply to test. The magnet is very weak compared to others I've tested. Its not strong enough to do the initial "shove" it needs to move the valve. I'll probably swap the magnet with another at some point. I drive 90% city and don't use the car in the coldest months....so my unplugged valve gives me plenty of heat for now.

__________________
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'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I was wondering if the magnet could be weak. I'll test it and see. These things are crazy expensive.

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1987 300 TDT
1990 300 SEL

The good ones I have tested will go "THUNK" and slam the valve down with good force. When I tested mine last week you could feel a weak magnetic field but it wasn't strong enough to push the valve against its little spring and move it, if you helped it it would stay down....but thats it.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Good to know, I will check it out for sure. I will post my findings just for FYI purposes. Thanks.

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1987 300 TDT
1990 300 SEL

The magnetic coil is what I am pretty sure it bad on mine, because the valve leaked the could got saturated. Then good old electrolysis kicked in and its bad. You can test it simply by removing the entire solenoid and hooking it up to the harness. Then just change the dial for temperature. The valve should react. But if you can bench test, all the better.

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

Another 126 with the disappearing highway heat....
Seems like 60 is the magic speed. Below 60 - facemelting... above 60 pneumonia hole....
I've had a variety of other cooling issues which have now been resolved, and will try it with monovalve unplugged next time I hit the highway, but seem to recall trying that a month ago with the same results.

Any other info on vacuum activated blend doors? That's where my mind is circling currently...

Where's BC or Tango when ya need 'em....

__________________
On some nights I still believe that a car with the fuel gauge on empty can run about fifty more miles if you have the right music very loud on the radio. - HST

1983 300SD - 305000
1984 Toyota Landcruiser - 190000
1994 GMC Jimmy - 203000

https://media.giphy.com/media/X3nnss8PAj5aU/giphy.gif

I wish I could add something to this thread. But I have not worked on it at all out of fear that I will have NO heat during these cold times. With temps at minus 20 around here, I want all the heat I can get! But I am ordering a whole new mono vale assembly. Solenoid and all.

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

I'm sure it mentions in this thread about a tear in the monovalve insert. I would still bet on that being the problem in the most recent post here. It can do all sorts of funky things. The vacuum question I don't think is it, that would be related to throttle setting (manifold vacuum) not speed. But the speed of the car might actually relate to engine rpm (water pump speed?). You could try holding the engine rpm at whatever you are seeing say at 50mph for 30 seconds or so, then try increasing to what you are seeing for rpm at 65 mph, as a test. But my best suggestion would be to inspect the watervalve insert.

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Visual inspection of the monovalve insert did not reveal any issues, and today it was cold enough that I fell into LandYachts category of not messing with it for fear of losing the heat I have.
Also, I don't do any highway driving to get to work.
I think tomorrow on the way in, I will try driving in third to raise the rpm's and see if the problem relates more directly to rpm, then try unplugged monovalve with similar rpms on the way home.

If there is a tear in the insert which I did not see when examining it, will unplugging the monovalve negate that issue? or will it still exist even without power?

__________________
On some nights I still believe that a car with the fuel gauge on empty can run about fifty more miles if you have the right music very loud on the radio. - HST

1983 300SD - 305000
1984 Toyota Landcruiser - 190000
1994 GMC Jimmy - 203000

https://media.giphy.com/media/X3nnss8PAj5aU/giphy.gif

OK.... today, drove along with great heat at 45. Down shifted into third and maintained speed at increased RPM's (tach is intermittent as well, so wasn't able to get #'s) and heat disappeared. This was with monovalve plugged in.
Drive home, monovalve unplugged, same exact experience. Heat at 45 in fourth, dropped to third and the heat went away.

Undoubtedly RPM related.

Was too cold today to jump out and start grabbing hoses to see what was hot at different times in the sequence, maybe tomorrow when the temperature isn't single digits....

Wondering if there might still be an air pocket in the cooling system....
I just changed the thermostat and radiator over the last two weeks, so the system has been open a couple times.

__________________
On some nights I still believe that a car with the fuel gauge on empty can run about fifty more miles if you have the right music very loud on the radio. - HST

1983 300SD - 305000
1984 Toyota Landcruiser - 190000
1994 GMC Jimmy - 203000

https://media.giphy.com/media/X3nnss8PAj5aU/giphy.gif

What was mentioned earlier about jumping power and ground to the valve electrical connectors might be a good test, to hear if the valve seems to be opening and closing as it should.

__________________
Click here to see the items I have up for auction at EBay

Click here to see a photo album of my '62 Sprite Project Moneypit (Now Sold)

yes, the magic RPM seems to be about 2000. Above that, it seems to lose heat. I'm working the same problem. So far, I have narrowed it down to something to do with the engine rather than the heating system. Checked flow through both heater core and mono valve. Removed mono valve connector and unplugged aux pump.

I now know there are no obstructions in the heating system. I have lower than normal temps around the engine though. I pulled the thermostat and tested it. Worked ok the first time, but did not open til way too late on the second test. I am going to replace that next. The water pump and radiator are also not that old. Based on temperature measurements, I am not flowing well through the radiator, but still don't think it's the radiator. I'll know more after the thermostat.

After that I'll have to start checking the pressure integrity of the cooling system to see if I have any leaks or if pressure builds during higher revs.

Tough nut to crack. Just hope it's not a cracked head.

Apply 12V to the pin closest to the center and ground to the outer pin. You should see the valve close through one of the two upper input ports. If it is unplugged though, it should be a non-issue as far as cutting off heat at higher RPMs. When it is unplugged, the monovalve is not stopping the flow. You would look elsewhere.

I would go back to the vacuum / blend door theory. Test it by taping a thermocouple to the heater core return line. If the temp stays hot with speed, you know you're getting water flow.

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Prost!

I had this same experience in my 126. There was a hole in the rubber diaphram of the monovalve, the increased water flow at higher RPM allowed for it to become a problem, at lower RPM with less flow the problem did not present. I would wager a replacement would correct the issue.