Hey ya'll,
Still having trouble with heat at highway speed. Just collected more data/ as follows. With mono valve two pin connector disconnected, I still lose heat at highway speeds but gain heat as I slow. The slower I go the more heat.

Does this rule out the mono valve coil/magnet? I'm starting to think thermostat, but engine temp is steady at 85C.

Was 18F this morning. What to do/What to do

Pat

Have you removed the monovalve components and visually inspected the rubber diaphragm? If torn you will have no heat at highway speeds. Or reduced heat.

__________________
-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)

Did the rebuild kit for the mono valve even though the old parts all looked to be in good shape.

Next

First things first: what is your engine temperature after running for approx. 5 minutes? You should be at 80 deg C +/- 10 degrees. If you are down in the 60s or below, your thermostat is stuck open. Not only are you freezing but that's bad for your engine.

With a properly functioning mono valve, if you unplug it, you will have more heat than you can stand since the valve is failsafe to open with no voltage.

I suspect you have a blockage through the mono valve. Get a piece of 3/4" pipe and substitute it for the mono valve temporarily then go for a drive.

If you get roasted then the mono valve is the problem. If you are still cold you have a blockage in the heater circuit.

I would next try bypassing the aux water pump with the pipe and see what happens. If the impeller is jammed it might be killing your flow.

If that doesn't solve it (both the mono valve and aux pump bypassed) then you have a blockage in the heat core or hoses.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

My magnet is bad in my monovalve on my 420, so I unplugged it for full heat, and when the engine is at 82C its HOT...with a coolant temp of about 180F, I bet the air coming from the vents is not far below 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)

I was told by my local indy who has worked on these things since they were new that the entire monovalve needs to be changed if disconnected and no heat at speed. The unplugged monovalve defaults to full heat period. He said it was a yes/no question and not to over think it.

He also showed me a 12v test light that he used to use for testing the temp control. It was basically 2 wires and a bulb cut from a harness. He said that the light would come on if the temp wheel was turned to high heat. If not, something is wrong with the dial.

He also said to change the auxillary pump if the push button needed changing. The pump can take out the push button CCU.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Thanks,
Good reply finally.

One more question, I live in the middle of Kansas and can't seem to find a complete mono valve. No salvage yards that I'm aware of.

His recommendation to replace the whole thing is not correct. The housing itself it just a static piece of plastic. There is then the magnet piece at the top, which can fail, and the valve piece with the spring and diaphragm. Those two components are the only "wear" items. No need to ever replace the whole unit unless its cracked or leaking etc...

Its possible you have a flap issue with the heat-door in the dash that is causing it to improperly close at higher speeds, leading to no heat. Especially if its doing it with the monovalve unplugged.

I would do the test with a bulb to see if the klima unit is sending power to the monovalve at the correct temp wheel settings.

__________________
-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)

If the magnet is the problem, would heat just be ON/OFF and not controllable? Also, I've not seen that magnets are available. Are they?

Question/ Trying to learn: If heat is controllable at low speed, wouldn't magnet be OK? Why would speed cause low heat. As I quoted earlier, mono valve innards looked OK before rebuild kit was installed.

Pat

At this point the next step must be to inspect the monovalve itself and make sure the diaphragm is not torn. If it's torn then many other solutions are moot until you replace that part. Since tearing of the rubber is so common look there now. It takes about 2 minutes to inspect it and all you need is a screwdriver.

There is no magnet in the monovalve. There is a solenoid for valve actuation. Does not sound like the monovalve is your problem since power is needed to shut off flow.

Put your hand on the in/out hoses of the monovalve. If they are both hot, monovalve is not the problem.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Sounds like you rebuilt the monovalve with a kit already? The monovalve is a normally closed open/ close type valve actuated by a solenoid to control temp by cycling at different rates.

Is your coolant level ok? What is your coolant temp? Shut engine after a long drive and feel all the coolant hoses/ radiator and report back the relative temps. That will tell you if there are flow issues. Upper lower rad hose, hose in and out of the firewall are the important ones to feel.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

How will the solenoid actuate when there is no magnet?

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

By sending electricity through a coil of wire. Isn't elecity great?

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked