so i get no heat even with defrost button,ive chnaged monovalve ,took it out and put 12v it worked fine,had a spare ccu unit -still no change,whats next,ac works,blower is new works fine.even when car gets to temp no heat.if aux pump is suspect wouldnt there be heat at higher speeds?anyone have any ideas.maybe plug to monovalve does not get voltage-ill check with meter

If the monovalve is ruled out, the aux. pump is the primary suspect. However, you should still get some heat at higher speeds without the pump.

The possibility of a partially blocked heater core, coupled with a bad pump could produce your results. What's the history of the cooling system.........if you know?

And, just for grins.........unplug the monovalve and drive it.........see if you get full heat.

It is possible that the aux pump is clogged, in which case no hot coolant will reach the heater core regardless of the health of the monovalve. The heater core itself could be clogged but I'd check the aux pump first. The monovalve defaults "open" with no voltage applied; monovalve failure usually means heat all the time. If necessary, you can replace the aux pump with a piece of copper tubing and that will get you some heat while you repair or replace the pump.

Also remember that a frozen aux pump can draw so much current that the PC board leads in the CCU will fry. Adding a fuse to the aux pump circuit is advised (1 or 2 amps will be enough). There are threads in this forum on how to do that.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

i put 12 volts to aux pump and it worked,my cooling system is in good shape,engine runs at low temps even on hot days.

Does the pump work under normal conditions without putting 12V to it?

Did you remove the plug from the monovalve?

how do i check that pump works without direct 12v,should i hear it with engine on in defrost-or just defrost without engine on,so much noise from engine to hear it,though i could put my hand on it and feel if its on.

Well... Mine is an 85 and the aux. pump only runs if the heat is on high (control wheel in the detent) and or if the defrost button is pushed.

It is hard to tell with the engine running because the motor runs so smooth and quiet. I used a multimeter for the test.

Did you ever do the "unplug the monovalve test" as mentioned above? You should have more heat than you can stand in that test.

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AJ

1985 300D (SOLD)

i will do monovalve test in a few days as im changing glow plugs and cannot start car till i get the new plugs in the mail and in the car.

The engine need not be running. If the key is in position II, the pump will be running in almost all modes when heat is required.

aux pump is on in defrost mode.

i am getting no heat in any setting of ccu including defrost.new monovalve-tested that it works,no heat with monovalve unplugged,new aux water pump.car has 180k,had heat recently,ac works,blower is new,tried different ccu i had around with no change.cooling system is ok,what next.?

Have you felt the coolant hoses running into/out of the heater core once the engine is warmed up? Check out the hose around the monovalve assembly as well.

Have you checked the voltage AT the monovalve? This doesn't seem to be the main issue since unplugging caused no change, but it is worth checking to see if your CCU is functioning properly.

I'm assuming the car does not overheat and stays just abov 80C?

I guess you need to check your air flow next. In my SD there are crossover valves (I can't remember exactly what they are called) on the side of the middle console (footwell) on the passenger side (remove carpet panel to expose). You can check the pods here with a vacuum gauge. You may find one that does not hold vacuum and leaks down.

These systems can be weird. I've never been able to get heat on mine with anything but defrost depressed. I'm pretty certain it is either the CCU or the flaps/pods. I'm putting in a new dash with no cracks sometime in the near future (hopefully near...it is sitting in the basement) and I plan on checking/replacing all the pods at that time. I just use unplug the monovalve and run around all winter with either the system on and FULL HEAT or it off. I've just decided to live with it. I'll do some exploring while I have the dash out.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

MBZ 300D turbo,I have very little heat coming out on any setting. I hear a constant squeak, not very loud, when I turn on the fan (blower).

The real question is whether the auxiliary pump is running. Pull the plug and check for voltage at the pump if you cannot feel it running.

If the pump is running and the monovalve is unplugged, either the heat blows you out of the car or the heater core is plugged. I'm making the assumption that the cooling system is full of coolant.

An air bubble is not out of the question, particularly if the coolant has been removed and replaced recently.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13