I have a 1984 300 d turbo diesel and I cannot get my heater to shut off. So, my question is what could that possibly be? Could it be a fuse, or something else? I appreciate the help. I have encountered the same problem in the past but after a couple of days it will shut off when pressing the circle button if you know what I mean. It has always has worked in the past except last winter and a couple of days ago.

You probably have a bad monovalve. It controls the flow of hot coolant through the heater core. Its default position is "open" so that you always have heat to clear a fogged windshield. To turn off the heat, the CCU applies a pulsed DC voltage to the coil in the monovalve; as the width of the pulse is increased, the valve shuts more and more. Eventually it is all the way closed and you get no heat.

The most common failure is a torn rubber diaphragm in the monovalve. A torn diaphragm will allow hot coolant through all the time. I had this problem in my '85. This used to be fixable with a kit of parts but I have heard that it is NLA. Perhaps I am wrong. Replacing the whole monovalve is much more expensive.

It is also possible that the monovalve is not getting the signal from the CCU due to a bad connector, wiring, or a failure in the CCU itself. Check the 2-pin electrical plug at the monovalve for good connections.

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

It's not a fuse. If the fuse was bad it wouldn't work at all. It's a pretty simple system. (Switch, wires and fan). My guess would be the switch is shorted closed.

As counterintuitive as it might seem, a torn diaphragm often has just the opposite effect, especially at higher engine rpms. In my experience, the most common complaint associated with a torn diaphragm is "heat works well at slow speeds but blows cold at highway speed."

Haven't spent much time studying the ACC system, eh?

What, exactly, doesn't shut off? The heat and the fan or just the heat?

yeah more details the better

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Another failure point causing heat: if the temp sensor in the dash center (123) or overhead (124) does not work (unplugged, broken wires, failed sensor), you will get full heat unless you turn the temp dial all the way to the blue (cold) click-stop.

__________________

"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