I have not been posted in a long time because my 1984 300SD has been running so well and any repairs I did were easy. I must say that this heater problem has really confused me.

The engine warms up normally. When the car is idling or moving below 30 mph, the heat is very good. The temperature of the heat at these speeds is about 120 degrees. When the car goes faster than 30 mph, the heat temperature rapidly falls.

The same problem also happens when standing still. When the engine speeds up, the heated air temperature drops. I must mention that the engine temperature remains constant at all conditions.

My first attempt in trying to solve the problem was to find out what happens when the monovalve is disconnected. I removed the connector from the monovalve. The blower blows much warmer air, however, it blows hot air when the climate control is in the cooling mode. This is expected since the monovalve is wide open when the wire is disconnected. I also inspected the monovalve diaphragms, and they are in good condition.

I also disconnected the auxiliary recirculating pump. As I understand, this pump increases water flow at low speeds. The result of disconnecting the recirculating pump was that the air got hotter. This was unexpected. I was expecting that a non-functioning recirculating pump would reduce the heat. Some members said that the recirculating pump was not necessary since it was needed for more frigid weather.

I also flushed the system. I used Prestone Super flush. I drove the car for 2 weeks with the flush in the radiator and then flushed the system with lots of water. There was a little oil in the system, but otherwise, the system was clean. The flushing did not help the problem.

I also checked to see if any heater hoses where collapsing at high rpms. All the heater hoses held their shape at high engine speed.

Radiator cap is holding pressure. Water level is at upper limit in the expansion tank. Antifreeze is 60/40 Peak Global, the yellow stuff.

I noticed that other members have had same type of problem, but I did see any posts saying that they found the solution.

What advice do you have?

Thanks.

I would suspect the thermostat first...don't overlook the possibility of low coolant either.

I don't see how disconnecting the aux pump could be helping you.

Tom W

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..I also have a 427 Cobra replica with an aluminum chassis.

I agree. First I would look at coolant level. At speed, you could be getting air in the heater core. Do you hear any "gurgling"? If the thermostat is stuck open, you could be pumping coolant too fast for the engine to heat it properly.

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Dean

Mine does the same thing. I put new thermostat bypassed the aux. water pump and thermovalve and it still does it. I've tried flushing and adding fluid etc. Only thing i've heard is the aux water pump is required to have enough flow to heat the heater core when you've got alot of air movement over it. Mine works @ 60 degrees on min. when it's 20-30 outside but when it's not too cold like 40-50 mine works on higher fan settings. I did post and got abunch of suggestions but not much luck on fixing it other than the aux pump but can't see spending 150$ on it

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1982 300D Turbo 231K 120hp? 28.1mpg YTD

And what is that temperature?

A bad monovalve...

They fail in two ways:
* Broken diaphragm
* Magnetized shaft

Both cause strange symptoms like you describe.

PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1984 Mercedes Benz 300D Turbo
Part: Mono Valve Repair Kit
Note: Heater Valve
Electromagnetic heater valve.
R3023-15174 Mono Valve Repair Kit Bosch IN STOCK






Have a great day.

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The diaphragm in the monovalve may be fine but the magnetic portion may have failed....as whunter stated above.

Does the engine temp stay constant? Or is the engine itself cooling off too?

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Ive got a spare pump around here somewheres, PM me if interested.

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On BMW E23s, E24s, and E28s, which use the same heater control valve, this is a classic symptom of a valve malfunction. At low speed, the coolant flow isn't too high, but once underway, the force of the flow will force the valve closed. In this case, unplugging the aux pump should actually help the situation, but not substantially.

Unplug the valve, forcing it into the default "open" state. I suspect the behavior will be the same.

The cure is the replacement of the heater control valve insert.

As stated sbove...

Had the same problem with mine and just like whunter stated, I replaced the monovalve and now plenty of heat.

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85 300CD The FryrBird MY daily driver

These days, the engine temperature hovers around 80 degrees when the engine warms up. I think this theperature is warm enough to produce hot air.

I have been thinking about this for a couple of days. So far, the members have come up with 2 two solutions: thermostat or monovalve.

Thermostat is a very easy replacement. I am wondering if the engine is cooling off as the heater air temperature decreases. I know that in the manual, if the thermostat is removed, the engine will overheat. That should produce plenty of hot air. If the thermostat is stuck shut, the warm water will be diverted to the radiator and the engine will have a hard time warming up. I do not feel that the engine has a hard time warming up.

In this problem, there is a correlation between engine RPMs and heater output. The higher the RPMs, the less heat the heater outputs. I am thinking that it has something to do with water pump pressure because as the RPMs increase, the water pump increases the water pressure. On the other hand, as the RPMs' increase, the water pump suction must increase as well.

So, that leads me to the monovalve again. Is it possible that a weak spring in the monovalve causes the monovalve to be sucked shut, thus cutting the flow of coolant? But, I wonder if the pressure from the water can force the monovalve open.

Looking at the layout of the monovalve and recirculating pump, the recirculating pump is on the output of the monovalve. I know that when the recirculating pump has been disconnected, I have more heat. I guess what is happening here is that the recirculating pump is sucking the monovalve shut. I think this symptom supports the theory of the weak monovalve.

I will go check out the monovalve again.

Monovalve and Recirculating pump

Attached Thumbnails

 

The monovalve default is OPEN. The ACC closes the monovalve (to regulate the amount of hot coolant entering the heater core) by applying pulsed 12 volts to the coil of the monovalve -- that's the two wires going to it that you see under the hood.

A monovalve can stick shut due to corrosion, accumulated crud, and deteriorating rubber parts. The fix is to install a rebuild kit ($51.59, number R3023-15174 from FastLane -- click the "Buy Parts" link at the top of the page, they support this forum and deserve our support).

A jury-rig can be fashioned by replacing the monovalve with pipe but then your choice is between hot air and more hot air.

Jeremy

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IIRC, the aux water pump should be pushing water to the monovalve, not pulling from it, unless it is pulling from the heater core discharge. I also recall the monovalve has to work against the water pressure to close.

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