Loss Of Heat at Highway Speeds
 

I have a 85 300D. Seems as though when the heater is on, the slower I drive the hotter the air gets. As I reach highway speeds, the air cools dramtically. :(

What is the engine temperature doing?

I think the problem is in your monovalve. The monovalve sits towards the back of the engine on the passanger side and it has a black plastic top which is an electrical connector. Heater hoses run through this thing also, so it is easy to find. I think if you pull that top off, clean up the electrical connector it might well solve your problem. I had to do that about once a year on my 85 300D.

Nothing...............

Monovalve
 

You may also find it is damaged/leaking internally. There is a rebuild kit available.

If the temperature is normal, I agree that you should look at the monovalve.

x3 on the monovalve. it's very easy to install the repair kit and you have the classic symptom of decent heat at low speed and no or low heat once the road speed gets higher.

thats interesting...usually the monovalve breaks open. You know how when you stop holding something it tends to fall. The monovalve holds back the heat so when the diaphram rips it lets heat in. I suppose its possible to break closed but that would be a bit on the rarer side.

http://www.benzworld.org/forums/atta...-valve-001.jpg

Pull that puppy and check the rubber diaphram. Then give it 12 volts and see if it moves.

For whatever reason, they do have a failure mode where they work at low speed and not at high speed. Ask Brian, he's been through this.

Once a monovalve is replaced, how long do they last usually?
Anyone have a newer one break yet?

jeepers, if mine was that easy to access i would pull it out and put in a brass ball valve
summer off and winter on! win win

I replaced mine once, years ago; they should last for a while.

monovalves can fail open, closed, stuck in the middle... lots of failure modes.
another possible issue is the aux pump could be seized. if it's seized, the heat would work low, but not high speeds. if it's the pump, it can burn out your push button unit, so find out for sure.

That entire analysis is quite flawed. Hands down, the most common affect of a torn diaphragm is normal heat at low rpm and poor heat at high rpm. The diaphragm doesn't block the normal flow of coolant; it seals the solenoid side of the valve. A torn diaphragm allows pressure to build on what should be the dry side of the diaphragm. The pressure overcomes the force of the solenoid and causes the valve to close, resulting in poor heat when coolant flow is high, as in during high engine rpm conditions.

Actually, it's the other way around.

I think I'm on my 6th monovalve so that's an average of about 4-5 years each. Some lasted longer than others.