I have checked other thread on climate control but I havent seen this problem discussed. I have no heat in my '84 300 TD until I have been driving at freeway speeds for 20-25 minutes. When I located the Mono-valve I noticed that the 1.5 inch rubber hosebeside it is broken just as it come through the fire wall. I taped it up thinking it might be related to the problem but nothing changed. Any suggestions.

where did that hose originally lead?

Interesting idea! I had to go check. Would you beleive it goes nowhere? It is only about 5 inches long and attached to nothing. The end is falttened. I could not find anywhere it should be connected but I was looking by "braile" . I couldn't see but was feeling around for something.

It has been many years, but I think that car had a sensor on the engine to keep the heater from blowing untill the engine was warm. I would explore that angle! I had a 85 300TDT. BTW the default on the mono-valve is heat, it is normally open and closed by the heater control module. Dont know about the hose, could it be a drain hose from the vent, the flattend part would indicate that

Greetings,

The flattened hose you were feeling was probably your condensate drain for the a/c. What is your engine temp running at about five minutes into your ride and also at operating temps? If your engine thermostat is either stuck open or missing altogether, your engine will take quite a long time to circulate and heat the cooling water, and it still wouldn't seem that warm when using the heater in the car.

Just a Thought,

Charles

__________________
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan

My temp guage sits on 80. What should it be?

Greetings,

The temp seems fine, although most edge upward a bit from 80. How long does it take to reach this temp? Should be able to reach it in less than a five mile drive at 40-50mph. If it's taking 20 miles to obtain this temp I'd suspect the thermostat as staying open the entire time. Have you ever replaced this item on your car yet?


Charles

__________________
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan

Back again!

Yesterday was taken up with wood hauling and leaf collection activities. I did get a chance to poke around under the hood and to look at the shop manual. It show nothing similar to a sensor switch to keep the heater shut down until the engine reaches running temp.

This morning I took it out to get a paper. After 3-4 miles at speeds of 20-45 the running temp wasabout 60 but I HAD HEAT!!
Not roasting, but warmth. Later I went out again, still got heat at the 20-45 speeds however the temp stayed 40-60. When I got up to highway speed, 60, temp went up toward 80 but heat dissappeared. He heat came back as I slowed the speed down but the running temp went back down again.

This sounds like a probable thermostat issue.

The heat at lower speeds is a new development. Yesterday while poking around under the hood, I unplugged the plug on the top of the monovalve and plugged it back in makig sure it was seated good. Would this have changed anthing?

I love this car and I enjoy tinkering with it. But I'm sure that by now it's clear that I'm no mechanic! Your assistance is appreciated.

DEH

The monovalve is open by default, and closed by an electrical signal. A diesel by its operating parameters would and should warm up quickly, needs heat to explode the mixture, that is why they have glow plugs, to ignite the inital mixture. I dont have the shop manuals on my TD any more, but the delay somehow sticks in my mind. It could have been one of my other cars, sorry if I sent you on a chase. The thermostat idea is a rather cheap approach, why not go to the mb parts counter and ask them if the switch exists on that model.
Your temp guage should not be moving around once it gets to operating range no matter what speed , unless you are stuck in traffic and it goes up.

The auxillary water pump may be bad, too -- this will restrict flow and leave you with no heater core circulation at some speeds. I'm not sure how to check it -- mainly it should be running at low speed, anyway, and you can feel it buzzing.

Note that if it is bad, it can fry the pushbutton control unit from excess current draw.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

More confusion:

A variable I neglected to mention is that this car sat for 5 years in a barn. I got it this summer and this the first time the heater has been used. Part of this may be my difficulity in learning the mechanism.

This morning I had very good heat after about 5 min. at 20-40 mph. Operating temp was 60-70. As I got up to 65 in the interstate the operating temp increased to just about 85 and the heat went off. When I left the interstate the heat came back on and the operating temp decreased to about 75. At this pint I have very good heat around town but none on the interstate.

I think I will run it for a while longer before I make any significant changes. I wondering if the system just need to be used for a while.

Aux pump isn't working, possible collapsed hose somewhere too.

Check for aux pump operation, and if the engine temp drops on the highway, replace the thermostat, it's hanging open. Very common, one of the "legs" on the thermostat breaks free and the wax pellet cannot move the valve properly.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

As of last night my heater seems to be working fine in town and on the interstate. I really have done nothing to it.

The operating temp continues to vary 5-10 degrees depending on in town or interstate, higher on the interstate.

Peter, I did find a reference int he oporator's manual to the sensor switch you mentioned.

Guys, Thanks for all the suggestions. Maybe all the "good thoughts" fixed it!

DEH