300sd Heater stops at 45MPH

"My 81 300sd has good heat around town but loses it on the road above

45 mph. I have noticed that there is little heat at 2800 rpm but as I slow down at precisely 1500 the heat will suddenly burst out of the vents as if some one had opened a water valve. The heat will keep coming until you get above about 2400 rpm and then it just drops to a very little amount of heat. The engine temp is 80c and I have pulled the wires off of the mono valve so it will stay in the open position and I still get the same results so I figure that it is not a fault of any thing electronic. I think the water just stops flowing at high speeds. Any one got any ideas? Thanks, Bruce Buchanan "

The description of the heat bursting out has me confused. Does the volume of air remain constant? Is it coming out of the correct vents, ie footwells for heat, and does that remain constant?

If it's just the heat that disappears, with the monovalve out of the picture, there isn;t much left. I assume you've checked the coolant level and made sure it's full? Usually low coolant behaves the opposite, with heat loss at idle occuring first. The only thing I can think of is that there is some kind obstruction in the heater core loop that only gets blocked under higher volume flow, which would be pretty strange. I'd try flushing the cooling system using one of the adapters that allow you to connect a garden hose into the heating hose path.

The car is in very good shape and has had the coolant changed on regular intervals with Dexcool so I am not interested in dumping known good antifreeze. When I said bursting out I was trying to stimulate the picture of the fact that the heat suddenly not slowly comes back on. There is nothing wrong with the air flow or flaps. It is obvious to me that the core is suddenly cooling off and heating back up and it is connected to the speed of the car or the engine. It is hard to do diagnostics at 60 miles an hour LOL. The heat goes away at 45 and returns at 30 it is predictable as the transmission shifting. I was wondering if there is some way the mono valve can screw up with higher water pressure and start flowing with less. I doubt that the core could intermittently block off because of the way it is built it would take many obstructions to completely stop the flow. When there is heat it cooks you the heat comes out real good. Thanks, Bruce

Interesting problem. A heater hose that kinks under higher coolant flows is a possibility.

Good thought! I like this theory.

Group,

Thanks to all of you for the thoughts and info. That was returned to my recent posting. I am embarrassed to report that I gave misleading info in my posting about recent heating trouble with my 300sd. I must explain that I only work on my heat when it is cold, I am senile, and it has been a year since I last had worked on this heating problem. Last week I posted that I had tried removing the wires to my monovalve and that I was sure that my problem was not electrically related. Now today I am quite positive that it is electrically related.

I just returned from a trip to North Kansas and it is 30 degrees cold there. I live in South Texas and it is hot here. So I only have heating trouble when I take my annual northern trip. The last three years I have fought this damn heater on this trip. The heater works fine and will burn you out of the cabin if you don't turn it down. Until you get over about 45 mph then all heat is gone from the air (the core is cold). The climate control module has been changed and I thought the problem was fixed.

Yesterday as I was coming home and still in the cold north my heat quit as usual so I started to play with things and when I pulled the wires on the monovalve I got heat galore. Had to use the sun roof to regulate the heat. I stopped at a Radio Shack, got wire and a cheap volt meter, attached the wire to the reconnected monovalve ran it into the cabin and hooked it to the volt meter. As I drove the car in 40 degree weather, at 30 mph, with the control set at max the meter showed no voltage and we had loads of heat. As I approached 45 or so the voltage goes to 13 and the heat stops. Yes and when I slow down to about 30 I get no volts again and lots of heat. Well with this revelation I settled down to finishing my 800 mile drive home and had bountiful loads of heat with the monovale disconnected. Today I am at home and can now focus on the car. I hooked my voltage meter back up so I could play around with this problem some more, took the care for a drive and much to my frustration the darn car blows non_ stop heat hear in South Texas. When I move the wheel up and down the voltage comes and goes and the valve pulses like it should. The wheel turned to max and the voltage stays away full time with full heat. I am dumbfounded.

Bruce Buchanan

81 300sd 165K

Perhaps the cold makes some metal contract enough to open a connection?

Progress at least is always nice :~)[\