I presently own a 1983 300td. Love the car. Now that it is getting colder here in the east, I'm back to using the heat in the mornings. The car warms up rather quickly and starts to blow nice warm/hot air. I can drive all around town under 50 mph and still get the heat. When I get onto the highway and go over 50mph the heat stops. Cold air blows. When I get back inot town and drive below 50, the heat is back... Go figure?

any help would be apreciated.

thanks,
Andrew

check the monovalve.

Nic

Check your radiator cap to make sure it isn't maintaining the system in a vacuum. I've seen radiator hoses sucked flat. Also check for proper coolant level.

Otherwise, you have a circulation problem. The monovalve isn't working correctly, the circulation motor is bad, or you have a partial plug in the heater core, preventing circulation when the circ motor isn't running.

And, there is always the pushbutton control unit!

Peter

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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!

Could the thermostat be stuck open?

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold