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Old 12-14-2000, 12:03 AM
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My 1982 300d turbo looses heat when the blower is turned on high. When the blower is on low I (my wife) gets hot air. This occurs at all speeds. The engine thermostat is operating, the engine is at proper temperature, the blower is working, the pushbutton unit is also working correctly, (recently replaced) and the auxillary coolant pump is working(also new). Could my problem lie in that valve on the firewall? Thanks Kevin

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Old 12-14-2000, 01:26 AM
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What's the ambient temp in your area?
I've noticed on my 81 300SD than when it gets below about - 15 Celsuis that the Engine simply can't keep the cabin toasty warm. These things are too effecient.
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Old 12-14-2000, 07:12 AM
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The problem could definitely be the monovalve. Remove the four screws on top of it and see if the diaphragm is ruptured. You can get a repair kit at your dealer for about $35.

Good luck,
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Old 12-14-2000, 10:41 AM
Alan S's Avatar
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Join Date: Dec 2000
Location: Cary, IL
Posts: 94
I have had the same issues with my 380se, it just doesn't get toasty. I replaced the mono-valve last week, not much difference. I do live in Minnesota where it has been in the negative digits all week. Maybe that's just the way these cars are like?
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Old 12-14-2000, 04:20 PM
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not enuff heat

temps have been in the 20 to 50 degree farenheight(sp???) range. not really that cold compared to you cheeseheads out there, or our bretheren in the great white north eh.the common denomonator is the WIFE WITH COLD FEET!!!! I'll look at the monovalve now to see if it is working properly. Larry Bible--what am I looking for in a non-functioning monovalve, please. Thank you, Kevin.
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Old 12-14-2000, 04:39 PM
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Join Date: Feb 2000
Location: Germantown, MD
Posts: 106
Dosen't the monovalve prevent the not coolant from entering the heater core and thus prevent the heater from heating...if it were the monovalve then too much heat should be the problem...not too little?
My car has a similar problem and I suspect that it is due to cold air entering from somewhere it shouldn't not the monovalve. I fixed the monovalve in the summmer due to constant heat from the vents.

Your help is greatly appreciated.

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Old 12-15-2000, 04:16 PM
Join Date: Aug 2000
Location: Holland, MI
Posts: 1,316
There are a few possibilities for low/no heat in the cabin.

Lack of heat rejection from the engine; it's too efficient , not working hard enough to get hot compared to outside temperature, or the thermostat sticks open so that the block never gets hot. If you read good temperature (80-90C) on the gauge it's not any of these.

Lack of circulation to the in-car heater core; plugged core, pinched hose, monovalve stuck closed or not cycling often enough, or a water pump not circulating water fast enough. Since this occurs at all speeds, let's say your water pump is good.

If it's REALLY REALLY cold outside, the air on the HI setting will be moving across the heater core faster than the water can put heat into it. An example would be idling in -20F; not much heat is being put into the block, and a lot of very cold outside air has to be warmed up to 70F, which is like a 100F change!

If this happens a warmer outside temperatures, like above 0F, it still means that not enough heat is getting to the heater core, but this is NOT expectet.

So, from your post it sounds like restricted circulation through the heater core.

Check the coolant level first because an air bubble in the radiator won't let coolant circulate properly, won't give you much heat, and adding coolant is cheap!

Then, check the monovalve operation. A stuck/bad monvalve (probably), plugged heater core (rare) or plugged hoses (really rare!) can all result in not enough hot water circulation. Another possiblility is an automatic temperature control that doesn't signal the monovalve properly.

Good luck. I'll leave you with this happy thought. The Brits, when asked about their sports cars (MG, Triumph) notorious lack of heating, say there's no such thing as bad weather, just bad clothing! Your MB should do a lot better!

BCingU, Jim
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Old 12-15-2000, 09:35 PM
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My '79 300 SD W116 has a "heating water pump" right next to the climate control servo (which I believe is what is referred to as the monovalve by everyone here). This pump is responsible for pumping hot coolant through the heating system. If you unhook the wires from it and hook the pump up to the battery directly, you should hear it. It is possible it is not working properly. My W123 chassis manual shows one, I would start there.
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Old 12-19-2000, 08:29 PM
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Not Enuff Heat

This was a "rapid onset" as the doctors say. The ambient temps are in the 20 to 50 degree F. range the car is at the 80 to 90 degree C. range the heater was working fine water pump is pumping, the little elecrical circulating motor is working and the pushbutton unit is working and it won't take any more coolant into the system. Soooo I'm starting to lean toward the monovalve. Gonna try to get to it before the fat guy comes down the chimney. ( you know--that whole coal risk thing)Thank you for the advice.


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