Hi,my 81 300TD has lots of heat. The fan blows hot air all the time and there is no monulation in the temperature. The only way to make it cooler is to turn it off. Does the infamous "monovalve" also govern how hot the air gets?
Also, I remember reading about a piece of foam tubing that exists in the cabin temerature sensor line.. about 2 feet from the sensor..behind the glove box. My car has none. Is that important in keeping the temperature moderate?
great service for all us wantabee mechanics

That's exactly what the mono valve does. It is on the firewall next to the battery, and the repair kit costs about $30. You can install the repair kit in five minutes. If you take off the top and it is flooded with coolant, the diaphragm is ruptured and it needs repairing.

Good luck,

The tubing behind the glove box was on my (late, great) 85 300SD. It, and the small blower it was attached to, pull cabin air across the temperature sensor in the headliner. It has less to do with actual temperature than how quickly the automatic controls react. Before I replaced this small blower I would roast until the headliner and sensor slowly warmed up from contact with the hot air blowing up the windshield. Then things would settle down.

From the browsing I did in the service CD I think the tubing/fan was a later addition; earlier W126 didn't show it and when I went to the MB dealer the parts man could not locate the small fan on the computer. He tried some other year/model, found something close, brought out the part and voila! it was the correct part.

Somewhat off topic, my Suburban isn't making enough heat. Head gasket leak is putting air bubbles in system, and air just doesn't heat as well as coolant.

It's rebuilt engine time...

I wish others had a shop forum as good as this one, and I'll try to make my Brand A and Brand G postings related.

All I want for Christmas is no more automobile challenges.

BCingU, Jim

I just replaced the foam tubing in my '82 300D yesterday. If you take out the glove box liner (by removing the plastic "plugs"), the foam tube will be visible just to the top left of the glovebox hole. It looks like a piece of foam around the tubing, but is in reality the connection between the two tubes - the one on the left goes to the "thru the dash" ambient sensor, and the tube on the right goes down to the heater motor suction side. The foam will deteriorate and split, causing little or no air flow through the ambient sensor, which makes the heat control either nonexistant or very sluggish. When I touched the foam on my car, it broke into several pieces. It only takes a few minutes to replace the foam - good luck!