I've had a problem for awhile now in that in the cold weather I have to keep my knob all the way off for no heat to come out if I want some heat I just have to bump it a very little and a lot of times get too much heat out of it and roasts you out.I cannot keep the climate knob in the 18-22 range and expect a comfortable temp.Do you think my problem lies in the climate control knob in that it has some bad solder joints has been resolderd already ounce by my mechanic or I have read that there is a form tube that could cause this problem but I do not know if that pertains to my model.I think I have read that there is a cabin air sensor by my dome light that sometimes gets clogged with dirt+ dust could that just need some good cleaning? Do I have that foam tube on my car near the glove box that could need replaceing?I am in the process of replaceing my condenser on A.C. and hope to get these climate temp. issues resolved so what # I put the dial it will keep that temp.

Check your monovalve diaphragm. If there is a tear then you will get all or none heat. (that's a blanket statement ymmv)

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg

If you get heat on a hot day as soon as you move the temp selector from the "detent" cold position, the problem is not likely the the temp sensor. Could be the temp selector, the temp control regulator, or any on the circuits in between. That would include the solder joints.


There have been numerous reports on this forum that suggest a different outcome. A very common condition relating to a torn diaphragm is normal heat function a low speed/rpm and no (or reduced) heat at higher speed/rpm. That happens because the higher dynamic pressure of the coolant allows it to pass through the diaphragm (seal) and counteract (overcome) the spring force that causes the valve to open.

If you have a 300SD (w126 body) then there is no foam tube arrangement. The temperature sensor is an electronic component with a simple 1980's vintage thermistor. It's located behind a small grille in the roof, just beside the rear view mirror. it's about a half inch square grille and it simply pops out of the headliner.

The FSM has instructions for testing it and measuring its resistance to make sure it's OK.

I had the same symptoms as you and suspected both the CCU and the sensor. Despite the fact that I had replaced the monovalve two years previously, it was again the monovalve. I've had two problems with mine since replacement

1. "gunk" in the cooling system causing the new valve insert to jam from time to time. A cooling system flush and fill took care of that.
2. Loose electrical connection on the little black two prong plug that connects the control signal from the CCU to the monovalve. There is a posting on this forum on this very issue. You split the plug, crimp the connectors a little tighter and off you go.

Both of these problems were intermittent and resulted in excess heat. Mch swearing ensued before I tracked them down .......

There are many postings on diagnosing what is wrong with the climate system on these cars if you use the search feature. It's worth absorbing them - probably a couple of evenings of reading - and actually figuring out what is wrong. The FSM - available on CD - has a decent description of the system and where the parts are located. Otherwise you run the risk of spending time and money to replace parts that are actually fine!

The fact that the monovalve responds correctly and reliably (ie: remains closed) when the temp selector is in the detent cold position would tend to exclude that particular problem. That fact also makes it unlikely that the monovalve itself is at fault mechanically.

climate control is still acting strangely in that other day when I had control wheel in detent position air on when I hit the defrost button heat came on thinking of getting a replacement temp control unit or checking the monovalve + connections even though control unit has been resoldered and monovalve replaced within last 2 yrs. 85 300 SD

Not sure but I think the heat comes on automatically when you hit the defrost button. Sounds like it is working properly.

That's correct. The Defrost mode opens the ground circuit to the monovalve, regardless ot the position of the temp selector.

Tangofox I thought on all cars when you hit the defrost A.C. comes on also?I remember last winter sometimes when I needed the defrost it made my windshield even foggier then it was before I hit it.Unless I did'nt have the outside air control the correct position.

Not on all cars. But on your car, the Defrost mode should open the monovalve, engage the a/c compressor, run the fan on high and fully open the windshield vent flaps. In fact, the a/c compressor should be engaged at all times unless operating in the Economy or Off modes.