Question regarding my 1981 W123 300D heater. What are the symptoms for a bad cabin temp sensor/middle of the dash sensor? Is there a separate fan that draws air over the sensor or does the main fan do that?

My heater works flawlessly for about 10-15 mins. Heat is controllable at high/low mph. Fan starts strong then lowers speed as expected. But after 10-15mins it loses heat and wanders aimlessly mostly on the cool side. Even then it goes hot when defrost is selected or max heat is selected? AC not working, all this is in the EC mode.
Thanks ya'll
Pat

First thing to check is that the tube from the dash top down to the sensor is still there. They often disintegrate over time.

The dealer will sell you one. Most people use pipe insulation instead, roughly the same material.

The system is designed to give you full heat on defrost, whether or not the rest of the system is working as a climate control. It's a sort of fail-safe.

I believe there is a small fan of some sort that pulls the air down over the sensor, but my memory is fuzzy on that one.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Hello Presse47,

What Zacharias said is more than likely the problem. I have a 1981 W126 but it is more than likely the same. On my car I had exactly the same problem and the cause was the disintegrating foam hose. On the W126 it is about 15cm (6") long and connects the fitting on the base of the temperature sensor housing to the Neoprene hose that connects to the blower motor. When the blower runs it pulls a small amount of cabin air through the small grille on the dash and past the temperature sensor. It then signals the control module to manage the Mono Valve.

On my car I could just get at the hose by removing the instrument panel. You will just be able to see the base of the temperature sensor housing. As soon as you touch the hose it will crumble in your hands. As Zacharias pointed out a piece of water pipe insulation 15mm (5/8") will fit quite nicely. As mentioned, 15cm (6") should do the trick. Don't forget to glue or tape the joint if it is the split type of insulation.

Hope it helps,

Since this is an 81, I would assume it has the evil servo system?

What does your CC push buttons look like?

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Replaced the hose several months ago. Pretty sure the mono valve is working OK. This afternoon checked temp sensor in the heater box/heat exchange. If I operated the multimeter right it seems ok also. Just need to check the cabin sensor in middle of the dash. Have also inspected the CCU solder joints. Looked OK. Cabin Temp Sensor is about the last thing to check. How to you disconnect it to remove?

Pat

Access to the dash temperature sensor is also best through the glove box, but it is really a tight fit to reach the sensor.

There is a plastic retaining ring that holds the sensor in place. Be careful not to lose it or let it drop as you remove the sensor.

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2012 B Class

1981 300D - Now with greazzer

One more thing. How do you get that plastic ring off to remove it? And the connector for that matter. If it's like the sensor in the heater box doubt if I can get if with one hand.

Pat

Its been a while sunce i worked on a 123, but isnt there a small fan which draws air through that tube?

I can't even help because I don't even know what type of climate control this person has.....

I mean people before we start giving advice, shouldn't we first know what CC this guy has in his car????

I know total hair brain idea.....

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I have been suspecting the temp sensor on my car for a while.The inlet for the sensor is in the center of the sun visors, Does any one know how that is set up? Is the sensor up there or is there a hose running all the way around to the CCU or what?

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1983 300SD
200000miles