Regarding a 1985 123, where can I get the foam hose under the dash that deteriorates over time? I think this is the reason my climate control wheel seems not to work. I either get full heat or full cold, nothing in between.

Most of us use a simple piece of thick wall plastic tubing from a hardware store. Works fine, temp control may be a little slower but no foam hose hassle.

Don't think that is your problem in total loss of control. That issue is probably within your climate control pushbutton unit.

I used a length of foam insulation for pipes that I had lying around. The split kind with the tape.

X2. Foam pipe insulation worked well when I had the OP's model year Benz.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

What are the symptoms when this hose fails?

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1981 Mercedes 300TD, 1994 Honda Civic Del Sol
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The sensor will have limited effect. The automatic (closed loop) system will be out of phase with the actual temperature. It will seem to heat too heating to long and cooling too long.

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2012 B Class

1981 300D - Now with greazzer

That sounds like my problem. Is there a howto on this? I'm not sure where to look for the hose on my '81 300TD, or what to disassemble to get to it.

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1981 Mercedes 300TD, 1994 Honda Civic Del Sol
http://mefi.us/images/fuelly/smallsig-us/67195.pnghttp://mefi.us/images/fuelly/smallsig-us/103885.png

Not sure but if you pull the glovebox out you will not miss it it. Basically the old foam connector hose has disintegrated.

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2012 B Class

1981 300D - Now with greazzer

Sounds simple enough. I was afraid I was going to have to take the whole center stack apart or something.

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1981 Mercedes 300TD, 1994 Honda Civic Del Sol
http://mefi.us/images/fuelly/smallsig-us/67195.pnghttp://mefi.us/images/fuelly/smallsig-us/103885.png