I recently fixed my climate control system on my 82 300d and got all the flaps working except for the center vents (due to a vacuum pod issue that will be addressed later) and at first run, everything was fine. (Thanks to all that helped me with that) However, once the cabin gets up to temperature, the heat slowly dwindles away, and then the vents start blowing cold air until it is cold in the cabin, then the heat comes back on. I've heard that there is an outside temperature sensor on some of these cars, and with my newly working vents/flaps, I'm pretty sure that this is to blame. Does anyone know where it is/how I can test it/bypass it? The automatic heat worked great before I got the vents to blow anything but defrost, and now I'm chasing my tail. Anybody out there got the same problem? Guess I should have left well enough alone once I had the heat back on. Vents be damned.

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1982 300d -Adolf- Got it for 500 bucks from a guy that thought he knew how to make biodiesel. Even the best diesels won't run on burnt french fry ends! 318,000 and counting
1982 300cd -Erwin- 107,000 miles! The most fun you can have in a diesel. Wrecked by an idiot, massive insurance payout for me. Tough to see you go, my friend.
1983 300d -Katrina- Another five hundred bucks. Straight outta New Orleans, by way of Maryland. Work in progress. 291,000 and counting

I am not not certain but does your car have the automatic climate control ?
I found a thread here that shows how to clean the inside temp sensor mounted
on the headliner near the Rrview mirror.
Just a thought.

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92 500E Silver 66k
82 Porsche 911SC 84K
68 Cadillac ( Gone Now )
03 Suburban Z71 200K
85 300SD 217k From Original Owner ( Dad )

Pull out the glove box and see if the aspirator hose has deteriorated. I think that is the correct name for it. Regardless when my hose had disintegrated due to age, I had the same exact issue.

The hose is located in the upper left corner of the glove box hole at the underside of the dash.

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-Typos courtesy of my mobile phone.

HAH! I love this forum! There are two hoses under there that SHOULD be connected. Put them back together with some pipe insulation I had laying around the house. Will give it a try in the morning and let you know how it goes. Thanks man

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1982 300d -Adolf- Got it for 500 bucks from a guy that thought he knew how to make biodiesel. Even the best diesels won't run on burnt french fry ends! 318,000 and counting
1982 300cd -Erwin- 107,000 miles! The most fun you can have in a diesel. Wrecked by an idiot, massive insurance payout for me. Tough to see you go, my friend.
1983 300d -Katrina- Another five hundred bucks. Straight outta New Orleans, by way of Maryland. Work in progress. 291,000 and counting

The sensor is under the little grill just behind the center vents on top of the dash. Make sure it's plugged in while your in there. I suppose the sensor could be bad too. If so I'm pretty sure I have an extra one that I won't need. Let me know.

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So far, so good. Thank you both for you prompt and informative replies. Heat's working perfect so far. I'll start a new thread if something else comes up. I know it will. Joining this forum was the best decision I ever made, other than buy my car for 500 bucks. Thanks again.

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1982 300d -Adolf- Got it for 500 bucks from a guy that thought he knew how to make biodiesel. Even the best diesels won't run on burnt french fry ends! 318,000 and counting
1982 300cd -Erwin- 107,000 miles! The most fun you can have in a diesel. Wrecked by an idiot, massive insurance payout for me. Tough to see you go, my friend.
1983 300d -Katrina- Another five hundred bucks. Straight outta New Orleans, by way of Maryland. Work in progress. 291,000 and counting