SL Loses Heat?

[Paoli380]

Hi everyone-

I have a 1984 380sl and after about 15 minutes of driving in cold weather, I lose heat. The blower works fine, as do the buttons. I only seem to lose heat when the blower is on either high or automatic, but not when on low. I wanted to be sure I was fixing the right problem before I replaced the monovalve insert. Just a bit confused since I am getting heat on the low blower setting. Thanks for any help!

[TylerH860]

It often is the monovalve insert when cars have cabin temp problems, but this one seems funky. Will it reset every time you restart the engine? Does it change when you unplug the monovalve and plug it back in?

Its cheap enough to replace, and you can troubleshoot from there if needed.

[rocky raccoon]

Before you buy a monovalve insert (cheap but still about $45) try this.

If you get heat at all it is probably NOT the monovalve. Since you get initial heat but it will not regulate, check your cabin temperature sensor. The usual failure mode is the "foam" tube that feeds into it under the dashboard. The material deteriorates and does not conduct ambient cabin temperature to the controller. I had a similar problem and replaced that foam tube with a simple piece of clear plastic tubing and it has worked fine through two Winters since.

Remove the passenger side underdash panel and look up there. You will see it and access is good to repair. Let us know how you make out.

[Paoli380]

Originally Posted by TylerH860

It often is the monovalve insert when cars have cabin temp problems, but this one seems funky. Will it reset every time you restart the engine? Does it change when you unplug the monovalve and plug it back in?

Its cheap enough to replace, and you can troubleshoot from there if needed.

Thanks for your response. Yes, if I restart the engine I will get heat for about a minute or so. I will check the foam tube first, but could it be a vacuum problem due to the heat coming back after restart? Thanks again everyone!

[Paoli380]

Originally Posted by rocky raccoon

Before you buy a monovalve insert (cheap but still about $45) try this.

If you get heat at all it is probably NOT the monovalve. Since you get initial heat but it will not regulate, check your cabin temperature sensor. The usual failure mode is the "foam" tube that feeds into it under the dashboard. The material deteriorates and does not conduct ambient cabin temperature to the controller. I had a similar problem and replaced that foam tube with a simple piece of clear plastic tubing and it has worked fine through two Winters since.

Remove the passenger side underdash panel and look up there. You will see it and access is good to repair. Let us know how you make out.

Thanks for your tip. I took dash apart as you said and the foam tube was no longer connected. Very brittle and old. In the process I accedently broke the interior temp sensor and cannot find it on any of the websites so far. Does anyone know where to look for a 380sl interior (cabin) temp sensor? I saw others posted the same question for different models but I had no luck with those links. Thanks very much in advance.

[rocky raccoon]

My usual on-line part sources did not offer a sensor. It may be worth a phone call to FASTLANE Phil on this site. They have parts available that do not show in the on-line catalog.

Alternatives are:
(1)Junkyard - as you probably know by now, you can't just pull the sensor out of the dash. There is some sort of clip retainer. This part is common among many models and years.

(2) Benz dealer - probably a long wait and higher cost but I can't imagine that part could cost too much.

[ctaylor738]

There shouldn't be a long wait on the sensor. I wouldn't expect the dealer to have one in stock, but there should be one at the regional depot, or at another dealer, which means 1-2 days. When I replaced the one in my 560, it was painful, but under $100.