PeachParts Mercedes-Benz Forum - View Single Post

JimSmith · #12 05-08-2007, 06:56 PM

Several of us have gone through this exact scenario. Here is an old thread, with some link to other sites with ways to check.

http://www.peachparts.com/shopforum/showthread.php?t=125161

In my case the duovalve was the problem and you can tell by running the diagnostics on this site:

http://www.continentalimports.com/ser_ic40142.html

My results were posted here:

http://www.peachparts.com/shopforum/showthread.php?t=108173

The net result is when the heater core temperature is at the engine coolant temperature and you have the A/C on, there is a problem that is entirely independent of the freon level or the A/C circuit efficacy. In this case the heater can overpower the A/C (which is as it should be since the outside air can be mighty cold and by the time it gets to you inside has to be warm). In my case the driver's side heater core temp was hot as hell and the passenger side, where the A/C was working, was at "ambient" or so.

With that set of symptoms the problem cannot be fixed with the freon being recharged. It is a bum valve letting engine coolant circulate through the heater core when the control system says "close" or "throttle" and the valve actuator opens the valve all the way.

There is a most excellent post on how to fix the duovalve, which I wish I had stumbled across before I paid to have mine fixe, by Parrot of Doom, but it is on another MB website:

http://forums.mercedesclub.org.uk/showthread.php?t=25909

You will have to register there too to read the instructions and view the excellent step by step photos that accompany the step by step process. It looks like a reasonable job to carried out by someone with reasonable skills. I will sure try it the next time my valve fails.

Hope this helps if the freon thing turns out not to be the answer. It wasn't for me.

Jim