Yet another w126 climate control probelm! - PeachParts Mercedes-Benz Forum

mplafleur · #1 10-16-2006, 12:56 PM

Let's see if anyone has had this one before...

I get heat, but not all the time. I get cold air out of the defrost while in defrost mode. I'm sure the monovalve is working, as I can feel heat in the center console and knee panels. It's just not coming out the vents.

I then removed the passenger kick panels and tested the vacuum pods. They all work, no leaks. I did not verify the vacuum relays yet. I replaced the temperature control module with a spare and no change. I removed the pushbutton module and took it apart. It is in great condition. There were no signs of pitting on the relays, no solder reflows, no cold joints, etc. I could tell that it got hot, but is still operational.

Now it gets wierd. I took the car out for more tests and found that if I get down the road and while at a stop light I turn the car off and restart, I get heat! After a while, the heat goes away and if I cycle the key, I get heat again. It doe not do this if I turn the CCU off and then back on.

This testing was done with the CCU temp wheel in the locked-on heat mode. I found that once I had heat and if I then rotated the wheel to cold, it would immediately go cold, and if I immediately rotate to locked-on heat, - no heat.

Yesterday I removed the monovalve assembly with the aux coolant pump. I verified correct operation of both. In full heat, the plunger was open. In full cold, the plunger was closed. In between, the plunger was cycling 2 seconds closed, 3 second open. The pump was spinning when heat was called for, otherwise it was off. I also checked to make sure that the plunger was not getting stuck or was inhibited in any way.

I have more testing to do yet.

Any ideas?