PeachParts Mercedes-Benz Forum - View Single Post - W126 Climate control theory

tinypanzer · #10 03-30-2010, 08:37 PM

Hello everyone. Since I first started this thread, I have learned a LOT about 2nd generation W126 Climate control systems.

Here we go......

First off, there are three (not two as I previously said) vacuum pods that are prone to leakage. Luckily all three are accessible when you pull out the glove box. It's no coincidence that the three suspect pods are all double element pods, while the single element pods used in the footwell and diverter flap are much less prone to fail. Invariably, it's the middle element that fails on these double element pods.

My theory is pretty much dead on correct it turns out, but there's more to the picture than just the pods.

The vacuum valve unit turns out to have bad solder joint issues due to poor design of the input pins. To make matters worse, this unit is sealed shut. I wound up investigating the unit when I got done replacing my busted vacuum pods in the recirculator flap and somehow my recirc still wasn't working. Testing my pods and the circuit with a vacuum tester showed that everything was air-tight so there must be an electrical issue either in the CCU or the switchover valve. After trying my spare CCU and getting the same results, I grabbed my spare vacuum valve (thanks pick n' pull).

My spare vacuum valve worked on the recirculator function, but the center vents were now misbehaving..... What gives?

On a hunch, I started wiggling the wire harness that goes into the vacuum switchover valve, and tada! Things started working and not working when I wiggle the plug.

After taking the thing out and playing with the pins, I realized that indeed there were bad solder joints on the other side. This thing is melted shut so what do I do? Ahhhh...... See picture one, this will save you a lot of money....

Using a jigsaw, I carefully cut away the plastic behind the plug socket revealing the solder joints! See picture two... This is AFTER I touched up the joints. Indeed two were visibly broken and the rest just looked weak. Sorry the picture sucks, it's a cheesy phone. I nicked the edge of the circuit board with my saw, but didn't do any damage luckily. If I were more careful not to let me saw get so close to the board, that could have been easily avoided.

So, after re-touching the solder joints I noticed that this valve (dated '90) was ever so slightly different than my junkyard spare (dated '89). See picture 3. Notice that output #2 (for the recirculator flap) has a brass "resistor" installed in the nipple on the '89 valve, but that little brass "resistor" is missing or was never installed on output #2 of the '90 valve. The end result of this is that on the '90 valve, the recirc flaps open and close almost instantly, and on the '89 valve they can take up to 15 seconds to fully open or shut. What a difference! Even if this is purely by accident, I recommend anyone who digs this far into their system to go ahead and yank that sucker out of the #2 port, it just makes things work so much better.

So, I put everything back in and BAM! My flaps are working like brand new. The soldering fixed the intermittent valve, and here's the really cool part. See picture #4. That's the valve after it's been installed. Notice how the solder joints are visible and accessible even with the valve installed? If they ever break again, all you have to do now is pull the cover off the side of the center console and go to town with a soldering iron! The thing can now be re-soldered in place!

I hope this information proves useful to you. Please go back and see my YouTube video if you are just tackling this problem for the first time.

Cheers!