AC help mercedes 420sel w126 - PeachParts Mercedes-Benz Forum

benzo w126 · #1 07-30-2009, 08:09 PM

i have a 1986 mercedes w126 and the ac is constantly blowing out hot air and sometimes cold but it is always blowing out a little air even when i turn the ac off,and the defroster is always blowing out air to, also the aux fan is really loud and turns on when i turn the key to position two without the car being started, please help

mbdoc · #2 07-31-2009, 08:11 AM

Sounds like 2 or more issues.

1st) most likely is the heater valve not shutting OFF. Try blocking off water line.

2nd) sounds like a vacuum loss, not closing off the fresh air door.

Removal of the glove box liner & watching the vacuum elements that move the door above the blower would be a good starting point.

appatula · #3 07-31-2009, 12:49 PM

While we are on the subject....

What vacuum elements can be accessed (R&R'ed) without pulling the dash?
What vacuum elements can be seen from the glove box cavity?

(1986 300E)

Is this heater valve also referred to as the "mono valve"??

Thanks Doc!

gmercoleza · #4 07-31-2009, 01:24 PM

You're actually way off subject - he has a W126, you have a W124. On the W126, only the recirc and defrost pods can easily be replaced without removing the dash (actually, the defrost pod is kinda tricky)...

benzo w126 · #5 07-31-2009, 01:43 PM

i have pulled the glove box liner out and turned on the ac and nothing is moving inside, i saw a video on youtube talking about the climate control vacuum problem and he said those two vacuum pods are suppose to be moving but they are not

tinypanzer · #6 07-31-2009, 05:10 PM

Umm, yeah that was me that posted that video.

What I was describing in that video is a specific problem with the center vents. It shows up as intermittently switching to defrost.

MBDOC is correct, you have more than one issue. If you get heat when you're not supposed to, you probably have a shot monovalve diaphragm, which is a pretty easy fix. Search the forum here and you will see countless examples on how to check or replace it.

What you want to look for behind the glovebox are the two vacuum pods I showed in the video. They actuate when you switch from fresh air to recirculated air, or vice versa. With the AC on, toggle the recirculation switch. If you don't see any movement, then they're probably damaged like I show in the video.

The way to test them is to remove them, connect a hose and then use either a vacuum tool or suck with your cheeks on the hose to see if the pod moves or you just suck air endlessly.

In any case, if your monovalve diaphragm is torn, you will not get reliably cold (or hot) air so you want to check that first. You can order one here from fastlane if it is indeed shot, about $45 if I recall correctly.

good luck.

Hit Man X · #7 07-31-2009, 08:51 PM

First off, with the 126 most everything is vac related... buy a MityVac so you can test stuff easily. If not it's shooting in the dark attempting to trouble shoot issues. I tried to go around this and finally broke down like my old double barrel Le Fever and bought one... smartest thing I did for my 126 fleet.

Air only from defroster is a vac leak or torn diaphragm in the pod. Just go ahead and order all three of the double throw pods on the pass side... defrost and recirculation. This will almost certainly fix your air volume direction issues. Super common and always happens, not a big deal. I can swap all three in about 20 minutes start to finish now. Chances are they're original from '86.

The single throw ones rarely go bad, you can test them (I would) but they're usually not to blame. Obviously inspect and replace any of the three way, four way, two way vac hoses, check valves, etc. Climate depends on proper vacuum at idle. The ECONOMY needle should be fully left at hot idle in P/N... if not, time to replace rubber.

Dead monovalve diaphragm is to blame for the heat constantly, it's done for safety in cold areas... in hot areas, just roll the window down.

Budget $300-400 at the most for all parts plus tools and a case of brew to get it done. Lastly, if you haven't yet... get the glass tinted to help keep heat out of the car.

tinypanzer · #8 07-31-2009, 09:18 PM

Where's that third double-throw pod located and how do I get to it?

Thanks!

Hit Man X · #9 07-31-2009, 09:36 PM

Quote:

Originally Posted by tinypanzer

Where's that third double-throw pod located and how do I get to it?

Thanks!

Defroster one, just reach up in there. I disconnect both vac lines first, then disconnect it from the flapper door, then simply twist and remove.

People make it out to be WAY harder than it is to R&R... and I have huge hands to boot. Just notate which hose is for what side of the pod and off you go.

appatula · #10 08-01-2009, 08:31 AM

Didn't notice the "W126" until now.....

I just automatically assume W124 whenever I see "vacuum pod/element" for obvious reasons...grrr

My apologies!

ehm · #11 04-05-2018, 10:09 AM

I have an issue close enough to the above in this old post to tag it on (or at least arguably): 1991 420sel and recirc switch light will not come on; I don't know enough to tell whether it's recirc-ing or not; car's all black so could really use that recirc help in summer if it's not currently working. What would be the simplest way to see what's really going on?