w126; 300SDL AC vent controls
 

So driving home from work today......my Air Conditioner Vent "controls" are acting quite strange (weird).

Unit is off (far right button). Push in the 3rd button, AC air is coming out of the defrost vent ....then within 4-6 seconds, flap changes, and AC air is coming out the Dash Vents. YEAH!!!

But wait.

Then in another 16-28 seconds, the flap moves and the air is not coming out the Defrost Vents. :mad: So, what did I do? I pushed the far right button. Unit off. If the unit stays off for at least 5 seconds, then pushing the 3rd button, repeats the cycle --- AC air starts coming out of the Dash Vents within 4-6 seconds after unit is on.

OH. And another oddity, if I leave the AC air running coming out of the Defrost Vents.....the air gets HOT.:mad:

Any ideas what the issue is? I have spare vacuum pods and my dash has major cracks, so removing the dash does not scare me. I need to fix this before I wear out my right index finger!!!:D


EDIT --- I took out the HELP!!! part of the original post.

anyone experienced this?

Yes, I experienced this problem when my monovalve went bad. It had a hole in the diaphragm. My problem was heat for 30 seconds then only cold air out of the defrost vents. Turning the car off and then on again started the cycle all over again.
This is what I learned. By the way my car is a 86 300sdl. In 1986 MB redesigned the Push Button climate control unit to have a circuit breaker in it. You see in the systems prior to this when a monovalve or the aux water pump went bad and drew too much current, it would burn the circuit board in the Push Button unit. So in 86 they have a ciircuit breaker that will trip and then the unit goes into default mode, Defroster vents only.
I went to the Programa website and downloaded the troubleshooting pages for my specific unit and then using a digital VOM, I tested the various sensors that control the PB unit. I found my monovalve was not in spec. I opened it and found a torn diaphragm. I replaced the monovalve and all is well.
By the way I tested these sensors using the plugs in the wiring harness to the PB unit. That involved removing the ashtray, radio, wood panel and the PB unit to disconnect the wiring.
Good Luck. I hope it is something simple.

John

It sounds very much like an intermittent connection in the pushbutton controller. If I recall correctly, you have the controller with the circuit boards arranged in an "H" pattern. Over time the solder connections develop cracks where the boards meet and causes very intermittent problems. Resoldering usually cures this.

However, note that the controller changed, I believe in '87 but it might be '86. A search with ACC and your year model might help determine which one you have.

My 81 wagon did that sort of thing ...only slightly took my pushing the buttons as suggestions ...never as commands.... it would wait random times to execute..which might be on the next trip... I could not find an exorcist...so I sold it...

Hmmm......
 

Okay, thanks, John. This may not be too bad after all since my ashtray is already out as is the radio --- it's getting replaced with a Becker. Give me a radio with buttons I can see and my fingers can fit.

I picked up some extra vacuum pods out of a 1989 or 90 560SEL --- it was about the only thing left of the dash area.:mad: And, I'm not afraid to solder; however, I usually always burn myself at least twice. Of course, now, I don't own an electronics soldering iron.....

What are you using ...? A two pound sharpened copper ingot on a 12 inch handle.. and heated in a gas forge ? ... Like a tinsmith would use ? You must have lightning reflexes to get it on and off fast enough not to burn through the whole board.:D

Back when I worked at Texas Instruments in Houston....I used one of the electronic circuit board (with variable heat control). Nice equipment. Still, I could lose my gripe and then GRAB the thing to keep it from falling onto the circuit board.:D


OKAY....I got the ACC out of the dash......do the buttons have to come OFF before the circuit board will pull out of the ACC case? I need to be taking some pics.

OH --- looks like just one circuit board. The bottom side is exposed when I took off the cover --- there are 2 areas that look brown (overheated). I wonder what is on the other side. Do I need to look at the other side or just look for cracks in the solder joints on the bottom?

Thanks a million!!!!

Once you've confirmed that the CCU has issues, my advice is to get a rebuilt unit. I've gone the route of screwing around with solder and replacement units off ebay. You'll spend countless hours struggling with them...........and then you'll finally just buy a rebuilt and be done with it for the next five years.

Or, you can just save the time upfront.;)

Once you said some spots looked brown.... listen to Brian...
Have you seen the FSM manual for that rats nest of vacuum and electrical interactivity ?
Just for what you are dealing with at the dash controls...not shared stuff like the evap or blower... just the dash stuff.... I quit counting at 125 pages... and like the other manuals it was translated by a non English as first language person...
Be sure all your vacuum system is in good shape, buy a rebuilt CCU from someone trusted.. THEN attempt to get it all working together...

only one....
 

I only see one circuit board inside my ACC. Well.....there is a small one to the outside edge of the fan control buttons.

Sounds like you have a newer style of pushbutton controller. And with your comments, it seems there may be failed components (2 brown areas). Another option might be contacting these folks: http://www.gdl-online.com/begin1.html#contents

already resolved.....
 

I've put the thing back in my dash......searching for a reconditioned one. I looked at the hassle of taking off the buttons and that was enough for me.

On toward the next issue. These cars are full of adventures. Thanks everyone for the help.

The one in our 300D was acting odd....so I pulled it out, took it completely apart, soldered anything that looked suspicious, and re-assembled it. Put it back in, and it has worked great ever since. (4 months ago) Disassembly/soldering/reassembly took maybe 40 mins....

This adventure continues....
 

So, I found a spare ACC and installed it about a week ago and all the AC switching to HOT air out of the defrost vents cured itself.:rolleyes: Yeah, right!!! Maybe cured itself for one week.

Yesterday on the way home, of course its the hottest day so far this year....same thing happens. So, last night, the original board that was removed got soldered --- took about an hour; good, strong BRIGHT light helps. Darn eyes.

Tonight, try to go to dinner in the big car; started out fine for the first 1500 feet; then hot air. Never could get the AC to come back on --- this is with the backup ACC; not the one I resoldered last night.

I'm leaning toward investigating the mono valve --- maybe just a wiring connection? The weird thing about the backup ACC unit is when this HOT AIR Defrost event occurs, pushing the 0 button does not immediately turn off the blower. Is that weird or what? Of course, the AUTO blower setting doesn't work at all even when the AC is working.

Excuse me, (mam), but can you back up to about 5 feet away, so I can really see you?:D