w126; 300SDL AC vent controls --- HELP!!! - PeachParts Mercedes-Benz Forum

daw_two · #1 04-10-2008, 07:18 PM

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.

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.

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!!!

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

daw_two · #2 04-11-2008, 09:14 PM

anyone experienced this?

brandlj · #3 04-11-2008, 09:42 PM

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

daw_two · #4 04-12-2008, 02:14 AM

Quote:

Originally Posted by brandlj

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

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.

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.....

leathermang · #5 04-12-2008, 09:28 AM

Quote:

Originally Posted by daw_two

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.

daw_two · #6 04-12-2008, 11:17 AM

Quote:

Originally Posted by leathermang

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.

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.

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!!!!

SD Blue · #7 04-11-2008, 10:51 PM

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.

daw_two · #8 04-12-2008, 11:49 AM

Quote:

Originally Posted by SD Blue

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.

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

daw_two · #9 11-30-2008, 02:37 PM

Quote:

Originally Posted by SD Blue

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.

EPCnet indicates that this unit was "UP TO END OF MODEL YEAR '87"

I bet all the w126 spares I have are for the later years.

daw_two · #10 11-30-2008, 03:20 PM

I'm putting a link to this thread:
ac still not working after ac conversion 87 300SDL

since there is just so much information in it. I'm convinced that "bumps in the road" are causing the ACC unit to operate intermittedly.

Time for a new ACC.

daw_two · #11 12-21-2008, 01:28 PM

Well....guys; the heat has been flakey since I replaced the ACC, so just tore into the monovalve.

Guess what?

The monovalve diaphragm was torn. Got to take the kids out for a ride to see if it works.

leathermang · #12 04-12-2008, 01:09 AM

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...

leathermang · #13 04-12-2008, 11:38 AM

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...

daw_two · #14 04-14-2008, 04:28 PM

Quote:

Originally Posted by leathermang

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...

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.

pawoSD · #15 04-14-2008, 04:33 PM

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....