Dad has a 300SD W126. He recently installed a brand new pushbutton unit. Most of the cold air is going through the defroster ducts, but all positions switchover correctly when the buttons are pushed.

Interestingly, if I jump the center vent switchover valve, the center vent opesn all the way.

I'm thinking perhaps a faulty pushbutton unit?????

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

If all positions switched correctly, there would be only a small amount out of the defroster ducts in most positions other than the defrost position.

Since most of the air vents via the defrost, clearly, the defrost pod is not closing. This can either be the pod, the switchover valve, or the CCU (least likely). The default position is open so you need vacuum to close it.

You can find it by removing the glove box interior. It's on the left side and it's arm points forward and downward...........strange setup. It's easy to replace, however.

According to the diagram, the same pin of the pushbutton controller provides power to both the center and defrost switchover valves. From what I've seen of the ACC system, they usually switch the ground side on/off. A meter at the switchover valves should verify the electrical side.

However, as Brian mentioned, the vacuum side is more suspect. There is a four-way connection that ties the defrost and center pods together. If the defrost pod is leaking, the center pod will not get sufficient vacuum (due to an orifice in the line) to close as well. There are two vacuum sections to the defrost pods and you will probably find one section is leaking. It is best to replace the entire pod and it is one the easiest to access in the entire ACC system.

Here is a link to the vacuum diagram: http://www.startekinfo.com/StarTek/outside/12253/program/Climate/83-604.pdf
Scroll down to "cooling".

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

I re-soldered a climate control unit for my 83 SD, and magically, the center vent worked. It had never worked before since I got the car. Now my climate control works very well, including variable auto fan speed, center vent etc.

I use a suction gun to de-solder all the joints first, remove as much old solder as I can. When re-soldering the controller board, I also added some wire ends to re-enforce the "vertical" joints between the main board and the two small vertical external connector boards.

You only need to open up the back cover plastic piece to do the re-soldering, no need to disassemble everything.

__________________
Shane83SD

Thanks for the replies. I got to check it out when I get back up there. I still suspect his reman control unit, since I can jump the switchovers and make the vent work 100%...

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

I also know that on BOTH of my 300SD's the lever that the dash where the defroster pod connects to the flap is broken. Often times it will slip off directing all the air to the defrosters and little or none to the center vents.



In the 300SD there are three units responsible for climate control:

Push Button Unit
Temperature Regulator
Blower Motor Control

The Temperature Regulator has some control over vents too. I am not sure if its center vents or not. I have recapped a few units, might be time for yours?

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

I am working on a W126 at present, is this information W126 specific???

I have no trouble making the ACC system work, but I'm not all that good with the Monovalve system....

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Yes it is for the 126, first gen units only though. Not applicable to the second gen stuff

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

When you say "jump the switchovers" are you talking electrically or vacuum?

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

Electrically....

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

I took a look at another diagram and it shows the power side of the center vent switchover valve being supplied directly from F10 (this same fuse supplies power to pushbutton controller, aux fan relay and temp controller).

The ground side of the center vent switchover valve goes back through the pushbutton controller to the temp controller.

So, if you jumped power to the center vent switchover valve, it may be as simple as a fuse. According to this diagram, the center vent switchover valve should have power all the time.

Here is a link to the diagram: http://www.startekinfo.com/StarTek/outside/12253/program/ETM/ACC83-85.pdf

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

Here is a look inside the temp controller and PBU:

http://www.peachparts.com/shopforum/tech-help/321019-inside-box-w126-automatic-climate-control-first-gen.html

http://www.peachparts.com/shopforum/2968179-post3.html

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Temp Controller????

Is this in the Pushbutton unit??? I need to trace this to ground as I think this could be the issue.

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

No its a separate entity. The temp controller is located on the right side of the blower box.

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Could you get me a pic I could send to Pop????

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership