I am having an intermittent issue with my center vents opening and closing when set to “22”, but if I set the temp to 18 or lower, the center vents will always stay open, which led me to believe this was a soldering issue more than a pod issue. However, after pulling all the vacuum lines off the solenoid bank and testing them, they all hold vacuum perfectly except defrost 1 (red/white) which slooooooowly leaks down and then center flap, which doesn’t hold vacuum pressure at all. How could it be opening at all and be able to remain open if I set it colder if there’s absolutely no vacuum holding with a mityvac? I really don’t want to have to replace that thing after reading how difficult it is. Previous owner receipts say that all the pods were replaced 10k miles ago which was 2010. - Mike

If you can't pull vacuum on it at all, there's no way it'll open. Check how you did the test and visually confirm that the line you were pulling vacuum on actually goes where you think it does.

If it will open and stay open at "18" but works intermittently at "22" you have a pushbutton unit problem or an ambient temp sensor problem. Get a hold of the FSM and check to make sure that your vacuum lines are connected to the right places on the switchover valve unit. The switchover valve unit is also another common source of problems, there are some solder connections that can go flaky and cause erratic behavior.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

What model and year?

I don’t know what car this is but maybe it’s one that doesn’t send heated air through the dash vents. The engine vacuum pump ‘supplies’ considerably more vacuum than a MityVac so it can overcome a slow leak.

Sixto
98 E320s sedan and wagon

OP has a 560SEL, no idea what year. Gen II W126, gas engine = No vac pump.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Ha! Old habit Manifold vacuum and reservoir should cover it.

Since I think in *F, it’s plausible the cabin needs cooling to get to 64*F and heating to get to 72*F.

Sixto
98 E320s sedan and wagon

1988 560SEL w/ 210,000 miles. I only mention Celsius settings because the ACC is in Celsius. - Mike

Yup. I couldn’t understand why some MBs have outside temp display in *F and ACC thumbwheel in *C. The ACC box my ‘98 wagon came with showed operation setting in *F but diagnostics in *C.

Back to the point, do you expect heated air from the dash vents? Don’t.

Sixto
98 E320s sedan and wagon

No, I never expected heat from the center vents, but it has been my experience with a ‘90 w124 300E I used to have years ago with the same ACC unit that either the center vents are blowing cold or vented air or the foot well is blowing heat, but not both at the same time and certainly not the center vents cutting in and out randomly while the a/c is on on a hot day. Just super weird that the center vent opens up at all and can be manipulated to stay open if I turn the temp wheel colder, yet that pod doesn’t vacuum out at all with a gauge. I just pulled the vacuum solenoid bank and will resolder the contacts inside on the one end just to eliminate that variable all together. I’m driving from Phoenix to Buffalo and back in July and I want it all sorted it by then at the latest. It’s already heating up here too much to work on the car during the day. - Mike

The information given in your other thread in replies 19 and 21 still apply here. http://www.peachparts.com/shopforum/tech-help/392101-c-options-2.html

Since your center vent strangeness seems to depend on the temperature wheel setting, the issue is most likely to be one of the temperature sensors or the pushbutton unit itself. Solder joints would cause random operation all the time, regardless of the position of the temp wheel.

As stated above, if your vacuum pod failed to hold any vacuum at all, it would NEVER operate. Visually verify where the tube that you're trying to pump down goes, it may not go to the pod you think it does. If the center vent is opening AT ALL, it must be able to hold at least a partial vacuum or the diaphragm would never move.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Does this look right for w126:

From tippy-top to bottom:

POSITION - TUBE COLOR {dominant/stripe}
Main feed on top - (red/green)
Recirc 1 - (red)
Recirc 2 - (green/blue)
Feet - (red/yellow)
Center - (red/black)
Defrost 1 - (red/white)
Diverter - (green)
Defrost 2 - (red/blue)

This should help:
https://www.startekinfo.com/StarTek/outside/12253/disc%202/program/Climate/83-604.pdf

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

FWIW, when my ‘87 SDL air distribution and blower speed went off the reservation, it was time for a new PBU. The PBU from ‘86 is far more complicated to resolder than the 81-85 PBU, not that I can solder worth a d@mn. A rebuilt PBU (from George Murphy?) got things working as they should.

Does the cabin temp grate by the front dome light draw enough air to hold a 1cm piece of tissue paper when the engine’s running?

Sixto
98 E320s sedan and wagon

Yeah, the censor fan holds up tissue and I know it’s working because I saw it spinning when I removed the glove box. I resoldered the vacuum solenoid block and the problem still persists. I’ll order a new pod from AHAZ and a giant bottle of Skyy from CostCo and start the job after my tears have dried and the vodka has kicked in. - Mike

I now have beautiful cool air wafting regularly out of my 560SEL center vents, and it's oh-so-nice. I recall someone on the board mentioning that the diaphragms have to be seated perfectly within the rim of the pod in order to function correctly. So when I removed the acc panel and pointed the flashlight to the back of the pod which had been replaced in 2010 with all the others by a previous owner, low and behold, it appeared as if the pod was assembled incorrectly and one section of the diaphragm was pinched within the seam between the back and the cup, allowing intermittent functioning at best. It was too difficult to seat the diaphragm correctly with the pod in place since I couldn't see all the way around it, so out it came. When I put the pod back together at the bench, it held pressure perfectly. But, by then it was too late. Might as well use the brand new one. Tips and tricks; I would (probably) not have been able to do this job without a very long pair of curved surgical hemostats to separate and then later join the arm to the white barrel clip on the vent door. I also used a long & thin "stabby" tool used in book binding to get in there and rotate the barrel clip to just the right position to then squeeze the end of the pod arm into the clip. Also, I purchased a long 5.5mm socket along with a 1/4" u-joint swivel piece to get at the small socket screws holding the base of the pod in place. When I put the screws back in, I placed a very small piece of paper towel on top of the screw head, then jam it into the end of the socket so the socket screw does not fall off of the socket when trying to get it back there in the correct spots. All in all, I was really really dreading this job based on what I've researched on this forum and the confining space under the dash, and there were definitely some frustrating moments along the way with trying to get that arm correctly lined up with the clip through that tiny hole in the hvac box, but it wasn't all that bad and it didn't take as long as I thought it would. It's supposed to hit 105 degrees here in Phoenix by Sunday, so I am definitely grateful this job is over.