no center vent A/C-no bad actuator - PeachParts Mercedes-Benz Forum

Larry Delor · #1 05-12-2002, 04:27 AM

Hello all,

I decided to see why I wasn't getting any cold air through my center vents anymore. Last year I replaced the three easy to get to actuators, ( 2 for recirc. & 1 for def. flap). I suspected the 4th one to be the culprit...turns out its not. When I attach some tubing and apply vacuum, the center vents work properly. Following the tube that supplies the actuator, led me to a narrow bank, where it is neighbors with several other vacuum tubes. Seems like it's some sort of vacuum distribution device.
So, this leaves me wondering about the mechanics of this device, and wether its removable/serviceable. I think I heard some clicking a few months ago from that area, but figured it was the thermostat trying to make up its mind.
Oh yeah, the vacuum bank also felt quite warm to the touch.

any insight/advice is appreciated.

-Larry

JMH · #2 05-12-2002, 08:22 AM

Larry,
I am in the throes of the same thing... don't have any info for you but I am experiencing the same symptom in that my center vents don't work. Yesterday afternoon I was working on it and manually closed the defrost vent actuator while the car was running... the center vents came open and blew cold air... but it did not last long as it closed tight afterwards and the manual manipulation didn't work again (I am hoping against hope that it is not the center actuator but it probably is). Since I had both kick panels and the glove box out, I took a pair of plyers and grabbed a small white plastic piece on the a/c housing (through the glove box, turn left and just below the flex rubber duct material for the center vents. It felt stuck but stayed open after I opened it. I am going to see if this will keep it open but I think you are on to something as the defrost actuator holds vacuum but it is always on when I turn on anything from the temp control panel.. the Haynes manual is worthless in this endeavour and I don't have the cd rom discs that I understand are pretty good... so I have the same problem you do and I apologize I don't have any answers.... BTW, I am in Grand Island, FL, but not sure where Nokomis is..... is that in the panhandle?

joe p · #3 05-12-2002, 09:39 AM

Sounds like either you have a leaky actuator, a bad switchover valve block or a bad push button panel. There are pretty acurate tests for this along with the old reliable vacum test with a hand pump and a roadmap of the circuits.

Joe

JMH · #4 05-12-2002, 07:15 PM

Where do I find the diagnostic tests for this? I don't know where to turn to diagnose this as Haynes manual has nothing about it in there..... I have no way of knowing where it all runs to or how it routes and fits together.... all of the actuators/diaphrams seem to hold vacuum pretty well..... but they don't each open the way they should to get the airflow to where logic would tell me they should... any help would be greatly appreciated.

joe p · #5 05-12-2002, 09:18 PM

Odds are the roadmap for the vacum block can be had here but, I dont know the site that well. I think its in alldata as well. I've heard people talk about the rubber seals falling off the flaps inside the heater box and binding flaps too. (something to think about)

Whats the vacum like to the block as well? It should be right at manifold vacum, if not, you've got a leak elsewhere such as the vacum canister.

Joe

Billybob · #6 05-12-2002, 09:47 PM

Which vehicle are you working on? The Automatic Climate Control Manuals have the vacuum schematics for each model. I can e-mail you a PDF file from the 126 CD as an attatchment if that would help, you will need Adobe Acrobat reader to veiw it. Good Luck!

tcane · #7 05-12-2002, 10:39 PM

The automatic climate AC systems in the W123 is complicated and discussed a lot in the diesel forum (I've made several posts on fixing problems with this system). Do not be too specific with searching for info on the diesel forum because the various sub-systems work the same (center vent, floor vent, defrost, side vents, etc.), so a search for say "center vent" may not return info that will help you. You can find a very good M-B AC manual in the Mitchell automotive service manuals - I found copies at my library - and I consider this the best, or eBay for the M-B CD manual or perhaps the OOP print manual for the AC. Tools needed will be the Mighty Vac ($30-$35 basic kit $50-$60 deluxe kit at NAPA, Pep Boys, O'Reilly, etc.) to draw vacuum to check various components (I've found leaks in a 2" length of vacuum line and a plastic Y-fitting that leaked) and a volt/ohm meter. The AC system is a combination of vacuum and electrical components (the swtich over valves are both) so you need both tools to check things out. Finding the problem is a matter of taking the time to trace out and check the components in the sub-system that is not working (here, the center vents). There are not any "silver bullet" fixes on these systems.

Good Luck!
Tom

jcyuhn · #8 05-13-2002, 09:47 AM

It's been a couple of years since I got rid of my 123, but I recall chasing down a similar problem. If the center vent actuator checks out OK - it holds vacuum and the vent opens - then you need to look for large vacuum leaks elsewhere in the system. There is one vacuum feed to the entire climate control system; it is shared by all five vacuum actuators in the system. The single vacuum feed contains a small orifice to limit flow; this is so that leaks in the climate control cannot negatively impact the power brakes.

Anyways, you need to look for a large leak in another actuator. The most likely culprit is the defrost actuator. The defrost vents are normally open; applying vacuum to that actuator turns off the defrost function. Hence to get cold air out the center vents, both the defrost actuator and the center vent actuator have vacuum applied.

As well, the defrost actuator on the 123 is a funky two stage device. It has two separate sources of vacuum, one from the vacuum block behind the pushbuttons, the other (I think...it's been a while) comes from the center vent actuator. Even though I had the 123 climate control manual, I could never figure out how/why it was built this way. I think the climate engineers at Mercedes are more insane than the rest of the engineers.

Larry Delor · #9 05-20-2002, 02:22 PM

Hello again,

The vehicle in question is the 420SEL.

So far all evidence points to the vacuum distribution bank. A narrow rectangular item, with quite a few lines leading away from it.
I am wondering what the guts of this thing look like, and if I should even bother to take a gander inside to see what makes it tick (suck). The device also feels quite warm to the touch. I am also wondering if this is a common item to fail, and if I ought to find one at a junkyard, or purchase new.

Has anybody else replaced this gadget?

I did notice, that recently the "economy guage"/vacuum gauge reads less vacuum than usual. The needle returns to its normal position when I push the "Econ" button.

Any comments are appreciated

thanks! -Larry