Hello,

Im getting back into the world of working on w123 cars, and have a new job that I never had to tackle on my old one - some of the settings on the ACC do not work, so I need to replace it.

In the interest of time and sanity, Ill be buying a rebuilt, warantied unit, so I can swap it in myself...

That said, I don't really understand what is involved, since Ive never had to mess with that part of the system. My old 83 had a 100% functional climate control and AC system.

How does the pushbutton ACC unit correspond to the vacuum system to operate the dash vents? Is the ACC unit an electronic hookup only type of unit? Is there a functional portion of it which routes vacuum and moves flaps that ought to be replaced too?

While Im in there, are there any other smart quick win types of replacements that I can do? I knew from the start that the ACC was something that didnt work well, but I dont know what Im getting into as far as replacement steps to complete the job, and other things that would be smart to do while inside. The vacuum system is 100% tight, so I know that there is a reason why my center flaps dont work, on top of the fact that the ACC pushbutton unit doesnt turn the blower on, etc.

Ive read a lot, but some of the more basic info seems to be lacking. Please help an old time MB fan who is just getting back into it out!!!!

Thanks,

JMH

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)

I believe that your car is the same as mine (see my sig) and has ACC type II.
In this setup, the push button unit doesn't directly control the vacuum pods that open/close the flaps, the vacuum pods are controlled by electric switchover valves behind the climate controls that remain closed when unpowered but when powered open and allow the vacuum pump to evacuate the line that is connected to them. I would say that you probably want to take out the upper dash panel (with the sunroof control, etc) and the wood trim and plastic cover at the ACC unit to get to the changeover valves.
While you are back there, you want to test the changeover valves (they are called both changeover and switchover valves). I don't have a very systematic way of testing them, but basically find a wire that is clearly activated and plug each one in turn in to it. Test each one disconnected to see if it holds a vacuum, make sure they open when connected to the active wire, and make sure that if you cap them while they are on the wire, they hold (I had one that leaked while it was open).
You them want to use a vacuum gun again to test the lines and pods that are opening the vents. The lines from the switchover valves are:
1 - defrost full
2 - footwell flaps
3 - center vents and defrost halfway
4 - recirculated air flap
5 - main air flap

One other thing you may want to try that may save you the cost of a push button unit - open it up and check the solders. It can be hard to see, but check the large contacts for the four magnetic relays for cracks. Mine had one relay not making contact, and I managed to resolder it (however, the plastic in my buttons and the mechanical side of the unit was too far gone it seems).

If you have trouble getting it open (I did) I can provide a description there for you.
I might also ask where you are getting the remanufactured unit as I need one after a mechanical failure (after I put it together after the resolder, pushing the off button doesn't release whatever button is currently pressed).

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1984 300DT turbo 138k mi

Still hauling me to school and back.

Handy Site:
http://www.dieselgiant.com/

I assume your referring to the ACC as the push button unit. This is most like due to broken contacts which can be easily resoldered. Don't try and figure out which ones, just resolder them all according to the DIY here : http://diymbrepair.com/

The climate control has 3 main units. The push button unit, the climate control unit which is a silver box with an analog circuit inside and 16 or so pins, and the blower regulator which gets it's signal from the CCU and the toaster (resistors located on passenger side of engine compartment).

On that are the ancillary units that do the real work, the vacuum pods, blower motor which get their signals from the aforementioned units.

Good luck

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1982 300TD 210K miles ("The Replacement" aka "The Anvil") - SOLD
1979 300SD 245K miles (never ending project)
2007 Pinarello F3:13
1995 Ducati 916 (SOLD, sniff)
1999 Ducati 900SSie (SOLD)