ACC Question - Dead Center Button - PeachParts Mercedes-Benz Forum

#1 06-25-2002, 06:28 PM

The center button on the ACC control on my '82 300D (W123) is dead. As in, push the button and nothing happens--no blower motor, no compressor kicking on, nothing. The A/C functions with the 2nd (from the left), but only blows out of the two outside vents and the defrost vents.

Is there a way to repair the electrical part of the pushbutton ACC control panel? This seems to be an electrical problem to me, but could there be a vacuum fault as well?

I've looked through the archives for the last couple of hours, and didn't find anything specific to this problem. I'm looking for the most cost effective repair that will get my center vents blowing again. I haven't taken anything apart yet, and want to be as educated as possible before pulling the dash apart.

Cheers,

Brad

Ken300D · #2 06-25-2002, 08:53 PM

I had complete success in fixing a similar problem with my '87 300D, which uses a similar setup for ACC controls. I removed the unit from the dash, took it apart, and resoldered all the electrical connections for the pushbutton switches and the temperature setting roller potentiometer.

My symptoms were A/C and fan operation with the unit's OFF button selected, and A/C operation in ECONOMY. Now it works perfectly.

I'm getting slowly motivated to do the same thing with my '82 300D, because the fan is mostly dead on everything except DEFROST - deteriorating during my time of ownership from working most of the time in the other modes.

I certainly was under the impression that the ACC switches were entirely mechanical in nature. I originally thought they were vacuum control valves of some sort. They are actually just rather plain electrical slide switches like you'd find on a radio or something electronic. What I think happens is that people are too violent with pushing the switches, especially when their operation starts to fail.

My theory is that the mechanical impacts of pushing the buttons and also having them pop back out when you push another button, is that the solder connection eventually cracks and fails.

Taking the ACC control unit out of the dash is not too complicated. You do have to carefully and slowly pry off the big electrical connectors - they are pretty massive plugs. Then you can pry off the bottom cover carefully by finding all its holding tabs. This should reveal all the pushbutton solder pins. On the '87 300D, each pushbutton has two parallel rows of three pins.

Its really a simple rebuild, and I'll bet a lot of ACC units have been replaced at $300+ with nothing wrong but bad solder joints.

When I do the '82 300D, I'll post some pictures if I get the chance.

Oh, the vent directions may be vacuum controlled - an actuator may be bad - but go for the electrical fix first....

Ken300D

haasman · #3 06-26-2002, 11:50 PM

Ken, great post. It is good hearing about these types of repairs

nellotare · #4 06-27-2002, 12:02 AM

Ken,

Thanks for the post and will look for pics. Just this week.. when I push my EC button, nothing happens. If I push it in just far enough to disengage the OFF button but not far enough to actually make the EC button stay depressed.. I get Air Conditioning. It makes me depressed.

#5 06-27-2002, 09:27 AM

So I took the unit out of the dash last night, took it apart, and all of the solder joints were intact. I cleaned all of the contacts with contact cleaner and emery cloth and put it back together.

Once installed in the dash, the only change is now when I hit the center button, I can hear the compressor clutch engage. That's new. But still no fan or cooling. I got everything to work one time by running the AC off of the defrost button for a couple of minutes and then switching to the center button. The center dash vents did not open, but air did continue to blow out of the corner dash vents and the defrost.

Where do I go from here? Could I still have a bad ACC controller? The fact that I get no fan with the center button leads me to believe that this is an electrical problem first, and a possible vacuum problem second. Is there something with the fan motor control that differs with each "button" setting on the ACC?

Thank you for the excellent advice thus far!

Brad

Ken300D · #6 06-27-2002, 09:46 AM

The only additional comment I have before actually doing the job again (hopefully this weekend) and posting pictures, is that I could not visually tell there was anything wrong with the solder connections. But I didn't care, I reheated all of them and touched them up with a little more solder.

I think the printed-circuit board is plated on both sides, meaning that there is a connection run on top of the board and on the bottom. You might be able to spot a crack in the solder on the bottom, but it would be tougher to see one on top where the actual switch sits - and that's where the most mechanical stress is anyway.

So, visual indication or not, my recommendation is to reheat all the switch solder joints and touch them up with a very small amount of additional solder. This will make them mechanically and electrically sound again - better than the original wave solder method.

Ken300D

Ken300D · #7 06-28-2002, 08:14 PM

First picture is the W123 300D ACC unit shown still in the console with the woodgrain cover off. The woodgrain is removed by gently pulling it outward - it pops off.

Ken300D · #8 06-28-2002, 08:18 PM

Next remove the four shiny phillips-head screws. This drops off the larger black cover plate and leaves the ACC unit hanging in place - shown here pulled out a bit from the console. Note the sophisticated use of electrical tape to hold the illumination bulb wiring in place - somebody's been in here before!

Ken300D · #9 06-28-2002, 08:26 PM

Undo the illumination wiring and detach their associated bulb holders. (Now is a good time to test the bulbs - just turn on the parking lights.) Carefully pull off the black plug on each side of the ACC unit. The ACC unit is shown below on the test bench, with the plastic tabs highlighted that are holding the circuit board cover onto the unit.

Ken300D · #10 06-28-2002, 08:30 PM

Got the cover off now? Good. Here's what you are going to see. It turns out the W123 vintage cars have less electronics in the ACC unit and more mechanical stuff. You'll see that in a minute. The parallel rows of pins along the far side of the circuit board represent the pushbutton switch connections (there are more, as we shall see). Now you have the chance to look for cold solder joints and broken solder joints.

Ken300D · #11 06-28-2002, 08:38 PM

The next picture below shows the ACC unit considerably further disassembled. On the '87 300D, the ACC unit has more electronics in it, and all switch connections are on the bottom circuit board that you can access by removing just the one cover. Here, the ACC unit is mostly mechanical, with larger switches and a second circuit board for another set of connections. It was on this upper circuit board that I found a cracked solder connection.

To get to this state of disassembly, you have to get the cover framing the pushbuttons loose - it is held on by tabs. Then you work down one side, popping off the buttons gently one by one. Once I got all the "MODE" buttons loose, rather than pop loose the fan buttons, I just removed that entire module with the whole pushbutton frame attached. The temperature wheel module on the other side of the ACC unit also pulls off (carefully, there are some tabs holding it).

Finally, the circuit board assembly is leveraged out of the remaining plastic case starting with the back end (away from the pushbuttons).

Ken300D · #12 06-28-2002, 08:46 PM

What a mechanical nightmare! Those shiny copper-colored components are called "relays". "Ree" "Laaaaazzzze" You won't see them anywhere else in a control unit except if you visit space station MIR as a tourist!

So far, I've found one cracked solder joint on the smaller sub-board and one cold solder joint on the larger board. I can see where someone reworked a few of the joints previously - either a rework facility or a previous owner. I'm taking no chances - all solder joints are going to be reheated and touched up.

Now - what to do with those REE-LAAAAZZZZEE ? I'm kinda shakey on burnishing them with anything other than a business card or matchbook cover. I don't think you want to touch them with any kind of metal file (if you should have one that thin). Better to be gentle on their contact surfaces.

haasman · #13 06-28-2002, 09:19 PM

Ken Thanks for this info. Great help. Great pictures.

ladomani · #14 07-01-2002, 06:25 AM

Hi Ken,
I love the way u have documented the acc problem in the w123 . Do you have any idea about the w124 acc switch. It is a rotary switch. The problem i have with mine is that the ac only starts working when the switch gets to level two(II) nwhich is usually too cold for me.

I have a feeling the first level(I) has a cold solder or partial contact because if i leave the rotary switch on 1, and i get into a bump on the road , the ac suddenly roars into life.

So do u know how i can access the acc control in a w124?

Any solutions will be appreciated.

nellotare · #15 07-01-2002, 09:30 AM

Yes, Ken.. thanks for the great pics and describing the process.. it always helps to have the pics as well as instructions.. now I'm not scared to tackle the job. I do think my button problems can be fixed by your method.. or at least I'll try that first and see what happens. Thanks again!