Wednesday, October 10, 2007

Automatic Climate Control Unit Repair

I haven't had time to post some of the older tasks I've done, so I'll just jump in with what I'm doing these days...

Task and Diagnostic Procedure
This task about repairing my automatic climate control unit (the ACC). These have a typical failure mode were some internal solder joints break, rendering it partially functional. Different units break different solder joints causing different failure modes, but one of the most common is that only defrost works, and often only works for a while after starting the car. This was exactly how my car worked. The non-technical diagnostic procedure is to wait until the ACC is in its failure mode, and then give the front of it a whack to see if it starts working again for a bit. On mine, that would cause defrost to start working again, for a while at least. Note that there is another unrelated failure mode where wiggling the ignition key restores functionality for a while -- if yours responds to this procedure, then this repair probably won't help you.

The repair for this unit was to use a soldering iron and reflow (remelt) the solder. To get to the joints requires disassembly of the unit, so that's what we'll do.

Tools Needed
To do this repair, you'll need the following tools:

  • Flat blade screwdriver
  • Phillips screwdriver
  • penknife (my tool of choice for easing click-together plastic latches apart)
  • needle nose pliers
  • soldering iron
  • rosin-core electrical solder


Step 1: Remove the fancy wood-grain front. I don't have a picture of this, but it is held on with friction pins that fit into the holes circles in
red. Above the radio (the radio bezel may have to be removed to access them), are two flat-blade screwdriver slots in the the bottom of the bezel for you to
gently pry it off with. The friction pins fit into the holes circled in red in this picture.

Step 2: Remove the unit from the car. There are two screws (circled in green) that need to be removed. After that, on the w123, the unit will hinge
forward on its bottom edge until you can lift it out. There are two large multi-pin cables attached, one on each side, that need to be
unplugged from the unit.

Step 3: Remove the front, plastic mounting bracket from the unit. Three screw (circled in purple) hold it on.

Step 4: Next, remove the temperature wheel It plugs into a set of pins on the side of the unit like the cables did, and has some little plastic tabs
holding it in place. Gently (!) squeeze the two white plastic tabs together using a needle nose pliers just enough to get them to slip down
the hole (see orange circle and arrows) and ease the temperature wheel unit off. Don't force these tabs, and don't squeeze them together too hard or that 20 year old plastic will break.

Step 5: Remove the cover of the fan switch unit. The switch unit can't be removed yet because the buttons lock it to the front piece. Taking the
cover off makes it easier to remove the buttons. The cover is held on by two tabs, one on the top (see purple circle) and one on the bottom.
Gently ease the cover off past the tabs. I prefer a penknife for these kinds of operations as they are thin enough to slip between the plastic
parts without mashing and marring them too bad. Just be careful not to slice pieces of plastic off.

Shown here is the unit with the temperature unit removed and the cover of the fan buttons removed.

Shown here is the inside of the fan switch unit. The "T"-shaped piece of plastic is held in by the cover. If it fell out when you removed the
cover, this picture should help in getting it back together correctly when you reassemble. The round pivot in the middle is different lengths. When you reassemble, just be sure the fingers of the "T" fit into the backs of the switches like shown here.

Step 6: The next step is to remove the buttons and the front piece. The way I did this, they sort of need to be done together -- the front piece can't
be removed until the buttons are off, and I couldn't figure out how to pull the buttons off so I had to pop them off from behind, which I couldn't do until the front piece was off. The next two pictures show where the tabs that hold the front piece are -- here is one on the top (circled in blue) and two on the bottom (circled in blue on the next picture). Pop them them apart, but the front piece can't be removed yet because of the buttons. However, popping it free allows just enough access to slip the penknife in behind it and pop the buttons off by pushing them from behind and shown in the next picture. The fan buttons can be easily popped off by pushing from behind.

Shown here are all the parts removed so far. The rest of the disassembly is pretty easy from here on. Note also the clear plastic light pipes across the front and down the side of the fan switch unit.

Step 7: Next remove the bottom plastic panel. I've reused an earlier picture to show where the three tabs on the back that hold it on are. Once released, it hinges up toward the front until it can be removed.

Step 8: Finally, with the buttons off and the front piece removed, the fan button unit can be removed. Press the two tabs by the purple arrows together just until they clear far enough to slide out and allow unplugging the fan button unit from the side of the climate control unit.

Step 9:The plastic cover over the internal circuit cards can finally be removed. The circuit cards and guts hinge down and out from under the cover. Once the cover comes off, the two plastic pieces with pin numbers that the side pins stick through can be slipped off.

And here's the wrong ACC unit in all it's stripped glory -- yours from a w123 won't look exactly like this one (this one's from my practice w126 unit). There's a main base board, the top daughter board, and the two side wing boards with the pins. BTW, the w123 unit only has 4 relays instead of the 6 shown here, and the circuit traces are slightly different, but otherwise they look pretty much identical.

The Repair

With the unit disassembled, the repair can begin. Some have reported that they had cracked circuit traces on the daughter board where the pins from the front panel switches connect. Mine were fine there, but had cracks where one of the wing boards connects to the main board.

Shown here are the cracked solder joints on my unit, circled in red and with arrows pointing to them. I've even slipped a small piece of yellow-green paper down one crack to prove to myself that it was indeed not connecting reliably :). The solder on these needs to be remelted with a soldering iron, and maybe a little more new solder added, to rebridge the gap.

And here's the repaired solder joints, circled in blue.

Failure Analysis
I'm certainly not officially qualified to do a failure analysis, but I've had years of experience taking things apart and fixing them, often trying to make them better so they won't just fail the same way again. So, as long as I was in there, I figured I'd take a look and see if I could figure out why those particular solder joints had failed on my particular unit. Sure enough, there was. When my CCU was soldered together, the wing board with the cracked joints was bent and the solder was used to hold it in place. This put permanent tension on the solder bridges. Unlikely that the tension alone caused the failure -- it was probably the the vibration of the car and that heavy cable hanging off those pins that did it. However, the tension made them spread once broken and certainly helped propagate the cracking down through more of the bridges.

A Better Fix
Using lead solder as a mechanical attachment isn't a great idea as it tends to bend, stress fracture and disconnect. I tried using it for that a lot as a kid and it always failed in short order. So how to fix it to be better? Replace each solder bridge with a small length of stranded wire. Make sure that the solder doesn't bridge the gap so there is no mechanical attachment via it -- if there is, that bridge could break such that both ends of the wire are on the same side. I certainly didn't do this. Instead, I completely unsoldered that wing board, repositioned it so I wouldn't need to flex it as I resoldered it, and then made new solder bridges to hold it on. I figured the bridged on the other wing board (the one not flexed) had lasted 20 years, so this would probably be good for another 20 years now.
I also went a bit further and reflowed all the solder of all the joints on all the boards. Sometimes solder joints can have pratically invisible cracks through them in my experience. I figure it was easier to reflow all the joints then have to disassemble the unit again to repair some other failed joint.

I didn't take any pictures because there was nothing interesting. Just reverse the disassembly steps and it all clicks together. At the time I did mine, I didn't clean the pins and reassemble with dielectric grease, but that might have been a good idea. The only tricky part I has was remembering how the buttons went back in. They all have a side were the clear plastic inside them is exposed on one or more sides -- make sure you pop them back on such that the exposed clear plastic edges face a light pipe (I don't think they'll go on wrong, but I didn't try).


Douglas said...

I started a blog for my 84 300D mercedes too. If you read mine, I'll read yours. I found this from the shop forums.
If you click my name, it should take you too my blog. If not, its:

fuseboxgifts said...

Just wanted to take a moment to thank you for your work.

Your instructions are well written and extensive - I'm going to give this a shot tomorrow...for the past couple of years the car has blown either hot or cold air - just depends on it's mood.

Happy motoring.

fuseboxgifts said...

Just wanted to take a moment to thank you for your work.

Your instructions are well written and extensive - I'm going to give this a shot tomorrow...for the past couple of years the car has blown either hot or cold air - just depends on it's mood.

Happy motoring.

rick said...

Thanks a lot. I tore into my 82 300td clima unit. It worked, but only blew when the witch inside wanted to and since i'm about to replace the a/c compressor i wanted to have things in order. I even fixed a trace on the board that someone had patched wit a little solder and a lot of tape. I soldered in a jump wire.

thanks xx rick Athens, ga

Dan said...

A thousand thanks for posting this. I have just acquired an 85 300TD with wildly behaving climate controls and was googling for clue before I blindly started tearing into stuff.

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