The following VFAQ was created to help people replace their malfunctioning climate control unit. As always remember the lovely legal disclaimer that I do not accept any liability for yours or anyoneâ€™s actions who may follow any of my guides.
I don't know if anything is more annoying while driving than to have a malfunctioning climate control system, my personal favorite is when it is below freezing in the car and the heat is in the vents but not blowing through. The copper on the circuit board or the solders themselves are known to have problems from time to time, this is probably the number one reason why people buy a rebuilt unit. However with this DIY rather than paying the $150 or whatever it is with the little time to take this piece apart and a $10 soldering iron you can fix it yourself for good!
Before I go on to how to fix the unit itself I'd like to elaborate on symptoms of climate control issues and the most popular problems associated with them. First is the center vents don't seem to work right, perhaps while the a/c is on the windshield starts fogging or frosting up near the bottom even though defrost isn't on. That is a bad vent pod probably caused by a bad vacuum diaphragm or a vacuum leak. The replacement diaphragms can be found many places. If you would like more information perhaps my favorite forum
can help you. Another common problem is that the fan will only kick on if the climate control is set to defrost first with max fan speed, those are bad blower fan brushes. While you can buy a new motor for about $100 I find it much more appealing to fix it myself for $6 instead.
Another common problem is that the climate control only seems to work on max or min values, quite often this can be attributed to a disintegrated foam hose which runs from the dash to the temperature sensor for the climate control. Finally, the one that we are covering here. If your blower fan turns off and on intermittently, A/C is running when it shouldn't be, and several other very odd problems then bad solders could be at fault. On my own W123 my fan would work sometimes and sometimes not, but if I hit the center console near the climate control it would sometimes turn back on.
Ready? I hope so because off we go. First things first it has to be removed from the vehicle, no big deal here. As can be seen in the above picture after the wood is popped out you just take those two screws through the top face out and then tilt the top forward, after this you use a screwdriver on the bottom to lift it up and over the radio (what happened to your buttons?!? Well to be honest this was upon reassembly, I still had the buttons out for cleaning, we'll cover this later). In my case I also had to pull the trim ring around my Kenwood stereo off.
Now that it is out of the slot you'll see all the little light bulbs which provide the backlighting for the climate control, behind it you may see an odd shaped light housing. It provides the light for all the other buttons in the console via fiber optics. Back to the point, when disconnecting these lights don't pull to hard or you'll end up breaking the piece which holds it in (no huge deal but still...). Also don't try to walk away with your climate control before unplugging all of these lights. This is because you'll break the piece holding the light which provides the lighting for all your other switches and buttons, this is a much bigger deal. Now that you have all of those unplugged unplug the harnesses connected to the sides, one on each side.
Finally it is in your hands go ahead and carry it over to your workshop where you have the nice clear area and soldering iron ready. All there is left to do is start taking some covers off. I took off the switch cover (connected on the side) and the top cover to the climate control circuit panels, yes plural, three of them here. The problem is where these three connect to each other, this is what we'll be resoldering. I also removed the fan button assembly, it took a little more work but it should be clear what you have to do once you are doing it. This is where my actual problem was. These switches can be purchased separately but why not fix it? The cover comes off pretty easy if you utilize two flat head screwdrivers and lightly unlock the tabs which hold the cover in place. It will simply swing open and come off. Under it you will see this:
Ok, being the overkill person I am I got out the flux (used to clean surfaces for soldering) and extra solder. There was a spot or two that I added some solder to since it looked a little stingy but all we have to really do here is melt it and let it cool again. It is really that simple! Last thing to mention here is since you have the unit out anyway, why not pop the buttons off and clean them thoroughly so more light can shine through? Yes you may break a button or two but isn't it worth it? (I broke one after doing this to a few climate controls) Installation is reverse of disassembly. Remember to get the climate control positioned correctly on bottom so it stays on there securely. I also used die-electric grease on the pins to make the harnesses go on and off easier, as well as promote better conductivity.
If you read all of the above it took you longer to read this than to go do it, have fun and enjoy your fully functional climate control system. :-) If you have a W126 the steps required to remove climate control is different but I believe the soldering remains the same.
2 Flat head screwdrivers (one medium one smaller)
Soldering Flux (optional)
Electric solder (optional)
Small flat head screwdriver (only if you need to remove fan switch assembly)
Discuss this DIY here.