Fixing/resoldering a malfunctioning climate control module in 1987 W124 - PeachParts Mercedes-Benz Forum

Chris_87_300D · #1 06-15-2025, 04:21 PM

Redoing this because I couldn't find a sticky with the Climate Control (CC) repair procedure still attached that was specific to the W124 1987 and I ran into a few issues that took some time to sort out. Further, a lot of pics have gone missing over the years so I am going to be as descriptive as I can without posting any photos.

The climate control module (inside on the dash) of my 1987 300D went on the fritz - it would start the fan up for a few seconds and then run it super slow from then out. Occasionally it would start up again for a few second and then run slow for most of the remainder.

To ensure we are talking about the same controller, mine has a temperature wheel to the left, 5 buttons below for off and air flow direction, and three on the right - low fan, auto fan, and high fan speeds.

The problem didn't sound like the fan or the main resistor/speed controller that is under the hood so seemed like the controller was likely the cause.

Removing it is easy you don't have to take the radio out or anything. First disconnect the battery of course. Then, just remove the two screws that point upwards in the wood trim into the controller area (above the radio) and pull that out and tip it up - there are hooks in the top pointing upwards, towards the top of the dash, no screws.

Then take out the two small screws at the top and the two small screws at the side. And the larger two screws at the sides. The Climate Control module should come out easily. There are two large connectors on the right and left, get a small screwdriver or plastic trim tool and pry it up a bit so you can work it off from left to right carefully. Don't wrench on it as these pins are soldered directly to the circuit board underneath. Once free, take it to the workbench.

First thing, put the temp wheel all of the way to cold until it clicks so the rheostat doesn't move around once you take the circuit boards out. For all of the prying of things, I used a small, 4-5" long shaft thin tipped regular screwdriver and a short 90 degree pick. Remove the four small Philips head screws from the bottom and set aside. Carefully pry the two tabs on the back side pop the plastic flat tray tabs out enough so they stay out. Then side your screwdriver to the middle - pop that one up and the flat bottom comes off.

Next, gently pry up the trim around the temperature wheel, working it off evenly around all sides so no tabs break. Then, you can access the plastic "rod" (has a hex head on the end) that goes through the wheel and into the potentiometer on the circuit board. Put a thin screwdriver under that and pry it away and perpendicular to the temperature wheel. Move that out most of the way of that space (the width of the wheel, basically). Do not remove the wheel.

Inside are 4 circuit boards. One is the main on top, one is the primary sub with semiconductor/chips below and parallel to the main, one has the three switches for fan speed, and the last one is the light bar in the bottom front. All four are attached together with wire / connector "flexes".

Obviously, you will take the top circuit board off first. The board should more or less come straight up but the buttons will hold it back. Pull it up a bit and then put your finger behind each of the buttons and carefully push from the inside and pull them out from the other side. All of mine came off the button completely intact but there are several pieces to the button and just the front might come off and leave a white square behind (at which point the guts of the switch can come apart, so don't let that happen). I believe that the board will come out either way but if the covers come off, be sure to put them back on so the "missing" side faces the light bar so they light up after you put it back together. With the switch covers removed, if you moved the temperature wheel rod far enough out, the board should come straight up but will be held back by the multiple flexes.

Do the same thing as above for the fan switch - remove the covers and then fiddle with the board until it comes out of the groove and just sits there.

Third I removed was the large base board. Put your long, thin flat blade screwdriver behind it (back of the housing) and pull that up with a right angle pick or whatever. The front goes into two small grooves in the light bar.

Last, remove the bulb module from the front. Push the small black tab in the front on either side back and pull the board up with a right angle pick. Be sure to replace the bulbs as it's the easiest time ever to do it, they are "2721 1.2W 12V WEDGE CLEAR" without any plastic base.

Installation for all is the reverse, and in reverse order.

Mine had 6 electrolytic capacitors, all of which looked fine but I replaced them all anyway. The board is well marked but pay attention to polarity when you remove the old ones to be sure. The cap values were: 1x 47uf 25v, 1x 22uf 6v, 2x 4.7uf 63v, 2x 33uf 16v. You can buy an assortment on amazon that has around 700 caps (and included all of these values) for about the same price as you would buy the 6 caps + shipping from Mouser. Search for B0F2SM9B15 and look for the colorful assortment. Note that you can use any cap (that will fit in the space dimensionally) that has an equal or HIGHER voltage rating - so a 33uf 16v can be replaced with a 33uf 25v or 33uf 50v capacitor as equivalents.

The solder pads mostly looked fine but I resoldered most of the upper board and some on the lower. I looked for any telltale signs and fixed the ones that had too much / bulbs of solder, etc. And I resoldered all of the large power ICs (look like the bigger, vertical power transistors). I didn't do every solder point on the first pass because it takes a LONG time and I stayed away from the chips because they don't like heat that much. Basically, I hit all of the likely candidates that I could identify and figured if that didn't solve the problem I'd do the rest on a second pass. Best to use a solder sucker or wick and remove all of the old solder and resolder with fresh because it may or may not be a perfect match for the new solder you are using and the flux in the new solder helps ensure a good mechanical. Only desolder / resolder one side of a component at a time so it doesn't fall out but you can do long rows of them this way, one side at a time. When resoldering, heat up both the pin and the pad as briefly as possible and then touch the solder on both so you get a good connection.

Last, clean all of the contacts - I used a tiny squirt of DeoxIT Fader F5 in the temperature wheel rheostat/potentiometer and switch bars (squirt lightly on all components with the goal of getting most inside and run the switches back and forth for a bit and then put it together and then spin the wheel a bunch of times). I used DexoIT D5 on all of the contacts after cleaning them with contact cleaner and gently with a brass brush (I use a q-tip with some cotton removed doused with contact cleaner in the female side of the connector to clean them). If you don't have these already, D5 would work fine for either F5 or D5.

I put every thing back together and it worked great the first time so I didn't need to go back and reflow any more solder points.

Good luck!

jagboy69 · #2 06-16-2025, 01:04 AM

I sure could use some pictures here!!!!!!! I tried to get in mine push button unit on the 123 and I pushed and pushed on my buttons from the back and I swear they were glued on. In the end, I fired it off to the experts for a 75dollar overhaul.

Fingers crossed this fixes my issues. What was your solution for the monovalve?

Chris_87_300D · #3 06-18-2025, 03:16 PM

Yes, apologies on no photos but I have no way to host them (attempting to attach a photo requests a URL) and they disappear after a few years it seems so not very useful. I found other detailed DIYs, just none that still existed regarding the disassembly of the W124 (specifically, 1987 300D) climate control unit.

Buttons were pretty well mounted but they came off with pulling and tugging. It is just a plastic "pin" with a ridge or two that expands a bit inside the button's receptacle. Old plastic is hard to work on, though - always worried it is going to break irreparably.

I didn't have any issue with nor take any action with respect to the mono valve. My CC issue was intermittent fan speeds (that is, almost always very very slow) so no heat or AC in the car.

jagboy69 · #4 06-22-2025, 03:55 AM

It almost sounds like you have a resistor issue or your fan relay box is stuck or screwing up. Sorry I don't have a 124, but imagine it can't be a lot different than the 123.

My understanding of this asinine system is the temp controller decides what fan speed is needed and it tells the fan relay box which coils in the resistor to utilize. Something you could do is shoot the resistor and make sure you get different readings. Next would be to hot wire directly to the blower motor and see what it soulds like with 12vts. You could also pick a speed and read the volts at the fan. You can do this with it unplugged and measure at the plug. Just a DC motor, so less volts, slower speed. Hopefully on high, you'll read close to batt voltage there. My blower was getting loud so I pulled it to have a look. Everything under there was coated in black dust so I knew the brushes were shot. I bought a cheap uro blower. I did catch my system one day ramping up the speed when it was trying to reach whatever was set. Kinda cool I suppose, but honestly just an over engineered POS asking for trouble. I couldn't give a flip about auto personally.

Yes, posting photos here is kind of a pain. It really shows it's head when reading a 10yr old post with tons of holes. The photofuket fiasco really did a number on the internet. Hope the CEO of photobucket rots in hell for that one. Try hitting the go advanced button and look for manage attachments. Find your photo, hit upload, then close that window. It works, it's just the way vbulletin does things. :-/

Chris_87_300D · #5 06-25-2025, 06:05 PM

Hi jagboy69,

I guess I wasn't clear - I got everything working great on mine with the repairs that I did.

Thanks for the feedback on the photos - I didn't understand that. Unfortunately it's all back together now and so I don't have any photos to post... The only really tricky thing was figuring out that the plastic bar (axle) that goes through the temp wheel could slide out to the side of the potentiometer, freeing up the board for removal without removing the wheel.