This is a module underneath the rear seat on my W124. The one rear window won't go down. My mechanic traced the problem to this unit and said the window and switch work, but this module is not grounding for that one window. All the other windows work.

Inside the control module there's a bunch of little wire relays. Quite something to see really.

This guy says my best bet is to find a suitable replacement.

https://www.youtube.com/watch?v=asJHWHYyI80

1991 300d 252,200 miles

Pretty good video.

I spent a lot of time on a few of those control modules.

I ordered about 30 of the same small relays from Europe.

Purchased premium solder sucker vacuum and soldering kit. I wasn't experienced enough to resolder the new relays. I kept damaging the board due to the very high heat loss into the thick metal of the new relays.

Ended up sourcing a few used control modules for spares plus live with one window that doesn't work.

Yikes, open frame relays in an automotive application!

To solder large items, you need an iron with enough power and heat capacity to heat them quickly and avoid heat soak. If a socketed relay is available it's often a better choice, and you can do a remote socket with "flying leads" as well. The leads will solder to the board with ease.

Doing this also lets you swap a special relay for just about anything with the same rating and coil voltage.

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1993 300D 2.5L Turbo

Did you get an answer for this? I have just repaired mine as one of the connections appeared to have burned out and I cleaned it up and resoldered across it.

On my 126 and 123 the same failure seems to occur in all the 40yo boards. The relays that handle current (like window and AC control) all have cracked solder joints. My guess is these are the ones with the big temperature stress cycles since they carry a lot of current. It’s the classic thermal stress cone fracture. I think the electronics was designed well but few things can put up with so many fatigue cycles after 40 years.

The good news is it’s easy to spot. Here are some examples. Just clean off old solder with a solder sucker and resolder.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles