My driver's side electric windows not going down. I took out the switch and replaced it with another one and still it does not work. I tried a few switches and that window still doesn't work, while all the other ones do.

I checked the fuse and it was good but I still replaced it and still the problem persists.

Any ideas?

1991 300D, 213k

I suspect it's a regulator. You can buy replacements online, or - if you are near a junk yard working second hand units are a decent option.

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1995 E300 With:
OM606 Turbo
Merc 6 Speed Box
https://bit.ly/2sisie9

Does the right rear window work?

Front window is wired in with the rear on the opposite side. If your passenger rear is working but front driver side is not, it is something specific to the non-working door. The door card is easy to remove. Do a search or look in the FSM so you don't break any plastic tabs. They are brittle.

The wiring to the window regulator is accessible with the card removed. Test for current at the regulator. If good, you have a problem with the regulator - likely either the motor or the regulator itself. Motors quit working, regulator teeth get worn/broken off.

I'm too cheap to buy new and hit the yards. I can't speak to 123 fitment but anything out of a 126 will fit another 126 so long as yu get the motor and regulator from the correct door.

You can go on this site and look up the regulator and motor to determine the difference. There are different numbers of drive teeth on the motor and the drive gear is side specific.

Edit: buy some of the white slides in case you need them. There are 2 different parts for the 126. Check to find out whether the 123 is similar.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

They all work except the driver's side.

Sorry, I misread and thought 123. Likely applies to yours.

Why would you go through all the work of going to a junk yard, taking an old one out, and putting a used one in your car, hoping it will stay working long enough to make it all worth while??? You get a new one for $36 and you're set for the next quarter century.

I wouldn't if $36 is the price for regulator and motor or if I had examined and determined that $36 would fix it. Regulators for earlier cars are significantly more and have motors specific to the regulator and position on car.

There are several failure possibilities some caused by lack of lube. Having spare parts makes the process easy. I try to keep spares but clean the slides whenever the card is off and haven't had much problem - lately.

I grab monovalves, door checks & a few other things routinely. Haven't seen many diesels around here.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Before you get ahead of yourself and buy a regulator, you need to check the wiring:

Remove the door card. Somehwere under the card, there should be an electrical distribution junction with two connections. Do the following with the key in the run position:

1) Using your DVM on voltage, verify that both terminals have 12V+

2) Have someone hold the switch in the up position. One of the terminals will still have 12V+, the other not. Switch the dvm to measure resistance. Measure the resistance between the cold terminal and a good ground. Should be a few ohms at most.

3) Repeat step two with the switch in the down position. The opposite terminals should be 12V+ and ground

If any of these tests don't pan out, the likely reason is that one of the wires is broken, probably in the flex conduit from the A pillar. If all the tests are ok, go ahead and replace the regulator.

'Just like the W123 s before them the W124 s are/were subject to wire
failure at or near the door jam.
"Cyclic Flex Failure"
I.E. Open / Close (X s the Umpty Zump Gazillion times the door operated)

"Tinned Marine Wire" of the Most Conductors Available (The Most Wires)
should be used to repair in the failed area.

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'84 300SD sold
124.128

If it's the wires at the door jamb, can I just cut into the wire protector, fix the wires, then fix the wire protector, rather than taking off the entire door panel?

First of all, you can't diagnose the problem without taking off the door card. If the problem is a broken wire, then you should try to keep the flex conduit intact, because it's designed to keep dust and water out of the door and the A pillar. It's not all that involved to snake a replacement wire through and solder both ends. But you need to do your homework first.

My favorite replacement for the stock wiring is silicone covered high strand count copper wire. It's sold as ignition wire in hobby stores, or you can order it "direct":

BNTECHGO 18 AWG Silicone Wire 7 Colours Each Color 3 feet 18 Gauge Copper Wire