Why won't my power seat control work right?
 

On my 1991 W124 300d, the seat control button for the seat back moves fine, and will alter my seat back more upright, but it won't let it back down a lot of times.

Originally, I thought there was a problem with the control, I took all the stuff off to get to the switch, but ..... the seat back switch worked perfectly fine.

So I thought it must be the plastic cover piece that doesn't have a big enough slot for the control to move like it's supposed to so I drilled out a bit of the plastic to make the slot bigger. That did help some, but the same thing keeps happening. I drilled out more of the slot, but still the problem persists.

So I was thinking maybe temperatures are expanding and contracting the trim pieces around it or something but the other night I was driving home, temps were 75 degrees, I hit the button to tilt the seat back to a more reclined position, and ..... same problem. Wouldn't work. Next afternoon, about the same temperature, it worked fine. Maybe it's different humidity levels.

Any other ideas? Could it be that with different temperatures (the car interior has been hot all day and cooling down at night) that maybe the trim pieces are compressing the switch a bit too much, holding it in too tight or something?

At least with the w210 models, this is a common problem, and it's almost always the control switch. The w124 switch is very similar in design and construction to the w210 switch, so that is the first thing I would suspect. Yes a failing switch can exhibit intermittent behavior due to the reasons you've noted, temperature, humidity, etc.

The seat control switch is very similar to the window switches (copper rocker moved by a ball bearing) and is subject to the same corrosion. I took one apart and carefully cleaned all the contacts, applied some fresh dielectric grease, and it was good as new. This is a rather tedious

Those controls are about the best ever designed and can be cleaned and adjusted to last almost forever. Most likely you just have corrosion and need to steel wool off the contacts and then put it back together.