I have a gen 2 (1991) 350SD that is giving me an issue with the rear of the seat bottom not wanting to move down.

It has moved up all the way to the limit but now won't move back down. I don't know how to apply power to the motor directly because there seems to be a limit sensor system that does not seem to allow direct power for testing. Not sure if it is the switch or something in the motors and not really sure how to test it because of the limit sensors.

Has anyone figured this stuff out?

1991 W126 should be electrically similar to the W124s.

The memory seat on these cars is driven by a relay box under the drivers seat. The door switch sends low level signals to the relay box and the relay box does all the switching.

If the seat goes up but does not come down that means the motor is good. It's the same motor and connections, the controller flips the polarity of the applied voltage to make the motor go up and down.

I suspect the door switch is bad. I would try searching for threads on here, the switch is rebuildable. If you try to search in the parts yards be careful, there were 3 variants of the switch.

1st = direct power to the motor style, many thick wires on the back of the switch and no control box under the seat
2nd = early version memory switch, many thin wires on the back of the switch, and a control box under the seat - this one has a dedicated wire for each switch function down to the controller
3rd = multiplex control memory switch, only a few thin wires on the back of the switch down to the box under the seat. This one uses electronics (different resistance values?) to the control box to tell it what part of the switch was moved.

Then you get into the W210/W140 CAN bus based system, whole different animal.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech