I'm trying to figure out why and fix the cause of the passenger seat not moving back when using the electric switch (1987 300e/W124). All other functions work as they should.

I have pulled the switch, and it seems to work properly. That is, the resistance falls to zero for all of the switch positions between the appropriate narrow pin and one of the fat ones (presumably these are power). Yet when I try to jumper the connector in the door, nothing seems to happen for any of the "switches."

Do I understand properly how the switch works? If so, why did using a jumper not work? It would seem that either the connector or the wiring from the door to the body has failed. Is there a straight-forward way to figure out which, and which wire it might be?

It's part of the FSM (Factory Service Manual)

'Cause without Schematic (wiring) info,You be Lost!

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'84 300SD sold
124.128

Indeed. Is the FSM available easily on line?

BUT,
http://www.mercedesmanuals.com/Mercedes_Benz_W_124_Manual_p/w124.htm

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'84 300SD sold
124.128

Ugh. I suppose I ought to have one, and $50 is not unreasonable. I had hoped that there would be something "good enough" that would be available for free.

to see if it is the switch.

Also, the wires in the door hinge area will fail over long time use (constant
flexing).

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'84 300SD sold
124.128

I tested its function with a multimeter. It works as it is supposed to. What would swapping accomplish?

'Always trying to eliminate variables.

I guess next step(s) would be:
Ohm-ing out the individual wires.
Seeing if the Seat motor involved responds to Direct Current.

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'84 300SD sold
124.128

Right. I have the three connectors on door end. Where do they attach to the seat? Checking them is then easy enough, since they have a different number of wires each.

I checked my diagrams. On the switch it is 2 wires, one is violet one is blue, on the switch or connectors it will be numbered #1 and #2.
The color codes carry through all the way to the motor under the seat. There is a connector under the seat called X55/1. The connecotrs for the fore/aft is #3 and #4, again same color wires, from this connector it goes to the fore/aft motor.
Gilly

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Thanks, Gilly. I'll check wire continuity later today.

To close out this thread, I did finally get the seat to work as it should. It turned out to be the switch, my earlier measurements notwithstanding. A couple of spritzes with contact cleaner took care of it.