I am still working on the electric seat controls for my 1982 300SD. Upon getting a good connection, the power leads, both the red and the red/white had voltage. I have continuity on every wire from the switch to the seat connector. Before installing the seat, I tested every fuction by applying 12V to each wire pair, and the reversing polarity to verify reverse function.
Sill, I have no function from the rear half of the switch--the rear rise and fall and the recline.
What have I missed?

Have you taken the switch apart and cleaned it??

Theres a good post on here from donbryce that details it.

Did it to both of mine and they work perfectly.

__________________
1988 W126 420 SE

beware of fundamentalists!

Yes, I have cleaned the switch. I have even purchased an extra switch from a junker, and cleaned it--just incase there was a break in the circut--with no change in function. I am getting better at disassembly and reassembly of those switches than anyone should be.

since you have power and assuming your motor is ok, it could be the black covered plastic hex cable(from seat motor to gear) has stripped and is turning freely. try pushing on the cable as you activate the switch to see if it will catch.

__________________
1993 Mercedes Benz 300D 2.5 205K (ex wife's)
1984 Mercedes Benz 300SD 320K (SOLD)
2004 Mercedes Benz C240 75K
1995 GMC Sierra 2WD 5.7L 188K
1983 Mercedes Benz 300SD 239K (SOLD)
1987 BMW 325i 220K (SOLD FOR SALVAGE)

609 Certification

Thanks for the idea, but there is simply no motor function. I may wire a secondary switch just to prove to myself, again, that the seat is OK.

Did the motor run out of the car? I'm just not sure from your OP?

Ifit did and doesn't when you put it back into the car there has got to be a break in continuity somewhere (stating the obvious - I know).

Rear height and recline are 2 different motors - it would be odd, but not impossible that 2 motors are blown.

__________________
1988 W126 420 SE

beware of fundamentalists!

Yes, it did run.--all 4 motors.
I have continuity from the switch to the seat connector.
I show voltage in the red/ white wire to the switch. But I am beginning to wonder if I am getting sufficient current to run the motors. I may run a temporary jumper to that red/white wire right near the switch and see if that works.
The frustrating thing, is that everything seems to test OK, so I must be either overlooking something obvious, or "assuming" something I shouldn't.

OK, I am now convinced it must be the switch.
If I jump B+ and B- to each pair of contacts, I can make each motor function, and reversing the leads, reverses the motor, Therefore the wiring from the switch to the seat must be OK.

I have 12volt (B+) at the red/ white wire. If I check the terminals on the back of the switch with the 6 wire plug connected, and the 5 wire plug disconected, As I operate the recline, and the rear rise and fall, I get 12volt (B+) at each of the 4 terminals--I never see a B-.
I have checked continuity all along the ground brass strip inside the switch and can find no problems or breaks.
I have opened,and cleaned the switch about 6 times. I have put it together dry, and with dielectric grease everywhere, and with dielectric grease only on the top balls to keep them from falling off the springs. I have not returned the brass wings to the same slots, and yet every time I reassemble it, I have the same function--front half OK; rear half--dead.
Can it be anything else BUT the switch?

I changed the switch.......no change. I have cleaned and reassembled the original switch at least 6 times. I have substituted two other switches, both just as they came form the junk yard, and again after cleaning. No switch makes any difference in the seat operation.
I give up. I will be hard wiring two auxilary switches to control the rear rise and fall, and the recline.