Power seat problem
 

I have a 98 ML 320. I had a problem with the passenger power seat the other night. Seat would move forward but not backward. I kept fiddling with it and wound up with the seat all the way forward and it would not go back. I could hear the switch clicking but the seat would not move. Finally something caught and it started moving back so I moved it all the way back. My question is does it use relays to activate the seat motors? If so where are they? If not is everything controlled from the switch on the side of the seat?

There's a module under the seat, a black box, that controls all movements. You may have a bad motor.

Actually more likely a bad controller. The motors are generic dc motors (series wound with brushes and a commutator) and these are reversed by flipping the polarity of the applied voltage. This is typically done with an “H-bridge” circuit. (Google it). If one of the switching elements in the H bridge is bad then you will get motion in one direction but not the other.

If the motor was bad it would not move in either direction.