Originally Posted by jay_bob

Remember everything on the interior of the newer cars (W210 and up) is controlled in one way or another by CAN bus.

If you have a diagnostic computer it should be able to see which modules are not participating. Also you have to check for sensor errors in the seat motors.

In order to move the seat back, the following has to happen:
You push the switch that slides the seat back
The door control module reads the switch movement
The door control module sends out a CAN message “move the seat back”
The seat control module listens out for the CAN messages involving that seat, amongst all the other traffic on the bus.
When the seat control module hears the message “move the seat back”, first it checks its known position, and also evaluates where the rest of the seat elements are known to be (from the feedback encoders on each seat motor) to know whether it’s safe to move.
Only now does the seat control module actually put voltage to the motor to move the seat.

I have first hand experience with this on my W210. The drivers headrest would randomly stop working. Turns out the position encoder in the headrest up/down motor would stop responding intermittently. When the seat controller lost its position information on the head rest, it threw a code, and locked out any further movement of the head restraint until it was reset using HHT emulator in Xentry. Replaced it with an encoder/motor assembly from the JY and it’s working as it should again.

Another one. I had on that same W210 a failing left rear door window motor with integrated controller. It would report CAN bus errors and one line of the CAN bus shorted to +. Once I disconnected the CAN bus lines one by one at the junction I was able to determine that the CAN bus fault was being caused by that door motor. So I went to the JY and got a left rear window motor. Well when I opened up the door on the JY car the regulator had failed (no surprise) and the cable was all balled up around the spindle. Got the motor out, got home, and installed it on my car. Tried running the window up and down with the new motor but it would only go up and not down. So I got the computer connected and turns out the controller had sensed the jam and inhibited the motor from running down anymore. Once I cleared that code the motor ran both ways.

Long story short is that it’s pretty much impossible to troubleshoot one of these cars without a computer. But once you have the computer it will lead you right to the problem.