i just spent today replacing my drvr door motor[87 420sel]. but before i installed the new one i played a bit to see how the motor worked.
what i found was the motor on my 126 would turn in either direction as long as the switch was on. so that tells me that these motors do not have any limit switch or a switched shut off. what i believe is these small 12v motors torque out. there is a physical mechanical stop on the regulator assy for the down position and for the up is the window's door seal itself. i see this torque out/off in valve automation when electric motors have been undersized for a specific application.
what i have also seen on DC motors is they lose strength or power over time. all of my CNC machines have DC motors on the table and quill. in fact just had one go out on friday. likely it was only a diode but every so often these motor need to be repaired by the factory due to loss of power.
after all this said i'd look at the motor for a torque shut down. i would think if the motor begins to move then stops but you can make it go back in the opposite direction the motor is seeing more torque required than it can produce.
i would look at issues like... is the window moving freely up and down the tracks. the window could be getting wedged or it is shifting around and not moving evenly up or down. so... if it is not a mechanical issue then you only have the motor to point your finger at. my motor had 12v plainly displayed. one last thing is to get your vom meter and measure the dc volts at the motor's wiring.
1972 350sl Red/Blk 117k
1988 420sel charcoal/Blk 140k
1987 420sel gold/tan 128k
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