Lightly used Samsung SCX-4100 multi.

When powering on, LCD display says "Warming Up Please Wait" while the fuser
heats up. Meanwhile the scanner carriage does a quick "zero" just out a few
mm's and back, but continues to run the carriage motor endlessly, banging
against the zero stop until it gives up and puts up "Scanner Error" on the
LCD.

I got one good copy from it when it didn't do the "infinite zeroing" thing,
so I know it can work.

The scanner carriage is just a piece of tin with the plastic bulb/camera
holder, riding on a single rod. A toothed belt runs around the motor drive
gear and a spring-loaded tension gear at the end of the run. The belt is
simply "pinched" in a groove in the plastic bulb holder. When the carriage is
run up against the zero stop, the motor keeps going and the belt slips
through this groove.

I presume that this is the way it was designed to zero the carriage, just
running the motor long enough to run the full length of the bed and stop. But
it doesn't stop until it gives up.

There is no "home" sensor. No magnet, no opto, nada. Just a metal tab on the
carriage that bangs against another at the top of the run.

I tried holding the belt still, thinking that maybe the motor current is
monitored and when it goes high, that's zero. But the motor (a stepper) just
skips sync, and tries to keep going.

The under side of the glass has no marks that might be used for zero
indication by the camera.

I checked ground (it's tight) and for routing of the data cable from the
scanner carriage (it goes through its huge ferrite bead).

How do these el-cheapo models determine zero position?

Where might I look for a problem?

Yes, I know it's cheap. But if I can keep it out of the landfill and have a
simple document scanner, that would be great.

Thanks.