Request for Help trouble shooting Advanced DC

[Cal Schulz <cal.val.schulz@internetMCI.com>]

Hi all,

I  have apparently had a sudden major failure of the 9 inch Advanced DC
motor in my pickup conversion.  I'd appreciate your ideas or
confirmation of my diagnosis before I take steps to get it repaired or
replaced.  The best way  that I can describe the problem is that the 
motor rotates in "steps" of about 90 degrees.  At first I thought the
problem was mechanical, like a failed clutch or coupling as the pulsing
is enough to shake the entire vehicle.  However, all the drive line
components can be rotated smoothly by hand.   Then I suspected a weird
controller failure.  I detached the controller and applied power from a
single battery directly to the motor.  Unfortunately the "stepping"
behavior returned,... the problem has to be in the motor, yes ?  The
brushes and commutator appear fine to me.  I've tried to make resistance
checks to see if an armature winding has opened up but can't seem to
get  consistent readings.  Anybody have any tips on how to isolate the
problem further or can tell me what the problem is ?

Thanks in advance,
Cal Schulz