Most likely a pulsecoder on one of the axis. Hard to find, however I can give you one tip.
Make a AA battery holder (4) for each drive unit and unplug the battery connector from the Dcell box on the drive. Plug in the AA holder for each axis and see which one is draining first. You can also measure the current draw on each amp from the battery pack, the problem is you would need a VERY low range amp meter to track it down that way.