[Al Skierkiewicz]

It is entirely possible you have either killed a motor or a speed controller. Listen for a clicking sound from you robot. That is one of the breakers tripping. It should give you a good indication of what branch is at fault. Another method is pull all breakers feeding the drive. Then put them in one at a time. You will likely find one that is causing the voltage condition. Also check that someone has not made a wiring change on one of the motors. It is entirely possible to have shorted a controller output if wiring changes were made. It is also possible for something to be locking up the transmission. Check that the key has not slipped out of the pinion gear.