my job requires repair of machines that have stepper motors. I have come across a few units that have weak steppers... I.E. they stop spinning under small load and just make a weird noise, reset and start again. I have taken the motor apart, cleaned and inspected, all looks good. i suspect the power supply or motor control circuits on the motherboard are bad... How can i diagnose my problem? There are multiple wires connecting to the stepper, can i just check for voltage? how to know which ones?