Motor controller voltage dropping to zero when under load
The behavior you describe is typical for a current limit that is set too low (the voltage will be high as long as insignificant current is drawn, such as by a voltmeter, then drops under load).

According to the KBMD's manual, it has an adjustable current limit and an indicator LED showing when it is triggered, but you may need to remove the cover to see whether the LED is on (the manual does not say but it shows the LED being on the internal circuit board).

If you observe the indicator LED on when you attempt to move the motor, you need to increase the current limit trimpot setting. (Be sure to use an insulated adjustment tool and general line-voltage precautions since this is done with the power on.)

If this is not the problem (the CL LED remains off even when the output voltage is also measured to be zero), please edit your question to include the additional observations.



Note that your observations are also consistent with the motor being unable to turn — under that stalled condition one should expect a very low voltage and high current. You say you've tested the motor, but it might be worth also checking the motor current (using an ammeter in series) when it's hooked up to the controller. If it's zero or very low, the problem is with the controller; if it is high, the problem is with the motor.