Bench Testing the Wiper Motor
John Grady

August, 2019

When bench testing a wiper motor, it is critical that you ground the wiper switch housing. Without the ground there is no field magnetism and you will burn out the armature.

To properly bench test a wiper motor, run 12V to the hot wire, put a ground on the motor and a ground on the switch. You must ground the switch too because the return current of the field is through the switch ground and the dash metal, not through the motor. This is how the motor reverses to park.

With no field magnetism the motor may not spin. It just sits there across full 12V on the armature and with only the DC resistance of the windings to limit current and it cooks. This will not happen when installed in the car because the field is correctly grounded through the switch housing mounted to the dash. Further, the circuit breaker will click open if it does not spin.

I know about this as I was running a wiper motor on my bench and saw the ammeter on my power supply go through the roof. This led me to trace how it works, which is not obvious. The switch reverses the two field wires so it runs forward and backward (to park). After the reversing or the forward connection is made in the switch, by you shutting it off, it grounds the low side of field 12V, one negative wire right at the switch, not at the motor.