Bench Testing the Wiper Motor
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
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
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.