B737-NG windshield wiper uncommanded motion
 

A few days ago, in climb phase, I noticed the right wiper began to move without me or my cpt touching the related switch. Just a couple of sweeps and then it stopped again.

Has it happened to anybody else? What could be the reason?

PG

pax in the back running pprune on their phone

There's not much in the system:

CB, control panel and combined gearbox/motor/control circuit (behind the wiper)

The CB provides power to the wiper motor, but a second variable voltage supplied by the control panel to the the wiper motor assembly controls PARK/INT/LOW/HIGH.

For some reason, a stray control voltage is getting to the motor assembly. There are a few diodes in the control panel, maybe they were breaking down and affecting the output voltages from the control panel. Or maybe the contacts of the switch were faulty. The panel is the easiest to change. Given that the fault was very intermittent and unlikely to be faulted on the ground, the engineers would have probably changed that as a first course of action.

If the same as a car wiper, there is a home position switch. In this position the power must be applied by the windscreen wiper operation switch/circuitry to move the motor away from the home position. Once it is away, a constant supply voltage runs the motor to allow it to sweep return to the home position.

Thus when the wiper is away from home position, turning it off will not stop it operating until it returns to engage the home position switch.

If the home position switch is faulty, the wiper could be able to run by itself continuously.

I've had the wipers do an occasional sweep when transferring power from APU to engines or visa-versa. We chalk it up to stray electrons.