N-channel mosfet turn constantly on after a while
You need to prevent back emfs from the washer motor destroying your MOSFET when it deactivates. The normal method is a &quot;reverse&quot; diode in parallel with the motor such as this (equally needed for a BJT as a MOSFET): -

This suggestion is made assuming that the motor is never operated in reverse - if it is then a slightly different arrangement is needed.