This question applies to the wipers on my Albin 25 boat. It has two separate wiper motors wired to one switch. Three wires on each motor + - and S. These motors are typical hot rod style simple wiper motors with one shaft. They haven't worked right since I got the boat. If everything is wired up correctly both motors just oscillate a few turns in each direction, never turning the wiper shaft more than a degree or so. If I disconnect the port motor, the starboard motor still oscillates. If I disconnect the starboard motor, the port motor works normally. If I reconnect the starboard motor while the port motor is working normally, sometimes both return to oscillating and sometimes both work normally. If both are working normally and I shut them off and restart--they both just oscillate. I bought two new motors and wired in one new one in place of the starboard motor. Both motors still oscillated but for much longer periods in each direction, turning the main shaft about 10-20 degrees before switching directions. I'm mystified.
The new motor wiring diagrams show they should be switched on the negative side. When I hooked the other new motor up to a battery using the + and S terminals it operated fine.

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