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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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I bought a switch for the wipers on my 39 CE that operates both wipers and parks them at the same time....perhaps this will work for you. I got it off ebay I think.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I might have got the wrong end of the stick ('cos electrickery really ain't my thang) but could it be that the motors are wired in series and so you've essentially got two great big power drains / resistors that make the motors run slower together?

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I don’t have an answer but how is the “S” terminal wired?

My dad was into boats in a big way. He had several older ones over the years. They were all 12 volt DC. Fwiw, your comment implies reasoning about why his boats always had 1 switch per wiper motor. He didn’t install them but it was a consistent feature over 5 boats that had wipers.

I’d guess that you’d run a positive wire to each motor with a fuse on it and share a ground. Maybe the "S" terminal intended as a ground between the motors?

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Toyota is a leader only because their drivers block everyone behind them. Oh what a feeling.

I'm guessing that they work like an automatic radio antenna. but with positive ground...+ is ground, - is 12V always hot, and S is 12V - switched. When current stops on S, then power from - is still available to park the wipers.

Haven't had a chance to work on it. Will get to it in a few days. The new motors say they should be switched on the ground side.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

To close this out: Finally had a chance to reinstall the starboard motor once I cut the shaft to the right length. They both worked correctly once I did. Odd.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13