T-N-T Motor Diagnosis

[Oldslowandugly]

I obtained an unused OMC trim-and-tilt motor--that is dead to the world. I tested the armature for shorts and found none. The brushes are new and have no shorts. The armature shaft and motor case show continuity. The commutator was kind of dark, maybe from age, so I polished it to bright. I tested all the commutator bars, no shorts. Battery negative goes to the motor case for ground. There are two battery positive wires, each one goes to a brush. I get continuity between both wires. No short to ground. Applying battery positive to one is supposed to cause the motor to turn clockwise. Applying battery positive to the other should cause the motor to turn counter-clockwise. This determines if the tilt ram goes up or down. Neither work. What other tests can I do?

[Oldslowandugly]

OK, I got it. After researching the net about small DC motors I realized the OMC factory manual left out something very important. The two wires going to the brushes are just like any other DC motor. One is positive, one is negative.

To test the motor, the book said to apply battery positive to one wire or the other- but never mentioned that battery negative must go to the other wire. Even the schematic in the factory manual leaves this tidbit out.

Here I thought the case was the ground and each wire determined rotation. The way the motor reverses direction is controlled by the relays. The UP-relay sends battery positive to the blue wire and negative to the green wire. The DOWN relay sends battery positive to the green wire and negative to the blue wire. To test the motor you apply 12-Volts positive to one wire, and negative to the other. Then reverse the connections and the motor reverses direction. Beautiful. I spent all day doing this when what I needed was refresher course in DC motor 101.

[fno]

Did you get the motor to turn?

[Oldslowandugly]

Yes, it works fine. Put the motor back on the manifold and it extended and retracted the ram nicely.