So the question is for generic starters: I got a starter motor for maintenance work, but when checking its solenoid by applying power to its tab connector "only" (not attaching the battery fat wire to its corresponding pole), the solenoid engaged...and the motor spins!
AFAIK, the solenoid should only engage/release the Bendix mechanism, along to connect the two fat poles at the end of the run so the motor can spin. These motors draw a lot of current, that's why the need of a separate, fat wire directly from the battery. I seriously doubt the solenoid internal coils would suffice to support that current demand. So I opened the solenoid, no visual damages like shorten or stick contacts. I see, however, one - wire and two + into the coil. I suspect those are the "pull in" and "hold in" coils. But still I don't understand why it is energizing the motor in the first place. Any clues?
So the question is: aren't solenoids "solely" for activating the bendix mechanism and making the final contact to spin the motor? NOT to energize the motor itself?
The primary symptom I'm getting is that the starter engages/disengages repeatedly, or engages too weak. I suspect that solenoid got repaired or modified, so it makes the motor engage but has no enough wire thickness to keep the current demand and disengages before the main contacts are shut.
UPDATE: I realize this starter doesn't have the helicoidal grooves in the axle, that the bendix assembly would use to turn while being pushed towards the flywheel, so it can engage. This one doesn't have such thing, just a plain, flat, normal axle. Perhaps then it needs the solenoid to energize the motor while pushing the bendix?
FIXED: the problem was the hold-in coil within the solenoid. For some reason its ground wire got cut right in its spot-welded connection to the solenoid body. That's the why behind sudden and repeatedly engage/disengage symptom; it moved the bendix assembly but couldn't keep it engaged. I guess the spot-weld was too violent when manufactured, making the wire weak. My solution was to carefully scrap the thin wire and the spot weld, and solder them back together.