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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.

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The reason is that this is called “low power indexing” and is to reduce tooth to tooth abutment.

How it is done is that there are 2 windings in the solenoid: the pull-in winding and the hold-in winding. The pull-in winding is earthed by going through the starter motor, the hold-in winding goes directly to earth.

The 50A or so for the pull-in winding is sufficient to cause the armature to rotate gently as the solenoid operates.

EDITbased on comment: As the solenoid moves due to the pull-in winding then it eventually operates the main contacts, which changes the solenoid from the pull-in winding to the hold-in winding and supplies full power to the starter motor.

If those main contacts fail then the starter will not turn the engine over.

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  • but, if the main contacts are not sound the motor won't have enough juice to really crank the engine, and then the solenoid circuit may get the beating?
    – Aram Alvarez
    Jan 11, 2020 at 15:03
  • @Paulster: well in this solenoid (seems to be for a classic Lada starter) both pull and hold-in coils get + from the exciter connector; the pull-in seems to be in series between that pole and the + pole for the fat wire. The hold-in coil gets the other leg to ground. So both coils gets + at the same time and all the time while operating.
    – Aram Alvarez
    Jan 11, 2020 at 23:39
  • @AramAlvarez so if the pull-in winding is fed 12v+ from the exciter terminal and 12v+ from the fat wire (using your terminology to keep it simple) then since there is no voltage difference it will not produce any magnetic field or consume any power.
    – Solar Mike
    Jan 12, 2020 at 0:19

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