Starter woes! (Pics) (Solution Found)

[fspGTD]

I believe that terminal 15 controls the activity of the "start" coil portion of the solenoid.  There are 2 electrical coils inside the solenoid: "start" and "hold".  The "hold" coil gets energized whenever the ignition switch terminal (terminal 50) gets positive voltage.  The "start" coil only gets energized when terminal 50 gets positive voltage and there is a ground path present through "terminal 15".  When the electric motor is not energized, the "start" coil circuit travels out of terminal 15 and through the electric motor to find ground. (or at least I've heard is the case on another application of bosch starter solenoid, although ours appears to operate the same.)

When the solenoid electrical contact switch bridges positive voltage to the solenoid output terminal, this puts an end to the "start" coil's ground path, and current stops running through the start coil and at that point the only coil current keeping the solenoid in it's "bendix drive engaged, contacts closed, electric motor running" position is through the "hold" coil.  The solenoid apparently needs extra overall coil current to move the plunger against the spring, enegage the bendix drive, and close the electrical switch than it does to just keep it held there.

So it makes sense that a lifeless solenoid could be fixed by re-connecting the "start" coil ground path (terminal 15) to the solenoid output terminal.  Nice work!

PS - No, I didn't buy a new solenoid... I just swapped a good used solenoid out of another (bosch A2 diesel gear reduction) starter motor.