This weekend I had to replace the starter on my yellow car. Before I describe the problem, here's some basic starter info.
There are two wires that connect to a starter. The smaller wire energizes the solenoid when you turn the key to start. The larger wire turns the motor after the solenoid engages and closes a switch inside it.
Now here's the problem I had. When I turned the key, the little starter gear would spin against the ring gear, making a horrible whining noise. Needless to say, I didn't run it that way for long.
I took out the starter and hooked a battery charger + wire to where the small wire connects, with the - wire connected to the starter body. The solenoid engaged as it should, shoving the small gear forward. But the motor also started turning, which it should not have, because no voltage was applied to where the big wire goes.
This happened a few days ago, and the replacement starter, which I bought off Ebay, arrived this weekend. I did the same test with the battery charger, and the solenoid engaged, but the motor did not turn. So I put it in the car, and it starts just fine.
I have no idea what happened in the bad starter to cause the motor to run with only the solenoid voltage applied, and I'm not curious enough to take it apart and find out. I'm just glad my car starts again.