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I just replaced an old starter in a friend's truck, and the replacement was bad as well. Worked fine when the engine was cold, but dragged like a weak battery when the engine was hot. Took it back to the store, and was ready for an argument when they bench-tested it and claimed it worked properly, but fortunately, their testing machine was down and they didn't have any others, so I got my money back and went to a different store, which used a better rebuild house. That one worked okay.

Got me to thinking, though - what exactly is wrong with the starter in such a case? I've known about this for years, but don't know exactly what is wrong. The resistance in the armature windings goes up as the motor ages, and rebuild houses (as far as I know) only replace brushes, bushing and similar parts. I know of nobody who actually rewinds the armature.

Would that help? Is it the metal in the copper wires on the armature that somehow ages? Is it the iron in the armature? Both? Is there a way to fix it? Anneal the armature? Anealing hardens steel, but softens copper. What is really wrong in an electric motor with high resistance, and is there a way to fix it?

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    Why do you think the armature increases in resistance? In general it is only wear in the brushes and the commutator that cause problems. The commutator wears down so the insulation between segments can prevent the brushes making good contact. It is machined and the insulation undercut to restore operation.
    – Kevin White
    Mar 10 '19 at 2:18
  • Because the resistance is high even after the brushes are replaced. It's a classic sign of a worn-out motor - spins fine when the engine is cold, but spins very slowly when the engine heats up. The resistance in the starter goes up as the temperature goes up, as is the case for pretty much all conducting metals. In a good starter, the resistance is still low enough for the battery to turn it. In one that is old, the resistance is so high that the battery can't push through enough current, making it sound like the battery is dead.
    – Pete Danes
    Mar 10 '19 at 2:32
  • People often replace the battery in such situations, thinking they're fixing the problem, and sometimes it even does, temporarily, because the new battery may be stronger than the old one. But the problem is still the starter, and inability to crank will soon return.
    – Pete Danes
    Mar 10 '19 at 2:33
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    as I mentioned a common reason is not that the winding resistance has increased but that the brushes are not contacting the commutator well. There is an insulator between each commutator segment and it wears differently than the copper segments - if the copper wears below the insulator the brushes cannot contact properly - it may seem as the resistance has increased.
    – Kevin White
    Mar 10 '19 at 2:43
  • That is something that can happen, but that is not what I am talking about. If that was happening, the starter would not act in the way I have been describing. In fact, it would act the opposite way. Metal expands when it gets hot, but the composite between the commutator segments does not. A hot starter would have the commutator segments expend and be MORE able to contact the brushes. I repeat - a starter in such condition functions normally when cold, but cranks very slowly when hot. When allowed to cool down, it functions normally again
    – Pete Danes
    Mar 10 '19 at 3:21

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