240JOE,

The failure mode is: As starters get older, there is current leakage in the armature and field coils thru the insulation. This leakage current decreases the flux of the electro magnets in the starter, and causes the starter to pull more current yet crank the engine slower.

The best way to test a starter is to lock the rotor, apply a certain voltage and measure the torque and current draw. These specifications will be listed in a starter repair manual. I don't know how AutoZone tests starters, but if they don't do a current and torque test, I would be be wary of their results.

To be a "rebuilt" starter, the armature should be replaced with a new or rewound armature and new field coils installed if required. Then it will be equilivant to new. Otherwise it is just an overhauled starter and may not be much better than the used one.

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