Starter motor does not work

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Starter will not turn over. I thought the starter was bad, due to good battery, and no prior problems with battery. Starter will not click or make any kind of sound, it is as though the starter had died. I pulled the starter out, took it to a parts dealer, tested the starter on their machine, and it passed all test and the starter moter ran. Re-installed the starter, still nothing, motor does not turn over, starter does not work. I replaced the starter relay, still nothing. What else could it be?