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4g Alternator noise after rebuild

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I decided to rebuild the alternator on my 2000 Sable Duratec at 100k miles. The alt. was functioning perfectly, but after reading about problems with the thin walled slip rings wearing out, I decided to purchase a kit and rebuild it. The kit (from Performance Starters) included a new rotor, bearings, and voltage regulator&brushes. Every thing works fine, but there is a whistling noise, similar to a vac. leak. The noise is only present at idle and disappears above 1k rpms. I traced the noise to the alt. with a small hose for a stethoscope. It is not the noise usually associated with bearings. That leaves diodes as a possibility or maybe just air flow through the alt. Since it only occurs at idle, I am stumped. Any ideas?

Berry

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A single bad diode will make a "whine" high pitch mostly at idle. Higher rpm the whine gets above the hearing range. Small decrease in maximum output.

-chart-

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Thanks Chart. Since the noise was present before the rebuild, I must have damaged it. No good deed goes unpunished. I am not looking forward to removing the alt. again.

The original regulator was black and the replacement was gray. I questioned the vendor, but he indicated that wouldn't cause the problem.

Berry

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