Is your battery fully charged? If your battery is on its last legs and/or is severely discharged you may be putting a huge strain on that alternator you just installed. If it is a rebuild, and usually is the case, this may be enough to nearly wipe out the diodes.Be sure and charge the battery with a charger overnite before using the alternator.

Also, are you disconnecting the battery? Arcing, even slight, can damage the diodes and regulator.

I have seen both of these situations, and personally experienced, damage the alternator.

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Sam

84 300SD 350K+ miles ( Blue Belle )