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Hi guys I have some alternator problems my first alternator was replaced at 50k miles at gm dealer my next alt was a car quest ppv alt at 95k miles by now I am suspecting that they are going bad becouse of my 25% underdrive crank pulley so I replaced it with a 10% underdrive about two months later the alternator failed a few weeks past a year so I get a gm ppv alternator start the car and no charge engine running I get 11v at the back of alternator and with the 2 wire connector unplugged I have 5v at feedback wire and 11v at lamp so I believe its defective
But why am I going threw so many my battery is 3years old interstate agm and tests solid no issues
 

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Aftermarket alternators are notorious for being problematic with the G8's charging system. Always replace with a new genuine AcDelco unit. Check the battery and alternator connections. Wouldn't hurt to check the battery ground bolt, https://www.g8board.com/threads/battery-ground-bolt-read-and-check-yours.224746/. Does the serpentine belt have excess slack? Any sort of additional load being placed on the charging system? Are the replacement alternators new, used, or remanufactured?
 

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Normal alternator output is between 13-14 volts. What symptom(s) is the car exhibiting to be on the fourth alternator?
 

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Well, if an underdrive pulley enhanced the performance of a G8 it would be on there from the factory.

The pulleys are designed to be a particular size to optimise the performance of that accessory. As you suspect it is the pulley, put the standard one back on and a genuine GM alternator for that vehicle.
 

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Have you replaced the battery? If the battery is dying and having a partial internal short, that would put an extra load on the alternator and burn out the rectifier in it. Burning it out faster when you have less underdrive (i.e. alternator producing more power) points this way too. So before you replace the alternator again, replace the battery.

Sometimes even new batteries die fast. Some 15 years ago I've had a Sears battery go bad within 1 year and take the alternator with it (stopped buying batteries from Sears after that). The battery wasn't even completely dead yet, just self-discharging at a quick rate.
 
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