Ignition system can also cause this . But just to be sure have the alternator check for ac voltage ripple . Ripple voltage is when the DC voltage varies between (for example)12.5 Volts and 13.5 Volts
Question about 2006 Sierra
...its back to 14.2 You got a bad regulator in the alternator,maybe Also check diode leakage (current ripple) & grounds Those alternators have been rebuilt & you still have other peoples old ...
...I charged the battery and took it to advance auto to check the system and they told me there was a ripple in the alternator so I replaced the alternator, the whole time the voltage in the truck is ...
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