I have a 2005 GMC with a 5.3 vortec engine. The alternator failed and I replaced it with a used one out of a previous GMC I still own. It would not charge more than 12.45 volts below 1000 rpms. At over 1000 rpms it would charge between 14.2- 14.7 volts. I then replaced with a new alternator. Still having exact same issues. Any suggestiona anyone???? I tested both positive and negitive battery cable with voltmeter. And it they show no resistance. Both below .02. I'm at a lost.
Have you fitted the correct pulley, ie the original from the one you removed? The pulley size is crucial to get the correct speed so the alternator provides sufficient output.
However, the readings you suggest seem to be correct, at idle the alternator is usually capable of just enough to put the warning light out. The reading you quote for above 1000rpm also seems to be correct.
Have you measured the current output, this can be more challenging due to the control systems.