GM-GMC: GMC Duramax Charging system, light glows, ac delco

Question
I have a 2003 GMC Duramax that had an alternator light that would glow very dim at times. It was so dim that you couldn't see it during the day. This happened on and off for about a month (charging fine). The alternator went out and would only charge if you had the RPM over 2500 and then it quit charging all together. I replaced the alternator and now it charges fine but the battery light glows dim at times again (real dim). I am afraid that this may have been what caused the original alternator to fail. The battery light glows dim sometimes even when the engine has been shut off and the key is out of the ignition.

Answer
There are a number of posibilities.  One being, the replacement alternator is faulty.  I have seen rebuilt alternators from parts stores that were junk right out of the box.  The second possibility isn't quite as simple, there is a technical service bulletin which identifies similar symptoms to what you are describing and it names the Instrument Panel Cluster as the cause.  The diagnostic procedure for it is to simply install a scanner and observe the ignition1 voltage, it should be between 10 and 15 volts.  If it is, and the light is still on, the IPC should be replaced.  That is neither cheap or easy.  I would double check the alternator if you didn't install an OEM or AC delco.
Hope this helps