I have a 2003 LB7 Duramax and have been having electrical issues for a while. I just installed 2 new batteries and a new stereo(professional install). While driving I noticed that the Volt meter barely got to 13 and the last day it hovered about 12.5. Well after sitting for 2 days I went out to start it and it was totally dead. Took a reading and the battery tested at 2.7 Volts. Got it charged to about 12 with a jump and put the charger back on it and its slowly charging. In the meantime someone suggested watching a video on Youtube that said to test for drain by the alternator by using a test light. I hooked up the clip to the ground bracket and poked the probe into the wire from the alternator to the Red jump block. The test light lit up immediately. I disconnected the charger and did the test again and had the same result. So is this definitely a sign the the alternator is drawing power when the truck is off? Also where do I go from here? I did check the voltage drop on the stereo fuse and there was none so I dont think the stereo is the problem. Any suggestions?

 

Take your charge wire off, put a test light in line, clamp on the wire from the battery, probe on the alternator. If it lights up, you have a current draw. Then start pulling fuses one at a time to see which one puts the light out. When you find it, that's your problem. If it doesn't light up on the alternator, disconnect the positive on the battery and put the test light in series between the wire and the battery terminal and do the same thing until you find the fuse that turns the light out. Hope this helps.