Silverado Charging Issue

I have a 2003 2500HD 6.0L Gas 80k Miles

About 6 months ago the original alternator went bad on me. I bought a new one from Advanced just to have the same thing happen to me in a week. So they replaced it again. And once again it went bad. They ordered another and It has been on the truck for the last 5 months. Since I have replaced the power steering pump, idler pulley, tensioner pulley, battery, belt. On a Sunday the starter began to stick. The following morning on the way to work the voltage meter began to jump between 6V and 14V and the lights would go dim. This only did this for 5 mins of my 15 min commute. The rest the truck just acted fine. That night i replaced the starter, and didn't do the voltage surge. The next morning it did the same thing. Stopped at autozone and the worked tested my charging system and said all was good other than my battery was low. He noticed that when my A/C pump kicked (had my defrost on) in was when the truck seemed to die down. So I played with it for a little while in the parking lot and unplugged the pump. The following morning the same issue. So I took the alternator and the batt back to advanced and had them tested off the truck. Both passed, but exchanged the alternator anyway. Got home and put the batt on charger. The next day I spent 3 hours checking various plugs, wires, grounds and fuses. All were good. Then put the batt and alternator back on the truck. Started and idled just fine. Put the foot on the brake and put it in gear, lights went dim, voltage did the same thing for about 30 seconds then it idled back out. put it back in park same thing. I've been over the truck and Can't seem to find any shorts or loose connections. Any one ever had the problem or know what it might be or what to try..

I have a 2003 2500HD 6.0L Gas 80k Miles

About 6 months ago the original alternator went bad on me. I bought a new one from Advanced just to have the same thing happen to me in a week. So they replaced it again. And once again it went bad. They ordered another and It has been on the truck for the last 5 months. Since I have replaced the power steering pump, idler pulley, tensioner pulley, battery, belt. On a Sunday the starter began to stick. The following morning on the way to work the voltage meter began to jump between 6V and 14V and the lights would go dim. This only did this for 5 mins of my 15 min commute. The rest the truck just acted fine. That night i replaced the starter, and didn't do the voltage surge. The next morning it did the same thing. Stopped at autozone and the worked tested my charging system and said all was good other than my battery was low. He noticed that when my A/C pump kicked (had my defrost on) in was when the truck seemed to die down. So I played with it for a little while in the parking lot and unplugged the pump. The following morning the same issue. So I took the alternator and the batt back to advanced and had them tested off the truck. Both passed, but exchanged the alternator anyway. Got home and put the batt on charger. The next day I spent 3 hours checking various plugs, wires, grounds and fuses. All were good. Then put the batt and alternator back on the truck. Started and idled just fine. Put the foot on the brake and put it in gear, lights went dim, voltage did the same thing for about 30 seconds then it idled back out. put it back in park same thing. I've been over the truck and Can't seem to find any shorts or loose connections. Any one ever had the problem or know what it might be or what to try..

I think you have a bad battery connection/wire this may be the red positive or black ground cables. with a voltmeter test where the voltage is dropping. is it the battery at terminals ? not connector at battery since this could be the problem. check voltage using the alternator as a ground and then battery as ground see if you get the same result. measure the alternator red wire at the alternator stud.

loading down the system ac on lights on defroster on then check where its dropping the alternator is 105/130 amp rated voltage should be 14volts. if this is not 14volts at alternator using the alternator as a ground point the alternator is defective or the small connector to it in the rear of it , is the cause. this is the field exciter connection. sometimes this can get corroded or damaged.

my 2000 silverado draws 66amps everything on. I used an amp probe clamp around the alternator output wire. I , 2 years ago replaced my alternator. got a new not rebuilt dephi alternator with the new updated regulator. my OEM alternator failed because the regulator corroded. I took it apart and was going to repair it with the new regulator. brushes were only 50% worn ..

dash guages on your vehicle are a big problem ....I would not use them for testing.

also if you really had 6 volts the engine should not run....

If you have a spare battery, or a battery from another vehicle you can borrow for awhile, you may want to consider changing the battery just to see if this cures your problem. Over the years I have had a few similar problems on various types of equipment and it turned out to be a battery that was shorting out internally on an intermittent basis.
It’s just a guess but an easy thing to try before you go thru a lot of trouble trying to troubleshoot. Good luck!