RyanInKY

This is my first ever post here, and I wouldn't do it if I weren't completely stumped. I have been working on cars for about 30 years (not professionally, just personally) and my mind is stumped. Was wondering if someone here could help me. I will list the issues and what I've done so far in short and sweet terms.
2005 Silverado K1500 5.3L
Truck was dead one day. Charged battery. Truck started. Truck later died again. Wasn't keeping charge. Decided new battery. Truck started and ran fine. (didn't drive, was just sitting) Couple days later, dead again. Took new battery back to store. Got a new battery. Same process and result with 2nd new battery. Each time I would charge the battery up with charger, the truck would run fine at idle for 20-30 mins, but then I could watch the voltage meter in dash drop. All the way down to 8 or so before the truck would then die. Service charging system message in dash. Decided to replace the alternator. Charged battery with charger. Again, truck runs fine for the 20-30 mins, then volts drop again and it dies. Took new alternator and old one to store to be tested. Both checked out good! So something is keeping my battery from charging. This model has the built in voltage regulator in the alternator. Since it tested good, that can't be it. I have checked cables over and over for tightness. I have cleaned ground wires. I even took the ground from engine block and cleaned it. Still no luck. I am at a loss and wondering if someone could please help me! Thank you!

hanky

Hi Ryan,

Even if you worked on GM vehicles professionally , you would have trouble with this one.
There are a few things you can check, but there are different electronic parts that you do not have the equipment to look at.
You need a capable scan tool to look at voltages and percentages in order to determine if the right things are happening.

The charging systems now, some are controlled by a BCM (body control module) in conjunction with the Vehicle powertrain computer PCM.
Something like a borderline ign sw can prevent the system from going into sleep mode , thereby resulting in a battery drain.
The charging systems are so designed to try to keep the battery at approx 80 % SOC (state of charge) and attempting to run "on vehicle" , tests is pretty much out of the area of the usual mech/repair person/tech without the tools and knowledge of how the system works. For example there may be a separate " GBCM, generator control module,that with PCM function will determine if the alt will output how much and when. Duty cycles are involved and most don't have the equip and info to trouble shoot these systems.
I would suggest that you consider having a qualified shop check your vehicle.

2001 chevy silverado

Well, you ran a new battery dead then changed the alternator. That may have damaged the battery. Check the battery in another vehicle.

You know you have a bad short somewhere, a parasitic draw.

The pros would find this "easily". You have a fuse panel(s). Use them. Unplug all the non-essential fuses. See if you have a parasitic draw.

If the parasitic draw occurs only when the car is running - obviously your going to have to find what is drawning more amps than it should. That component should "not be working normally".

You might have hot wires - warm to touch, that shouldn't be. Say, leading back to the brake lights or something. You obviously cleaned all your grounds before posting, obviously. Since you pulled the fuses for the lights you won't find that until you've found your truck is working normally with all non-essential fuses remove re-placed, though.

A radio or what can be it. anything humming (fan, but that should be off). they key system can short and cause not only a draw but a fire hazard. (the key system sends allot of power to the starter then dis-engages that power directs it elsewhere. if it's shorted it can be quite a draw problem needing immediately fixed - can be dangerous)