Alternator takes a long time to start charging

[Lil Red]

Recently, my 2007 GMC 2500 HD Duramax will run the batteries dead overnight. After boosting and starting, the alternator takes up to 5 minutes or more to start charging. The volt gauge will start to move then return to zero. It will do this for a few minutes then suddendly it will jump to full charging ~ 14 volts and stay. It doesnt do this every night. I can shut it off for several hours and have no issues when re-starting but overnight or after around 12-14 hours sitting this problem occurs. Batteries test out good. Anyone have this same problem and what was the fix?

 

[Budneeds2beers]

Any previous electrical issues? You got something way off. Your alternator should produce as soon as the engine starts. Amperage will vary depending on demand. Check for rat damage or a major short. If you have both batteries going dead in that short time you should be more worried about finding that. Any loose connections? Any electrical mods? Lights winch anything?

 

[duramax on fire]

Your pulling some power to drain both over night. Although I would still pull battery's over night and see if Volta drop.

See if any of the relays are hot. You can also disconnect both grounds then use a test light from battery negative to ground wire. It'll probably light up. Start pulling fuses until light goes off.

 

[Lil Red]

I changed the alternator yesterday as I had a spare one and the truck started this morning after sitting for 12 hours no problem and the alternator began charging almost immediately so maybe it was an alternator problem.
Thanks for the replies anyway.

 

[THEFERMANATOR]

I was gonna say it sounds like a voltage regulator sticking on draining your batteries. If the regulator sticks on, your alternatorwill field even though it's not charging. It's easy to check. Shut the truck off for a few hours, feel the alternator, if it's hot, then it is most likely still fielded. Or if you disconnect the charge wire, you might see a spark.