Duramax Alternator

[RI Chevy Silveradoman]

I was having issues with batteries being dead. I fully charged both batteries and truck was fine. After running the truck, then letting it sit overnight, no start the next morning. I fully charged batteries again and got truck fired up. I put volt meter on battery and got 13 volts. Lat time I checked I was getting around 14.7 volts. Belt is good. Alternator is original.
I spoke to someone who was telling me there are 2 charging circuits for GM, low speed and high speed charging based on engine RPMs. Has anyone heard or know about this? Anything else I should check before getting a new alternator?

 

[ak diesel driver]

If the batteries are really low or internally shorted a tad it could bring down the voltage

 

[Will L.]

No 2 speed alternator I ever heard of. But every other one basically doesn’t start putting out until a certain rpm, then puts out more the faster it spins. That pretty much how all alternators work.
There is a circuit that “reads” battery power and will work like an on/off switch. When the batteries are full & no major electrical draw is present the alternator will stop putting out. When the draw is present wether headlights, battery, etc then it will begin putting out power again. Magic pixies.

Stop by most any autoparts store and have them come out and hook up a tester on the alternator. Around here they all do it for free in hopes you buy the new one from them. There is YouTube videos how they do the test so you can see whag to watch for to ma sure they don’t lie about it being bad.

 

[RI Chevy Silveradoman]

OK. Thank you. Guy was telling me there are 2 charging curcuits. A low speed and a high speed circuit. Tgey charge at different voltage rates. I will give a better look-see today.