Help: batteries dying in an hour and a half???

[duramaxxx06]

ok i work at autozone part time. and the rest at a shop. guy came in last night and said his 03 gmc dmax had been dragging to start for a few days. when he went to leave it wouldnt start at all. we pulled the alt off and checked it and tested good on our machine. checked batteries and one of them was bad. so he bought two new batteries and i put the alt back on, and it was charging at 12.7 volts on the truck. he drove it on to work, came back out about an hour and a half later and it was dead again. any ideas? im at a total loss. ive checked everything i can find on it but still no idea. . .

 

[Dozerboy]

How big was the spark when you hooked up the batteries? Sounds like he has a hell of a draw somewhere. I guy I know went through this for months and it was his remote start.

 

[LBZ_Duramax]

Shouldnt it have been charging about 14 volts?

 

[duramaxxx06]

well on our tester off the truck it made 14.7 i think? but i was thinkin the same thing with it on the truck. and as far as i can remember there was not hardly a spark at all when i put the batteries back on, but i do remember a good spark when they tried to jump it off. .

 

[JoshH]

The truck should put out at least 14 volts when running. Give it a little to make sure the glow plugs and intake heater are shut off, if it isn't putting out the power, make sure the wire that is supposed to turn on the alternator is powering up and make sure the wire going from the alternator to the battery has continuity. Sometimes they will have a fusible link, but I'm not sure if our trucks do or not.

 

[duramaxxx06]

it does have the fuse link. thats where i checked the voltage was at the junction. what wire powers the alt?

 

[SSchmi5519]

Something is pulling current off of the batteries when it is keyed off. A bad relay or shorted curcuit could cause this.

 

[duramaxxx06]

anything a common thing on dmaxes?