i just had to replace my 2.5 year old diehard battery today, it just went dead without any warning.

was checking the voltage after installing the battery, at idle and i am geting around 13.6 volts into the battery at the top posts, with engine off it shows 12.6 volts.

do those voltages sound about right for a 3.9 liter V-6, totaly stock engine ?

the service manual i have says as long as there are more volts coming in when running than with engine off it is good to go, but i always thought it was supposed to be about 14.6 or so ?

 

that sounds about right, with the engine off, the alternator isnt being driven by the engine to charge the battery, so with the battery at 12.6 volts, that is a perfect charge, with an engine running, the alternator should put out between 13.5-14.5 volts if im not mistaken, so at idle 13.6 volts sounds fine to me, with the engine speed up, she will probabally pump more voltage out, how many miles are on the truck?... when im doing a highway run, my alternator is pushing around 14 volts which is acceptable and raises no cause of concern.

with batteries, the average lifespan is 4-5 years, but there can be faults in batteries which they can discharge themselves overnight,

you can bring your truck into a service center and have them do a system charge test - they will test the alternator output, starter draw and battery load test. the abttery load test can be done at an autozone by just bringing them your battery

 

just wanted to add, thanks and it seems to be running fine, guess the battery just went bad on it's own.