I normally hang out with the 2nd gens but I'm helping a friend with his 2008 6.7. Initially the truck would not start unless fresh off a battery charger and it would frequently show low battery voltage. The batteries were going on 6 years old and the driver side battery was spanked, dropping to 6V under load testing. Easy enough, new batteries. Now the truck starts fine but for the first 2-3 minutes after a cold start the charging voltage would intermittently drop out for 5-10 seconds. After that it's stable. Wondering if the alternator is dying and the high charge current after a cold start exposes the impending failure. I'm planning to do some sort of load test of the charging system but maybe some have experienced a similar issue ?

Also related, the FSM mentions a generator decoupler pulley for "some vehicles" without going into detail which vehicles have it. Any more intel on this pulley ?

Thx !

 

Have a look at the alternator pulley, if its a decoupler style, should be readily apparent.

Could have the alternator tested OFF the truck. The testing stations at the parts store can put a significant load on them.

 

On an engine specific forum I learned that the grid heaters not only cycle before starting but also during engine warm up at idle. Seeing the voltage drop is a pretty common occurrence during that time. I'll still load test the charging system to make sure the alternator is still fine.