If the alternator checks out OK and the battery is strong are you sure you don't have a short somewhere? When the truck is running, have you checked the alt to make sure it's charging the battery? Voltage regulator?
1998 F250 LD 4X4
2004 Explorer EB 4X4
2006 KLR 685
Alternator while running should be higher than 12 volts with most vehicles. 13, or more, would be more like it. Check for the output of the alternator while running (even a bit off idle) and see if you need another rebuilt Alt.
1999 F-150 XL, Extended Cab, Long bed, 4.6L, 4x4, M5OD (geez, the truck's description is just about as long as the truck itself! )
When the ignition is off, 12 volts at the battery is normal. When the ignition is on but the engine is not running, battery voltage should be 11 - 12 volts. With the engine running, voltage at the battery should be 13.5 - 15.0 volts.
John, aka KingRanchMan02
`02 F-150 King Ranch SuperCab
Charcoal Blue/Arizona Biege, 5.4L 3.55 LS, 4x2, Diablosport Predator Programmer, K&N filter. FTE Ride of The Week, 4/20/09 !!!
`08 Mercury Sable Premier, 3.5L Duratec, 6 speed auto
KRM02 has got the right idea. My '98 has sat for weeks & the pretty fresh battery still read 12.16 volts. When the key is switched on that dropped to 11.80 volts. Quite obviously charging, after sitting so long & on high idle, it read 14.36 volts.
IMO if you "have a short somewhere" you usually find out about it pretty quickly, especially in the large wires between alternator & battery. Shorts will ultimately pop a fuse/fusible link or smoke the wire until it burns out. More likely is a bad terminal or connection, that produces high resistance. This can fool a voltage regulator, acting like a fully charged battery.
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