Alternators rarely fail out right, fuses blow, fusible links burn out but alternators generally fail by degrees.
Alternator bearing going out is also an issue.
Alternators generate power thru 3 fields in the case, usually 1 field will fail when alternator gets old and that's when its time for a change.
Alternator can still run the truck and charge the battery with 2 working fields but at night you would notice dimming head lights at idle, because at low RPM and 1 failed field the alternators power output drops below 13.5volts with headlights on.
Put a voltmeter on the battery
Engine off should be 12.8v with new battery, 12.3v with 5 year old battery, 12.2v or below means get a new battery
Start engine, battery should now read 14.5v to 14.9v, under 15v, that's the alternator recharging battery after starter motor drained it
After a few minutes of running voltage should drop down to 13.6v, voltage regulator is working.
If you turn on all lights and heater fan to High, also leave doors open and crank up the tunes, lol
Voltage should drop and then come back up to 13.6v, all 3 fields are working
Last edited by RonD; 08-04-2015 at 09:28 PM.