It's hard to check an alternator for a shorted diode, unless you take the alternator apart and use an ohm meter. You can't check for a shorted diode by taking the alt to a FLAPS. An alternator with a shorted diode will still charge the battery at 14 volts (volts, not amps). I have yet to see a flaps tester that you could rely on, to fail an alt with a shorted diode. You could have a shorted diode with a low battery drain in a car that you drove everyday and you would never know you had a bad alternator.
My first sentence is technically wrong. You can check an alternator with an oscilloscope.
[img]http://home.earthlink.net/~roadruner/diode.jpg[/img]
Check for a drain in the car/truck wiring as Toyvo suggested.
Check for a drain in the alt as wrenchbender suggested or just disconnect the alt and let the truck sit the same time period as before.
