Alternator Going?

[benderado]

The past few weeks, I've been watching my in dash volt meter real close. After my glow plugs kick off the truck will jump up to a normal charging range, but sometimes while driving it will be up around 14-16 volts and then idling at stop lights it drops down to 10-12 volts. What would be causing this? Its only spradic and its driving me nuts!!! Both batteries are good, I tested them both with the truck off and got over 12 volts. I'm runnin' outta ideas guys. Thanks for the help!

[Clay Henry]

I don't know how you tested the bats but you should be testing for amps.
the voltage on your alt sounds way low...bad diode. Test it at the dealer to make sure its tested right.