Alternator Diode fried

["Allan Weidenheimer" <allanw@domain.elided>]

Charlie:
  I seem to recall your testing the alternators included lifting the
battery
cable off while the vehicle was running.  This is a bad way to test the
alternator, but a good way to fry diodes.  The battery is a large resistor,
in a way, and with power to the field coils, the alternator is trying to
make 
voltage.  The regulator circuit relies on that battery being there, and
gets 
confused when it isn't.  The sudden large resistance seen by the regulator
circuit when removing the cable causes it to try and make more current,
so it drives more and more current through the field, producing more and 
more output to the point that the diodes cook.  

Use a cheap voltmeter to test your alternator, if you get about 14-14.5
volts,
its good, then turn on the headlights and make sure it doesn't drop too
much.  It might drop some at idle, but should rise to the above voltage and
then not go up any more than that when the engine is revved some.
If the alternator has a bad diode, you
will see a lower voltage.  If its really fried, the meter will read the
same or lower
than it did when the truck was not running.  If the voltage regulator is
bad, it will do odd things when the load (headlights) are switched on and
off,
and it you might see a high voltage 15.5+ as you increase RPM.  Get a 
$10-15 meter at radio shack.  They are plenty precise for automotive
voltages (unless you plan to work on a new fuel injection setup).

Good Luck
Allan Weidenheimer
72 1210 Pickup