I have a good battery and a brand new alternator. I replaced the old alt. because it wasn't charging and was a 3 year old rebuilder.
The new alternator isn't charging either. In fact, the battery voltage DROPS when you start the engine and the alt. is running, from 12.4 volts down to 11-something.

I know about the PCM voltage regulator and bypassing it but, from what I understand there could be many other things at play here.
Any advice on troubling shooting this issue is appreciated. Thanks

 

If the alternator tests good out of the truck, and the wires for the field windings pass a continuity test, voltage regulator is toast.