charging troubles

[nap159]

Hello all. I have a 1984 truck with the 6.2 that has been converted to 12 volts. The problem I'm having is when the truck idles it charges fine. But when you put the truck into gear AND take your foot off the brake, the charging volts goes down to around 10 volts and stays there. Even at highway speeds. I've tried testing and replacing the alternator with no luck. We looked all over the truck and have found nothing shorted to ground. Any ideas where else to look???

 

[319]

Where are you taking the voltage readings?

 

[nap159]

I have checked in the cab at the fuse box. Holding the meter while driving.