Disappearing Voltage, not charging

[NIsaacs]

Most auto parts stores can test alternators, I would take it back to where you bought it and confirm it is good.

 

[DavidC]

If the engine is running and there is not 12vdc on the blue wire I am starting to question the PCM unless the wiring to the alternator is shorted. A quick check of that you should have already done during the stepped tests. Does your tach work showing that the crank sensor is good ?. I have not found where the PCM gets it's 12vdc from yet other than the truck must be running.

 

[DavidC]

Looks like the 12vdc comes from fuse # 9 at the side of the drivers dash fuse block.

 

[DavidC]

https://mopar1973man.com/topic/8791-pcm-issues/