What's causing the voltage to cycle off and on??

[Hamma Down]

I didn't have this problem until I picked up the truck from the shop after they replaced the transmission. I didn't even make it off the lot. At first, I thought it was a faulty fuel shut-off solenoid. So, I removed that and got it home. I then replaced the PCB. That didn't solve the problem. I then replaced the 2 circuit breakers on the firewall. That didn't solve the problem. The truck starts and runs fine otherwise. Is there another circuit breaker, switch, or connection on the transmission I am missing? Could there be a problem with the alternator? It is charging. Has anyone else experienced this? It cycles off real quick every 30s or so. It doesn't affect the lights, but will shut the truck off, unless I disconnect the fuel shutoff solenoid. I've been advised from another group that there's a wire grounding out somewhere. Seeing it didn't start until after the transmission swap, I will check around there to see if there is a wire pinched. Helpful suggestions would be appreciated Thanks!

 

[NDT]

I would bypass the ignition switch to see if that is the problem.

 

[juanprado]

Swag
I wonder if they fried the voltage regulator by starting wrong sequence or running battery down and tried to start with weak batteries or overcharged with a manual charger?