Charging issue after engine install

[Jh1895]

Just got my car back from my engine builder and now it has an intermittent charging issue. Has aftermarket high amp alternator and had no issues prior to pulling the engine. Now it will
Charge for few minutes/seconds and then it stops and you can hear the engine rev higher. I assume that’s the ecm doing that because of the voltage drop but I can’t figure out why it’s doing it. Has anyone else had this issue before?

 

[olympic]

Mine did that at one point, turned out to be a loose connection in the alternator harness. Not the main charging wire but the 2 smaller wires with the waterproof plug. If you can get under the car while it's running and wiggle the plug and wires it should be easy confirm/rule out.

 

[BlckBox04]

Also check to sell if all the ground wires were reattached from the block to the frame etc. after the motor was put back in