G&G UMP doesn't like it's receiver
 

Have a UMG with a weird problem. When the gearbox is by itself, it cycles fine. But once I put it in the lower receiver, it's completely dead. No click, complete electrical failure. Occasionally I'll get one shot as soon as I put it in, but it always stops working again. Works 100% of the time when the gearbox is not in the body.

Nothing appears to be obstructing or pushing on any wires.

Ideas?

Maybe the motor height needs to be adjusted?

Did you re-shim it recently? The lower receiver might be a tight enough fit that it's squeezing on the sides of the mechbox to cause binding. I had that problem with my Systema v2 box with an original Systema M4 metal body. MadMax helped me to diagnose and fix this many, many years ago.

The ump uses a v3 with a motor cage motor heights not gonna matter cause its going to be the same out of the body and in

Try moving the selector plate around also if I remember the ump uses old style tm based selector with the copper band. See if that's not burnt out.