APM 2.8 ARM problem

Regards to all,

I need help around the APM 2.8 + Power module.

I installed the multicopter and connected everything in the house and ready to go flying.
However, I could not arm him on the fly today.

By coming home I check all the connectors OK, I connect to the PC and open the Mision planner and how much I see everything OK.

I plug the battery while it’s still attached to the PC and I try Arm whenever it does it all right.
Straight out into the yard to try, when it will not Arm again.

So I scrambled a bit and came to the conclusion:
1 - As long as an APM is connected to a PC via a USB port, Arm can accept the commands.
2 - When you turn off the USB from the APM switches to RTL and I can only use the ruder and for a few seconds then the engines stand.
3 - Power module is attached to APM and skip jumper JP1
4 - I do not know what the problem is and I ask for more help.

Thanks in advance.

that could be a defective diode

You will need to go into parameters and set logging to log from power up without arming.

Power it up and attempt to arm.
Download the log file from the APM and post it here.

Have you done all the calibrations?