Hi

The battery pack has two switches

it senses the shutter release
it sensed the body cocking lever moving off park

It is pretty independent of the body really

So

problem with battery pack
or
problem with motor back

check the battery connectors are clean is easiest 1st step

check the connectors on the power cable from battery to motor pack next

borrow a motor pack and battery pack is easiest afterwards

screw driver battery pack last resort.