Is it possible to get a meter on the pick up cables while it's running?
I'm not sure if what I'm suggesting is even possible? I don't know what sort of current you would be looking at nor how to measure it, and probably rather difficult as it would be pulses, not a flow.
Beyond going to all the trouble of lengthening the cables to swap them I don't know what to suggest, except I really do think it's the vertical pick up failing to work at anything over tickover.
__________________
|