I have had my 520c since january but only recently started picking up interference using structure scan when i put my foot on the trolling motor. Tried changing frequency but that's didn't help. Ranger already has the ground wire from the cranking battery to the trolling motor battery to eliminate the interference and i checked that is was still connected tightly. This only started after I had the boat serviced and the dealer forgot to attach the main cranking ground and I had a weekend of accessory issues until I realized it wasn't connected. Any thoughts?

does the Fortrex have built in sonar 83/200khz. If so, it has a fuse in the top that may have blown. If not, when the boat was serviced, they may have connected the ground to the wrong trollingmotor ground.