If you have one of the US2 trolling motors you should check the following:
1.) Check the fuse in the head of the trolling motor with an ohm meter (multimeter) to make sure that it is not blown. Don’t just look at it – test it to be sure.
2.) Make sure that all of the little round metal connectors for the US2 cable are tight. If they are loose they can also cause trolling motor interference – again: even if you are not using the US2. If you are using the US2 transducer than be sure and check the metal connector that is on the Minn Kota to Humminbird adapter cable to make sure that it is tight as well.

There are some instructions and pictures here on the Humminbird web site about this:
Humminbird: America