OK I have done everything I can do, checked and cleaned grounds turned the noise up to (high 3) and I still show interference and I noticed also that the depth will jump up and read like 60 to 70 feet when the interference happens then it will go to the right depth reading. My trolling motor is a 24 volt system and the graph is a 788 ci di hd. I even took the transducer of the head of the tm and still had the interference. I am thinking I need a filter. I dint no what else to try, and I dint no how to check for a common ground because the tm goes from 12 to 24 at the front were the tm plugs in.