I'm sure glad to see this topic. I hope there is a resolution here soon. I have been fighting my 788 for about 3 weeks now and though that it might be helpful to post the trouble shooting steps I have used so that others might not have to go through what I have.
Fortunately, I bought the 998 and a high speed transducer at the same time so I have some other equipment to use for troubleshooting. I am also using a Minn Kota Fortrex 101 36v
The following is what I have tried with no success:
Running a ground from the cranking battery to the last in line negative on the trolling motor batteries.
Trolling motor ground to sonar ground at the bow.
Trolling motor housing to sonar ground
Various combination of all
I have chaged th transducer frequencies
used another battery
removed all electronics from the cranking battery and tried it again there
Used a different transducer (high speed sonar)
used a different unit (the 998)
Used the interference kit for the sonar power cable
Used the interference kit for the GPS / Interlink
Removed the interlink
Used an RF choke on the transducer
Use an RF choke on the trolling motor power
Ran several RF Choke in line on the transducer and the trolling motor
Used the TMC-1 from humminbird in multiple configurations and even went so far as to connect it directly to the control board on the trolling motor to eliminate the excuse for not getting it close enough to the foot switch.
Connected power to the boats fused board
Ran separate lines directly to the cranking battery with the inline 3A fuse
Was asked to inspect the trolling motor head for the fuse that doesn't exist since it is not a US2
Minn Kota had me check to see if the skeg was in line and it was
Checked continuity in the trolling motor housing
Ran 12Volts through the trolling motor control board and still got interference
I installed a new Minn Kota Maxxum trolling motor
Ran a ground from the negative side of the brush plate to the ground that the internal light uses (this was the suggestion from the Minn Kota CSA)
Pulled all batteries out to remove line discrepancies and provide shorter leads
Checked and confirmed good power
Finally, I took it to a friends house and installed it in a new boat using a motor guide trolling motor while swapping the transducers as well and got the same result.
The ONLY thing that worked at all was to bypass the control boar in the trolling motors footswitch and engage the motor directly. I used 36V and 12V and the motor spun just fine and didn't cause any interference. So, from where I stand, thy both work great together if you don't mind losing the ability to raise or reduce your trolling speed and turn the unit on and off.
Anybody heard anything that would help get this resolved? I'd love to hear what someone has done to get it to work correctly.