Hey BBC! It's been a while since I've posted, but this was worth sharing.
My MEGA 360 began having INSANE interference any time the trolling motor was activated, with slightly
different interference when the trolling motor was turned side to side.
I've spent literally over 100 hours reading, watching videos, isolating and troubling shooting. I've grounded and regrounded and ungrounded everything, rebuilt the trolling motor, etc. All best practices and dedicated 8ga wires are in place, ferrite mix 77 rings, charger disconnected, fuses replaced, etc, but interference WAS STILL PRESENT even after running everything off a separate standalone battery sitting on the garage floor!
I've isolated the cause and the solution clears up 100% of my interference! In my case, the power system was not the issue. The transducer or receiving aspect is where interference was being introduced into the system.
After all this time and money, I HAVE CONFIRMED THE FOLLOWING PROBLEMS:
1) The Ultrex cable bundle emanates interference picked up by my MEGA 360 in the 1mhz range when the trolling motor is activated, but not the 455 range.
2) The servo that steers the Ultrex at the shaft is especially loud with respect 1mhz interference, and the steering module is an interference hotspot.
3) The latest update to Helix G3N MDI changed the noise filters from 4 options to 6 options, but these 6 options seem to do literally nothing. Before the update, noise filter 2 had cleared most interference and each noise filter setting actually made a visual difference. Did Humminbird break their noise filters with the most current update? Perhaps, or maybe it's just a coincidence.
4) The MEGA 360 in-line connector must have no shielding or failed shielding, and its like a literal EMI antennae if anywhere close to the Ultrex steering servo/module or Ultrex cable bundle. The further I hold the connector away from the Ultrex in general, the less the interference (even when on it's own 12V battery outside the boat).
5) The CABLE SHIELDING ON THE 360 TRANSDUCER WIRES DOES NOT WORK or is not effective, even down into the shaft itself (so it can't be a kink?)
The problem is a combination of the noisy Ultrex components and failed, defective, or ineffective EMI/RFI shielding on the MEGA 360 transducer cable.
The SOLUTION is this: DOUBLE FARADAY/EMI/RFI SHIELDING.
According to EMI/RFI experts, braided copper over insulated foil shielding is best for capturing and eliminating problematic EMI and RFI. Here's a snippet from one of the papers I read on this:
Capture.JPG
Another source stating the same:
https://www.lapptannehill.com/resour...parison-charts
So with this in mind, here's what I did:
1) I bought FARADAY TAPE (it's a metallic mesh fabric tape) and wrapped the wires from where they exit the fiberglass shaft all the way past the in-line connectors. I soldered a dedicated wire to this shield and made it the same length as the rest of the wires.
2) I then wrapped the faraday taped cable bundle with fabric tape to make a barrier between this and the next layer.
3) I bought 3 meters of 28mm TINNED COPPER BRAIDED SHIELDING, very similar to a rod sock but heavy and metallic. I slid this all the way up and over the in-line connectors and down to the MEGA 360 shaft, but this isn't far enough! SHIELDING IS REQUIRED IN THE MEGA 360 THE SHAFT TO BELOW THE DECK OF THE TROLLING MOTOR! It's very easy to slide the mesh shielding down into the shaft a good bit, and I discovered that not doing this results in lingering interference. I have my shielding mesh inside the shaft and about 10" above water level and a good bit below the steering module of the Ultrex. I soldered a separate ground wire to the braided wire mesh,and made it the same length as the rest of the power cables.
4) I bought a black nylon braided mesh, just like a rod sock and slightly larger than the braided mesh shield, and ran it over everything down to the MEGA 360 shaft and zip tied it tight.
5) Both of these faraday shields are then grounded at my 8ga negative primary at the bow, along with the graph and MEGA 360 ground/negatives.
6) I RAN THE SHIELDED CABLES ALONG THE TROLLING MOTOR WIRE BUNDLE, which I already tested as very noisy with interference, but this double faraday shield is 100% effective at removing ALL interference from the Ultrex.
7) The shielded bundle runs into the bow panel, and just a few inches of the wires exit the bow panel and connect to the Helix G3N MDI.
I also shortened my MEGA 360 shaft by 7" like Mr. Ott Defoe, which leaves lots of clearance for everything including the more-rigid and insulated 360 cables.
The only negative is that this probably added about 2lbs to the front of my boat, but to me it was worth it for crystal clear MEGA 360 while on the trolling motor!
Hoping this information helps someone else troubleshooting similar issues, especially out of warranty.
Good luck!