I replaced a 7" Lowrance Elite on my 36 volt Motorguide with a Humminbird 9" Helix. The Lowrance worked great for years but developed a "dead spot" in the graphics. I never had a problem with the Lowrance and TM interference. Took the boat out today for the first time with the 9" Humminbird up front. Right after I stepped on the Hot Foot, the screen went haywire. The depth line jumped from 4 feet to way over 40 feet. As soon as I took my foot off of the Hot Foot, the graph line "fell" back to normal. It does not seem to do this on higher TM speeds for some reason.

There is no difference in the install of the older Lowrance's and the newer Humminbird. I used the same power source and stored the excess transducer cable under the front console cover, just like before. I took the new transducer off of the TM, stuck the TM in the water, held the transducer in the water, and stepped on the Hot Foot. Same interference.

I can move the excess transducer cable a little bit, but I do not know what else to do. The TM plug wiring is also underneath the front console cover, which is where my power source for my Humminbird is.

I have had numerous trolling motors with transducers mounted in them. This is the 1st time I have ever had any problem. Any input would be welcome.

I had a Helix do the same, step on the button and the depth would go crazy, tried several easy things. It kept getting worse. Wound up being the transducer. I know you didn't want to hear that.

PM sent.

I hit you back. Since it is new maybe you can get the transducer warranteed.

I bought and installed chokes on my lines. I was getting some major feedback from my main switch in the transom of my boat. I took the wiring away from that, then I made sure I wrapped my lines away from the other ones. After that 99% of my interference went away

Are you talking about RF filters?

I believe that he installed inductor coils to absorb errant AC interference that has gotten onto the conductor somehow.

Ferrite chokes. You loop your power wires through them, some can go on the transducer cables as well. Garmin sends some with their graphs, but you can find them on eBay or just google search.

Yes,
RF Choke 31200 Ferrite Filter Core Material 31 Mix ID 3/16" 5mm
I put one on each wire coming from my graphs , both the power and transducer cable.
The main switch in the transom of you boat, https://www.basspro.com/shop/en/mar...ller-400a-ez-mount-on-off-battery-switch
I was getting bad feedback from that also. It was also blowing my fuse in the head of my Ultrex. I took the power from that switch and put it directly to the battery and that solved a lot of interference and the blowing of the fuse in my trolling motor.
When you run you cables from your transducers to your graphs make sure they are not zip tied all together with all your other cables. Wrap them and coil them as individual cables and try your best to separate them.

Every time I stepped on the trolling motor my two bow graphs would go haywire and I would have so much interference they were useless. Once I did the things noted above, I would say 99% of all my interference went away. I might see one thin line on the screen when i push on the trolling motor at the very first but that is about it. Other than that, it is as clear as ever even when I am keeping my foot on the trolling motor.