I think you guys are worrying about RFI. I doubt that the trolling motor generates radio frequency interference. What you do have though is a powerful electric motor generating a powerful magnetic field. You should consider EMI. Interference from an electromagnet.
The first thing to try is increasing distance between devices. Where is the HB transducer in relation to the TM motor? Can you raise the TM to increase the distance, run it and still see the same problem?

Good luck.

IF I put hooked up my helix on my deck with a battery next to it and the transducer hanging off the side of the boat next to it, wouldnt that be rfi??

It would be interference. We don’t know from where. Your original said trolling motor ran and interfered with the HB. Is that correct? If so the TM is interfering with the HB. that’s where I am.

I wouldn’t know how the TM could generate radio frequency interference. Move the transducer away from the TM and see what happens.

You cant move a transducer further than the back of the boat, thats where mine is..

Anyways Ive spent well over 100 hours trying to solve the mystery I believe its rfi, I dont think anything else is going to go thru the air and cause interference when I hooked it up as I said.

Ive long given up, its not bad and I only have it on side imaging now which I very very seldom use while on the tm.

One of my old boats it was as simple as moving the FF 6" to the right of the deck to stop my interference when it never interfered with the previous FF.

Has anyone checked for updates on Humminbird’s website? That helped a lot of my interference issues. That’s the first question they always ask me

Im two behind thanks for the reminder, I usually do mine in Jan.

My hummingbird 859 will search for depths when in shallow water, under say 10 feet but if I put it on manual depth readings, it will show the depth correct on the chart but the depth numbers will still search. Humminbird said to increase the interference setting but that doesn't help. In deeper water it works fine. Seems funny that it would have interference in just shallow water. I just live with it.

Ok, I see. TM on front, transducer on back. Gotta live with that. The interference coming from TM has to be coming by wire. You need two RFI filters, one near the TM on the power wire and another on the HB power wire near the HB. They’re called ferrite beads and they come in different sizes. You need a bead big enough to loop the power wire through the bead, back around the bead and through the bead again. Then the assembly is customarily covered with heat shrink. If that doesn’t do it, add a third bead on the transducer wire.

Good luck, sometimes you just can’t filter it all out. Be sure that you are not sharing any wiring between the affected devices.

Trying to figure this out on mine I completely overlooked the fact that the temp and the depth were no longer displayed on my unit. I went thru every manual and video on my unit I could find and finally decided to just chunk it and buy a new Lowrance. Then I happened to think I had the same transducer on my 2-man boat. Hooked up my head unit to it and lo and behold it works. Called Humminbird talked to them about it and they are sending me a new transducer.

I was having the same issues as the OP. As soon as I would turn on my trolling motor my screen would go blank but show readings of 2, 3 or maybe even 900 ft. Then when I would let off it would go back normal. I tried running new wires,attaching a fused wire from starting motor battery to my trolling motor grounds and nothing would work. As I stated I finally noticed that my temperature readings were gone so I tried hooking my unit to a transducer on a 2 man boat I have in storage and the temperature readings were back. I called Humminbird and explained it to them and they sent me a new transducer as my unit is still under warranty. I installed it on my trolling motor and took it to the lake today and everything worked perfect. No interference, Temp reading and depth readings work like it did new. So to the OP if you know someone who has the same transducer and would let you hook yours up to it to try that may be all it is.

The transducer on the rear of the boat changes the problem for me. I assumed that you were using a transducer mounted to the bottom of the TM.. my situation. This has to be a power issue IMO. I don't think that any kind of RFI or Magnetic Field Interference from the TM to the DF could make that trip. I agree with a previous post that ferrite beads or cores on the power leads will probably be the solution, but you have to choose the correct ferrite mixture based on the frequency of the interference.. which I have no idea what that might be. You will have to experiment with different cores to find out which one will work. I think the culprit is the circuit that controls the speed of the motor.. its more or less a DC dimmer. If you ever tried to play electric guitar in a bar that had cheap wall dimmers on stage lights, you know what kind of noise those things generate! Good luck and please report back what your solution is.

BD