Have an old M K 'all terrain'  that I had initially set up with two 29 batteries in parallel after removing an older 24v Evenrude. 

 

Being a novice I added a junction box with (4) breaker protected circuits to the same batteries for my house circuits not knowing about depth finder interference. I picked up a New in box Hummingbird and mounted the transducer on the trolling motor.

 

At first there wasn't any noise in the finder. Working with two batteries I had attached the motor to the opposing + and _ with jumpers to make a series connection and then hooked the junction box to the opposite opposing + and - with good results. The first summer I ran 7 - 8 hours in windy conditions.

 

Starting out this year, I start hearing the finder alarm, repeatedly, when moving in the highest position. This had gradually worsened to the point where the finder screen fades out but comes back when I release the pedal. As I read up on causes of interference, I decided to divorce the batteries making one just the TM and the other just the junction box supply after a night run where, at the end the TM was pulling enough power to dim out the LED running lights.

 

Testing the batteries showed both still had over 12v so I charged them and re-arranged the wiring.

 

Sunday I'm out and the Finder still constantly sounds the alarm with out fading out until I shut it off to stop listening to the constant sound.

 

A bit later I notice that the TM appears to be running out of power so I used the gas engine to run up wind and drift along the shore.

 

Arriving home I grab my old analog meter and find the TM battery still shows over 12V.

 

Would used up motor brushes start to build up resistance enough to drive a motor mounted transducer nuts after I separated the power supplies?

 

Would that also slow the TM down?

 

The Batteries are WM 29 deep cycle new 9/17 used more the end of 18 and again this year so far and always recharged at 10- amps as soon as I return from the lake.

 

I'm thinking the brushes could be shot and arcing enough to make the motor mounted transducer notice.

It's hard to say.  But brushes would be a good place to start, especially if the motor has a lot of miles on it.  And clean out any junk on the commutator.  Sometimes they can pick up shavings from the worn brushes and cause arcing, or dirt and grease that just causes intermittent contact. 

 

Also, are the cables for the transducer and trolling motor running right next to each other?  That can cause them to pick up interference from one another.  The trolling motor won't mind, but the fish finder might.  If possible, you might separate them a bit.  I don't know if they use coaxial wire for these things, but if they don't, I imagine interference is possible.  Sometimes there's a ground connection on the trolling motor.  You might look for that and see if you can ground the motor to the skeg, or check it to make sure it has good contact with the ground.  If that connection has become corroded or disconnected, it might not work as it's supposed to.  

 

I'll admit that I'm not that familiar with boat electronics.  I've never really had issues on a boat that I've had to deal with.  But I've done a lot of electronics work, and eliminating line noise can be extremely frustrating.  Though in this case, I think you know where the noise is coming from (the trolling motor).  So it's just an issue of where it's being picked up.  

Just a little 14 alum boat, I mounted two battery boxes in the rear. I ran the motor wires up one side and placed the junction box on the other.

 

Looking at the transducer wire i do see a small nick that exposes the shielding, have to look at that. 

 

finder on the left, motor on the right with the transducer coming across the bow and follows the shaft down the motor doesn't come close to the motor wiring.

 

i did read about ground wire with a 3 amp fuse grounding the skeg to the trolling motor negative may help.

 

The brush kit comes tomorrow or Thursday I will leave the transducer off the motor at first then mount it and see what happens; I already had most of the components to replace the steering cables, etc. so a refresh to the motor was a good idea anyway. 

Sure helps to read the book, the old Hummingbird has a voltage alarm...battery time and it won't be Walmart.