Downscan Interference w/ Trolling Motor Mount

[Talon]

Has anyone successfully solved interference issues with Lowrance Structure Scan transducers mounted on trolling motors? I have seen it universally with Motor Guide's and Minn Kota’s. Hoping to not reinvent the wheel with a solution. My transducers are mounted as follows:


The issue is always speed dependent whether running fixed control or variable speed motors. At slow speeds the interference worsens with Downscan; typically opposite of conventional sonar. The attached screen shot depicts split screen Sonar (200 kHz)/Downscan (800 kHz) at three trolling motor speeds…slow to fast reading left to right. Downscan interference is always greater on 800 kHz than 455 kHz. The high bypass setting on both Motor Guide and Minn Kota produces the least amount of Downscan interference.


This screen shot depicts typical interference witnessed while applying short bursts of trolling motor power to hover over brush.


Thanks in advance for any help and solutions.

[CatFan]

The fact that it is better at higher TM speeds shows that it is related to the pulsing of the TM power. At 100%, there is no pulsing.

The first step is to figure out how the interference is coupling. Make sure that your TM batteries and the battery powering the unit and their wiring are as isolated as possible. No running the TM/FF power wires anywhere near each other. Same with the TM power cables and the transducer cables. Keep them apart until you have to bring them together at the TM. The only connection whatsoever should be a ground jumper between the battery with the negative TM wire and the negative of the cranking battery to eliminate noise.

A good first step is to disconnect the transducers from the motor and hang them over the side of the boat. If that clears the noise, mount them back to the TM with the transducer wires away from the shaft and try again. The point where the noise returns (or not) will tell you about the source of the problem.

[Talon]

CatFan,

Thanks for the quick reply with some great information.

All of my electronics cabling (power wires and transducer cables) run down the starboard side of my boat to a dedicated battery. I have a separate battery for engine start, lights and pumps. All of the trolling motor battery cables run down the port side of the boat. I do not have a jumper wire between the engine battery and the trolling motor batteries, but when tested with an ohm meter it appears this circuitry is wired into the boat. I will retest it and add a jumper to test effect.

Should the separate electronics battery be jumped as well?

Recently I replaced a bent shaft on the trolling motor and played with the interference relative to the transducer positioning. It really worsens when you approach the head of the trolling motor with the transducer. The cables up the shaft did not have a lot of influence.

[CatFan]

Some TMs benefit from the addition of a bonding wire inside the motor. You might look into that.