Trolling motor interference on my fish finders

Bronzer

Cadet
Joined
Dec 18, 2016
Messages
14
I just purchased a boat with a Humminbird 859 on the front and a Lowrance HDS-7 on the dash. The problem I'm having is that every time I run the trolling motor, I get a lot of interference on the screen of both units. The instant I let off the trolling motor, the interference goes away.The fish finders are running off of a separate battery, not the trolling motor batteries or cranking battery and I don't get any interference on either when running the big motor, just when I run the trolling motor. Any ideas??
 

ondarvr

Supreme Mariner
Joined
Apr 6, 2005
Messages
11,527
The manuals for the trolling motors typically have a section discussing interference, check those things first.

​How old is the trolling motor? the internals may be worn out.
 

jimr

Senior Chief Petty Officer
Joined
Mar 21, 2004
Messages
723
On my old boat I had a hummingbird and the trolling motor would interfere with it. I bought a new finder and it would mess with that one too. After I sold the boat the new owner put a new trolling motor on it and fixed interference problem.
 

Bronzer

Cadet
Joined
Dec 18, 2016
Messages
14
The trolling motor is a Minn Kota Maxxum 74 lb. thrust. It is only 5 years old.
 

gm280

Supreme Mariner
Joined
Jun 26, 2011
Messages
14,585
Bronzer, I tried, unsuccessfully, to see if your trolling motor was a variable digital controlled (Digitizer or PWM) TM or not. But I can't get a good review or info whether it is or not. If it is merely a preset speed control system, then you may need new brushes in the TM. If it is a PWM digital design, then that circuit can interfere with other electronic accessories on board.

You can install some filter networks on the other power lines that power the locators and that may help, unless the noise is EMF generated by the motor. If it is that type noise, it is going to be a lot harder to fix. Since you stated that the TM is running on its own battery setup, then it is looking more like EMF noise and not following the power lines back. Are the grounds from each battery connected together? The noise can travel that connection as well.

I would be looking more at the motor in the foot section for the possible noise source and service that foot with new brushes, bearings and seals. But without being there and doing some tests myself, that is merely a wild guess. JMHO
 

ajgraz

Lieutenant Commander
Joined
Mar 1, 2010
Messages
1,858
When you open the foot on the TM to check the brushes and such, also look at the magnets. I've had two MK TM's in the last ten years in which the magnets have either come unglued or just plain cracked into pieces.
 
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