Interference between trolling motor and fish finder?

[LazyIke]

Anyone else's fish finder go haywire when its mounted under their trolling motor? Mine seems to either "zero" out or register something crazy, like 416 feet, when my foot's on the pedal. I've got the transducer attached with a circular thingee (that's the technical term!) from the hardware store. Can the motor and fish finder's wires cause interference to fritz out my fish finder?

[RK]

Hiya -

A couple questions/ideas:

- Is your depth finder's power cord attached to your trolling motor battery? If so, that's almost certainly the cause of the problem. The power control modules in trolling motors really play havoc with graphs. Run the power to a start battery if at all possible.

- A simple step that may help. Take a bicycle or old tire innertube, cut a square out of it, and put it between your transducer and the motor's lower unit. Sometimes that alone is enough to cut the interference. Not always, but sometimes, and it's the easiest fix.

- If the power supplies to the TM and graph are on different batteries, do the power cables run side by side down the boat anywhere? Sometimes just having the power cords next to each other can cause interference. If it's not too much hassle, try re-routing the wires. (maybe this step is a last resort...)

- You may need to ground the trolling motor. This is a REALLY common solution to interference problems. It isn't hard to do, it's just a little putzy to track down. You didn't mention whether your TM is a MotorGuide or MinnKota. If it's a MotorGuide with a metal shaft, just clamp a wire with an exposed end to the tm shaft with a hose clamp or electrical tape, and run the wire to the (-) post on your cranking battery. You can test whether or not it's a grounding issue in the driveway most often. Get a length of wire long enough to reach from the (-) post on your battery to the trolling motor. Take the transducer off the trolling motor, and pull it away from the motor a ways. Turn on the graph and the trolling motor then move the transducer closer to the motor. Start at the top of the TM (the head) and move the transducer down the motor until you start seeing interference. If you see clutter, touch the wire to the closest exposed metal on the trolling motor and see if the interference goes away. On motors like MinnKotas, sometimes the only exposed metal is in the mount or the foot control. I think some of the newer MinnKotas actually have a grounding wire for just this reason but I'm a MotorGuide guy so I can't say for sure. You may just need to loosen a mounting bolt or screw, put a ring terminal on the grounding wire, and screw the bolt back on.

Good luck. Interference is a real nuisance and can sometimes be a bigger nuisance to isolate. Start with the easy fix (a piece of rubber), and get more drastic from there.

Cheers,

Rob Kimm (<---has had this problem a time or two...)

[evinrude19]

i have a little 12' ft boat and a 10 hp motor and sometimes when the motor is on a high idle the transducer wont pick up anything and will be like 300 some odd feet

[LazyIke]

Thanks a million, RK. I have been running the TM and the graph on the same battery. The TM is a Motorguide Bull Dog. Can I hook the graph to the cranking battery? I thought I was supposed to attach anything else to the cranker ... does the graph has such a little drain that it shouln't be a problem.

Thanks again!

[RK]

Hiya LazyIke -

Yeah, graphs have pretty minimal drain on the battery - the alternator should more than keep up. I have all my boat's accessories hooked up to the cranking battery - and that's a pretty long list of stuff. Just remember to shut the power off on the graph. I have a master power switch that shuts off all the accessories (power goes through a main fuse panel...) The other thing to be sure to do is put an inline fuse holder in the power wire for the graph...

Bet that solves the problem.

Cheers,

Rob Kimm

[LazyIke]

I do have an inline fuse holder for the trolling motor but not the graph ...

My boat is a 1959 Larson with a 1960 40-horse Johnson -- not sure if that changes your outlook on the power-drain issue with the cranking battery.

[TonkaBass]

Awesome RK! Last night when I was out I notcied my locator was getting interference, I'll have to try these tips!