This is something i came across might be of some help
Section 9.
Depth Finder Interference
Variable speed motors with a PWM speed control / Maximizer circuit can sometimes
interfere with the operation of a depth finder. This can be due to RF (radio
Frequency) or electrical interference.
Here are a few steps to reduce or eliminate the interference problem:
If the motor interferes with the depth finder when both are being operated:
Step 1.
Determine if supply voltage for trolling motor and depth finder is provided by the same / common battery.
A.
If yes, disconnect depth finder battery leads from the trolling motor battery and connect them to the
engine cranking battery.
B.
Test depth finder for interference while operating motor.
Step 2.
If interference is still present at Step 1B, proceed as follows:
A.
Connect a light gauge wire (18 gauge is fine) from the negative post of the trolling motor battery to the
negative post of the engine’s cranking battery. We suggest installing a 1 or 2 amp inline fuse in this
ground wire. (Minn Kota P/N 2060310)
B.
Test for depth finder interference while operating the motor.
Step 3.
If interference is still present after installing the ground wire, proceed as follows:
A.
Check the routing of the depth finder and trolling motor battery leads. If they run parallel to each other
for any length of distance, separate the leads as much as possible or run the leads to the trolling motor
and depth finder on opposite sides of the boat.
B.
Test depth finder for interference while operating the motor.
Step 4.
If interference persists after completing the previous steps, proceed as follows:
A.
Check mounting location of the depth finder transducer. If the transducer is on the trolling motor’s
lower unit, try temporarily moving it way from the lower unit while operating the motor and observing
the depth finder display.
B.
If the interference is reduced or eliminated when the transducer is moved away from the motor’s lower
unit, the problem is due to the transducers proximity to the lower unit.
C.
To reduce / eliminate this type of RF interference, a ground wire can be connected to the trolling motor
lower unit. This can be accomplished by means of either an external or internal connection.
Grounding the motor case in this manner creates a “shield” between the motor brushes and the
transducer, trapping / shunting the RFI to the ground.
C-1.
To ground the motor case externally, drill a small diameter hole (1/8”), in the motor skeg.
Attach one end of the ground wire at this point by using a self tapping stainless steel screw
(18 gauge wire may be used for this purpose). Run the ground wire up the motor shaft along
with the transducer coax cable. Connect the other end of the ground wire to the motor
negative battery lead or post.
C-2.
To ground the motor case internally it will be necessary to disassemble, reseal, and
reassemble the motor lower unit. (We recommend this be done only by a Minn Kota
authorized repair center) With the lower unit disassembled, connect one end of a light gauge
wire to the motor brush plate mounting screw (18 gauge wire may be used for this purpose).
Run the ground wire up the fiberglass motor shaft along with the red and black motor brush
leads. Connect the other end of the ground wire to the motor negative battery lead or post.
Reassemble and reseal the motor lower unit.
Depth Finder Interference 10/31/02
9-
2
Step 5.
In certain situations, when the above steps do not take care of the interference. One additional step may be tried
(NOTE: this is an external ground situation only, as in above Step 4 C-1)
.
A.
Because the center section (magnet shell) and the skeg (rudder) are electrically isolated from each
other due to o-rings and/or paint, you may need to run the ground wire to the center section and the
skeg. This involves either baring a spot under the transducer clamp (stainless steel hose clamp), and
placing the ground wire in contact with the bare spot under the loose clamp (or a piece of aluminum
sheet may be used between the motor and transducer - ground the aluminum sheet to the motor skeg).
By following these steps the problem of depth finder interference is usually resolved.
Oftentimes, simply connecting the depth finder to a separate battery will address this
issue. At other times, a separate battery and common negative ground is all that is
required. For other installations, all of the steps outlined will be required. Keep in
mind that the steps should be followed in the order they are written and that the final
steps should be attempted only after completing steps 1, 2, and 3 and testing with the
depth finder.
