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pump, or radio, try to isolate the problem. You can usually reroute the sonar
unit’s power cable and transducer cable away from the wiring that is
causing the interference. VHF radio antenna cables radiate noise when
transmitting, so be certain to keep the sonar’s wires away from it. You may
need to route the sonar unit’s power cable directly to the battery to isolate
it from other wiring on the boat.
If no noise displays on the sonar unit from electrical equipment, then make
certain everything except the sonar unit is turned off, then start the engine.
Increase the RPM with the gearshift in neutral. If noise appears on the
display, the problem could be one of three things; spark plugs, alternator,
or tachometer wiring. Try using resistor spark plugs, alternator filters, or
routing the sonar unit’s power cable away from engine wiring. Again,
routing the power cable directly to the battery helps eliminate noise
problems. Make certain to use the in-line fuse supplied with the unit
when wiring the power cable to the battery!
When no noise appears on the sonar unit after all of the above tests, then
the noise source is probably cavitation. Many novices or persons with
limited experience make hasty sonar installations which function perfectly
in shallow water, or when the boat is at rest. In nearly all cases, the cause
of the malfunction will be the location and/or angle of the transducer. The
face of the transducer must be placed in a location that has a smooth flow
of water at all boat speeds. Read your transducer owner’s manual for the
best mounting position.