Eagle Electronics SeaFinder 250 DF - Manual (Page 66)

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If no noise is present, turn the pump off, then turn on the VHF radio
and transmit. Keep doing this until all electrical equipment has been
turned on, their effect on the sonar display noted, then turned off.
If you find noise interference from an electrical instrument, trolling motor,
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 trans-
ducer owner's manual for the best mounting position.