Humminbird 515 manual Finding the Cause of Noise, Isolation, time to see if the noise reappears

Finding the Cause of Noise

Electrical noise usually affects the display with many black dots at high speeds, and high sensitivity readings. One or more of the following sources can cause noise or interference:

Possible Source of Noise	Isolation
Other electronic devices	Turn off any nearby electronic devices to see if the
	problem goes away, then turn them on one at a
	time to see if the noise reappears.
The boat’s engine	To determine whether the boat’s engine is the
	source of the noise, increase the RPMs while the
	boat is in neutral and stationary to see if the noise
	increases proportionately; if noise appears when
	you rev the engine, the problem could be the spark
	plugs, alternator, or tachometer wiring. Replace the
	spark plugs with resistor plugs, install an alternator
	filter, or route the control head power and
	transducer cables away from the engine wiring.
Cavitation from the boat’s	Turbulence created by the propeller can cause
propeller	noise; make sure the transducer is mounted at least
	15” (38 cm) from the propeller, and that the water
	flows smoothly over the face of the transducer at
	all times.

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