Humminbird 998c SI - Manual (Page 148)

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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
The boat’s engine
Cavitation from the boat’s propeller
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 re-appears.
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.
Turbulence created by the propeller can cause 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.
Troubleshooting