Humminbird 1158? combo, Product Manual

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Troubleshooting
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.