Humminbird 1157C - Manual (Page 186)

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.
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