Humminbird 1155C - Manual (Page 119)

Display Problems
There are several main conditions or sources of possible interference that may cause problems with the
quality of the information displayed on the control head. Look in the following table for some symptoms of
display problems and possible solutions:
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
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 cable away from the engine wiring.
Problem Possible Cause
The control head loses
power at high speeds. If the power output of your boat’s engine is unregulated, the
control head may be protecting itself using its over-voltage
protection feature. Make sure the input voltage does not
exceed 20 Volts.
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