Humminbird 785C2, 755C manual Display Problems, Finding the Cause of Noise, From the engine wiring

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:

Problem Possible Cause

The control head loses If the power output of your boat’s engine is

power at high speeds. unregulated, the control head may be protecting itself using its over-voltage protection feature. Make sure the input voltage does not exceed 20 Volts.

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 re-appears.
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.

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