How To Troubleshoot Depth Sounders And Fish Finders

By John Payne

When your depth sounder or fish finder isn't up to par, these procedures will help you make a diagnosis.

Modern electronics have significantly improved the performance and reliability of depth sounders and fish finders, but sometimes they do fail. The power output of a depth sounder unit is important with respect to the range and resolution of the unit. The higher the output power, the greater the depth range and signal return. Depth transducers are normally constructed of a crystal composed of various elements and it's the crystal, or a piezoelectric element, that converts an electrical signal to an acoustic signal. Never shake or strike the transducer as it could damage the crystal.


The majority of depth sounders operate at a frequency of 200 kHz. Acoustic signals are affected by seawater and bottom formations, as well as biological material (algae and plankton) and suspended particulates (silt, dissolved minerals and salts), along with water density, salinity and temperature. Before troubleshooting, read the product's owner's manual. Verify that the settings are correct, as straying fingers are a common problem. Ensure that settings are selected to auto or default to the factory settings. For each problem below, if all else fails, try a hard factory reset.

Problem: Malfunctioning Display

If the display has faded, is flashing or just blacks out, follow the steps below. If the display is foggy and shows signs of condensation, the unit requires servicing.

1. Electrical system checks: Verify that the battery supply voltage is normal, around 12 volts. If it's too high when the engine is running, there might be an alternator regulator problem.

It's common to see displays black out if the sounder is connected to an engine start battery. Start the engine and there may be a voltage surge or a dip. Connection problems are a major cause of voltage drops and this can be at the distribution panel or at the battery. Use a multimeter to check the voltage level at the depth sounder plug with the engine both on and off. If the voltage level at the battery is much higher than at the sounder, then there's likely a connection problem.

Check that all power supply connections are properly coupled, are tight and the plug's pins are straight with no sign of corrosion. Check both the fuse and fuse holder for any signs of corrosion. If the unit is supplied from a circuit breaker, check the terminations at the breaker and also operate the breaker several times.

2. Interference tests: Checking for electrical interference initially means turning off all other electrical equipment. In many cases, interference is caused by the engine, in particular outboard engine ignition systems and charging systems. Power down all other systems and then progressively start up the engine and, one at a time, turn on the other equipment to determine the interference source. If interference still remains when all other systems are off, the depth sounder may have a malfunctioning automatic noise rejection function. In some cases, when you have done everything to eliminate interference sources, you need to run the power supply through a noise suppression unit.

Always ensure that cables are installed well clear of heavy current-carrying cables. Never install sounder cables next to speed log cables, as the interference problem can be significant. Interference may also be due to cavitation (see below).

3. Cavitation checks: Cavitation is caused by water turbulence passing over a sounder transducer head and can significantly affect transducer performance. At slow speeds, water flow is relatively smooth. When the boat starts to travel at speed, air bubbles are created over the face of the transducer. Transmitted signals reflect back off the air bubbles, and this causes noise and masks the acoustic signals. Underwater turbulence is caused by the boat's hull form or underwater obstructions, as well as the actual water flow over the transducer, and from propulsion.

Transom-mounted transducer units must be properly sited and mounted to avoid turbulence. Transducer installation is crucial and it must be installed in areas of minimal turbulence or well clear of the main hull flow areas.

If you've done a factory reset and all of these actions fail, you may need an authorized repairer.

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