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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Problem: Power, No Display

If the depth sounder powers up but there's no display, there are four elements to check.

  1. Perform the basic electrical system checks above.
  2. Inspect the connectors and pins of the display unit and the transducer, checking for corrosion. Check that both the connectors and connector pins are straight. Use caution when straightening a bent pin as they are relatively brittle. Connectors are often not inserted properly or the connector is not tightened and this leads to corrosion.
  3. Perform a check of fuses and circuit breaker connections.
  4. Disconnect the plug to the depth transducer and verify that the sounder unit powers up. If it doesn't, you need the manufacturer's assistance.

Problem: Display Freezes

If the sounder display freezes, check the following:

  1. Perform the basic electrical system checks above.
  2. Verify that the sounder unit powers up.
  3. Check that two sounders or fish finders are not operating at the same time. Two boats using sounders within close proximity can cause mutual interference if using similar acoustic frequencies.
  4. Perform the basic interference check above.
  5. Check all cables for damage, cuts or fatigue. The transom-mounted depth transducer cables are very prone to damage particularly from trailering. If the display is still dark, you will need manufacturer's assistance.

Problem: No Bottom Readings

If the sounder operates but has no bottom readings, perform the following tests:

  1. For a new installation, check to see if the transducer is positioned correctly.
  2. Perform the basic electrical system checks for "Malfunctioning Display."
  3. Inspect the transducer for marine growth, damage or any obstructions around the transducer face. Inspect the transducer for signs of damage or paint coating. Clean the transducer face using warm soapy water, never heavy abrasives. Don't apply antifouling paint to the transducer surface. Small voids and air bubbles in the paint reduce sensitivity.
  4. Inspect the connectors and pins of the display unit and the transducer, checking for corrosion.
  5. Verify that your sounder is not trying to read a depth beyond its range. The upper and lower limit settings could be set to a region where there is no sea bottom. Consult your operator's manual for the unit's sensitivity specs and check that your settings are correct. The sensitivity control tunes in or tunes out returns. If the unit is set with low sensitivity, it will not detect bottom details. High sensitivity settings return signals on everything and clutter the screen. Most sounders and fish finders have automatic sensitivity adjustment, which compensates for ambient water conditions and depth.
  6. Check for excessive propeller cavitation. Also, nicked or bent props on high-speed boats can aggravate this condition.
  7. Where the boat has a transducer switch, unplug each of the transducers and plug into the sounder. Switches may cause problems.

If all of these actions fail, you're beyond the DIY zone and you need the help of an authorized repairer.

Problem: Incorrect Depth Readings

If your sounder is indicating the incorrect depths, do the following:

  1. If it's a new installation, verify that the transducer positioning is correct.
  2. Inspect the transducer for marine growth and damage.
  3. Verify that there are not two sounders operating close together.
  4. Inspect the transducer connectors and pins for signs of corrosion.
  5. Perform a basic cavitation check.
  6. Verify that the upper and lower depth limit settings are properly set.
  7. Inspect transducer cables for damage.

Problem: Erratic Depth Or Bottom Readings

For erratic depth or bottom readings, refer to previous troubleshooting tips, then try the following:

  1. For a new installation, verify that the transducer positioning is correct.
  2. Verify that there are not two sounders operating close together.
  3. Perform a basic interference check.
  4. Inspect the transducer connectors and pins for signs of corrosion. 

John Payne is the author of The Marine Electrical and Electronics Bible. This article originally appeared in Mad Mariner's DIY Boat Owner Magazine. This article was published as "Troubleshooting Sounders" in the January 2010 issue of BoatUS Magazine.

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