How to Rewire Gauges on a Boat

Written by matt marsh
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How to Rewire Gauges on a Boat
Even a complex helm can be rewired by a handy owner, given enough patience. (boat steering wheel image by EW CHEE GUAN from

Older boats frequently have faulty wiring systems, and a common sign of trouble is gauges that give inaccurate readings or don't work at all. In many cases, the gauges themselves are fine, and corroded or broken wiring is at fault. If your boat's instruments are failing, they can sometimes be brought back to life with a bit of wiring repair. In some cases, though, the only solution is to rip out all the wiring and start from scratch.

Skill level:

Things you need

  • Fine sandpaper, 180-300 grit
  • Needle-nose pliers
  • Screwdriver
  • Multimeter
  • ABYC wire size and colour tables
  • Tinned copper wire, BC5W2 rating
  • NMEA 2000 cables and fittings, if using digital instruments
  • Crimping tool
  • Ring or captive-fork wire terminals
  • Adhesive-lined heat shrink tubing

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    Troubleshooting the Old Wiring

  1. 1

    Identify the gauges that are causing problems, and note their positions on both the face and the back of the instrument panel. Working on just one gauge at a time, note the colour of the wiring attached to the back of the faulty instrument.

  2. 2

    Unplug the wiring from the back of the faulty gauge, making sure to note which colour goes to which pin. If there is any corrosion on the connectors, clean it off with fine sandpaper and reconnect the wires. You may need pliers or a screwdriver to loosen the connectors.

  3. 3

    Follow the wiring from the faulty gauge to its end at the engine, tank, or sensor corresponding to that gauge. Unplug the connector here, and clean off any corrosion using fine sandpaper. Reconnect the wires. Note how flexible the wires are. Good multi-strand marine wire should bend easily. Non-marine-grade solid core wire is stiff and tends to hold its shape. Corroded wire often has greenish-grey powder building up at the connectors, and may feel stiff or brittle.

  4. 4

    Start the boat and check that the gauges are working properly. If they still do not work, you may have a faulty sending unit or a faulty gauge. Use a multimeter to measure voltage at the sending unit, then at the gauge: similar readings at both ends indicate OK wiring, and a 0 or 12 V reading can indicate a failed sending unit. If you noticed significant corrosion or stiffness in the wires, you probably have to install a new wiring harness.

    Installing the New Wiring

  1. 1

    Remove all the old wiring, if you have not done so already. If you are going to spend the time and money to rewire your boat's instruments, there is no point in repeating any mistakes or reusing any faulty parts from the original installation.

  2. 2

    Draw a schematic diagram of your new wiring system. You don't have to follow any particular drawing style, but your drawing should be clear and easy to read. The American Boat and Yacht Council (ABYC) publishes a standard colour coding scheme for marine engines and gauges, along with charts to determine the correct size of wire for a given length and current. Look up the appropriate colours and sizes for each wire and instrument on your drawing, and write them down. For wires where the standard colours don't apply, use red for positive and yellow for negative. If you're at all unsure about the details, ask a marine electrician to help find any errors in your drawing.

  3. 3

    Pull new wires, of the correct colour and size for each analogue instrument, from the helm panel to the engine and the individual sending units. If you are installing modern digital instruments, choose a route for the NMEA 2000 backbone cable and secure it in place. If you have manufacturer-specific engine gauges, choose a route for the appropriate proprietary cable and secure it.

  4. 4

    Find the negative bus bar, and move it to a more convenient location if necessary. Pull a yellow wire, of the appropriate size for that gauge, from the negative bus bar to each gauge. If you have lighted gauges, pull red wires from each gauge to a shared positive terminal, usually one that is energised when the navigation lights are on.

  5. 5

    Crimp a ring or captive-fork terminal on the end of each wire, and seal the crimps with adhesive-lined heat shrink tubing. Connect these terminals to the corresponding pins on each analogue gauge and sending unit. If you are using digital instruments, plug an NMEA 2000 branch cable into each sending unit and instrument. Connect these branch cables to the NMEA 2000 backbone cable with T-fittings or an NMEA 2000 junction box.

  6. 6

    Connect each sending unit's power supply cable to the fuse box or circuit breaker panel. Check that each instrument's power supply cable is protected by the fuse specified in its manual, and that the fuses don't exceed the current rating you used to calculate the wire size in step 2.

  7. 7

    Test the system by turning the ignition on. Watch for appropriate readings on each analogue gauge. Program your desired settings into any digital gauges you're using.

  8. 8

    Protect your new wiring harness against chafe and vibration by securing it in place with conduit and wire ties.

Tips and warnings

  • Use only marine-grade tinned copper wire. Good quality boat wire usually has a "BC5W2 Oil Resistant" rating. Solid wire cracks, and non-tinned wire corrodes quickly, in the marine environment.
  • Plan carefully, and follow your schematic drawing! Most "rat's nest" wiring systems are the result of poor planning, or no planning at all.
  • Even a 12 volt system can produce enough current to seriously harm a person, and can quickly start a fire. Follow the ABYC wiring standards, use only good quality components, and get assistance from a marine electrician if you are at all unsure about how to do something.

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