Diagnose Boat Trailer Wiring Problems

Diagnose Boat Trailer Wiring Problems

Every boater should do a “walk-around” before pulling his boat trailer out onto the highway to head for the lake. A quick visual inspection of the tires can prevent a roadside emergency. You can also check to see if the tie-down straps are tight and secure. What about the lights? Probably no other feature on a boat trailer causes more headaches than a boat’s wiring. If you take a systematic approach to your lighting/wiring problems, you can prevent many long-term frustrations.


Isolate Symptoms

1. Hook the tow vehicle to the boat trailer. Connect the light connectors on the trailer and vehicle.

2. Position yourself behind the trailer where you can see the lights. Have your assistant sit in the tow vehicle.

3. Tell your assistant to turn on the headlights. The tail lights on each side of the trailer should show with equal brightness. Any running lights on the trailer as well as the license plate light should turn on.

4. Turn off the head lights.

5. Instruct your helper to turn the vehicle key switch to the “Run” position. Activate the right turn signal. The right tail light (and only the right tail light) should begin blinking

6. Instruct your helper to activate the left tail light. The left tail light (and only the left tail light) should be blinking.

7. Instruct the helper to turn off the turn signal, and step on the brakes. Both tail lights should glow.

8. Instruct your assistant to turn the headlights back on. With the key switch in the “Run” position, he should activate each turn signal, one at a time. As the turn signals are activated, the light on the side being activated should blink intermittently brighter than the opposite side.

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9. Tell your helper to leave the headlights on and to step on the brake. Both tail lights should glow equally bright when the brakes are activated.

Simple Fix First

10. Check the connectors where the trailer wires plug into the vehicle’s wires for corrosion.

11. Use a small screwdriver and emery cloth to remove any corrosion present.

12. Reconnect plugs and repeat the failed test or tests.

13. Visually check the for broken wires or cracked insulation on the wires leading to the connectors on both the trailer and the vehicle. Pay particular attention to the white wire which connects to a grounding screw on the trailer. Uneven light brightness, strange blinking patterns when turn lights are activated or tail lights turning off when brakes are activated are all signs of a poor ground.

14. Remove the grounding screw. Use the emery cloth to eliminate any corrosion on the connector or the connecting site on the trailer.

15. Reconnect ground and retest system.

Test Trailer Light Fixtures

16. Examine the wires and connections going into each trailer light fixture for breakage, corrosion or cracked insulation.

17.Use a screwdriver to remove screws holding the lens to the light fixture if present. (Some trailer light fixtures are factory sealed and impossible to open.)

18. Remove the light bulb by rotating in its socket 1/4-turn counterclockwise and pulling.

Check the bulb. You should see two visible filaments inside the bulb. There should not be any burned area on the inside of the glass.

19. Replace if necessary.

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20. Use a 12-volt test light to check the wires going into a sealed light fixture by grounding the test light, perforating the insulation on the wire with the tester’s probe and having your helper activate the light controls in the vehicle. The brown wire (present on each fixture) is the running lights. The yellow wire (found only on the left fixture) is the left turn/brake light wire. The green wire (found only on the right fixture is the right turn/brake light wire. If the wires are all powered going into the light fixture but the light inside does not turn on, the whole fixture will need replaced.