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Boat Trailer Wiring Diagrams
4 Way Systems
4 way flat molded connectors allow basic hookup for three lighting functions; right turn signal / stop light (green), left turn signal / stop light (yellow), taillight / license / side marker (brown) and a ground (white).
5, 6 & 7 Way Systems
On a boat trailer a blue wire is a brake lockout circuit. It requires a flat 5 pin connector rather than a flat 4 pin. It is connected to the back up lights of a vehicle and energizes a solenoid that prevents trailer brakes from activating while reversing.
FAQ's And Trailer Wiring Problems
When I have the headlights on and apply the brakes, all the lights on my trailer go out.
You may have a weak ground problem. A poor ground that is strong enough to provide some lighting functions but not all. When the taillights and brake lights are used at the same time, it creates the maximum amp load of the lights on the trailer. If a ground problem exists, it will show at this point. The places to check grounds are as follows.
- The connector on the tow vehicle should have a ground wire secured tightly to a clean unpainted surface on the frame. A wire attached to an insulated body sheet metal panel, a surface with undercoating or rust can cause a ground problem. Test with an ohm meter for the path of least resistance. (battery disconnected from vehicle)
- The t-connector from the tow vehicle should have a ground wire and should be secured tightly to a clean surface on the trailer.
- Each tail lamp assembly at the back of the trailer also must be grounded. This is done in one of two ways. The first is by a separate wire coming from the back of each taillight lamp assembly being secured to the frame. The second, and most popular, ground is achieved through the bolts of the taillight lamp assembly. In this case, the lamp housing must be attached to the metal frame of the trailer. If the lights are mounted on wood or plastic material, the light circuit will never receive a ground.
- Note: The trailer hitch ball of the tow vehicle should never be considered sufficient ground.
When I use one turn signal the other one blinks.
This is usually related to one of two things. The first could be a short somewhere in the system. All trailer wiring and the tow vehicle connector should be checked and tested. The second, which is the most common problem, is a weak ground in the system.
Should I unplug my boat trailer when I back my boat into the water?
Yes. You should always unplug your boat trailer when backing into the water. When the trailer lights are submersed in water, it can create a short causing the vehicle fuses to blow and / or result in damage to your vehicle or wire harness.
I am blowing fuses / circuit breakers on my tow vehicle.
Possible live wire shorting to ground. Check all wires for bare copper or places where a wire or cable could be pinched. Possible overload of too many lights on the trailer. Calculate the amp load your trailer is pulling, see below. Most cars are equipped to handle only standard lighting on trailers; one rear turn / brake signal on each side and 1 set of left and right marker lights. Large trucks, vans or suvs can handle more. Check your owners manual.
If you know the wattage of the bulbs on your trailer you can use the formula P = I X E
P = watts
I = amps
E = 12 (volts)
NOTE: For estimating use only. Amperage is rounded to the nearest whole value. May not apply in all cases. Always check with your tow vehicles owners manual about adding additional loads to your electrical system. Some 12 volt batteries produce up to 14 volts on full charge. Use a VOM (volt ohm meter) to test.
Repacking Trailer Wheel Bearings
This task only takes and hour and will avoid a costly trailer repair.
If you have the tools shown, the process should go without a hitch. If you find severely corroded or worn parts, take it to a service center.
- Once you have the basic tools and materials, you can repack many bearings at almost no cost.
Step-by-Step
Project overview
Many trailer manufacturers recommend this maintenance every year if your trailer gets normal use, or at least every 20,000 miles. To do this job, you'll need a jack, a pair of jack stands, a large screwdriver, a hammer and a needle-nose pliers, clean rags, a small pan, about a quart of kerosene and a spray can of brake cleaner.
First, loosen the lug nuts on one wheel and raise the side of the trailer with a jack. Support the trailer with jack stands and then spin the wheel and listen to the bearings. If the wheel spins freely and quietly, proceed with repacking the bearings. If you hear friction or a growling sound, you most likely have a bad bearing or spindle. If this is the case, take the trailer to a service center to get the bearings replaced.
Figure A: Typical trailer hub assembly
Note: You can download and print Figure A from the Additional Information section below.Bearing repacking – start to finish
Photo 1: Remove the wheel
Remove the lug nuts and the wheel. Be sure that the trailer is supported with jack stands and the opposite wheel is blocked.
Photo 2: Remove the dust cap
Pry the dust cap loose with a large screwdriver.
Photo 3: Remove the cotter pin
Straighten the ends of the cotter pin and pull it out with needlenose pliers.
Photo 4: Remove the hub
Remove the retaining nut and washer, then pull the hub off the spindle. Be careful—the bearings will come out with the hub. Set the bearings on clean newspaper.
Photo 5: Remove the rear bearing and seal
To remove the rear bearing and seal, tap along the rim of the bearing with a block of wood and a hammer. If the seal is rusted to the back of the hub, spray a little WD-40 on the back to help loosen it. If the seal is damaged, replace it. Clean all the grease from the bearings, races and seal with a brush in a small pan of kerosene. Inspect them for signs of wear and replace them if they look suspect. Once the bearings and parts are clean, spray them with brake cleaner and set them aside to dry. Wipe the spindle and the inside of the hub with a clean rag to remove all the grease, and clean them with brake cleaner as well.
Photo 6: Pack grease into the bearings
Push the grease into the bearings, working from the wide side of the bearing. Keep pushing the grease into the roller until the cage and rollers are filled. Next coat the inside of the hub with grease. Tap the seal back onto the rear of the hub with a block of wood and hammer, and install the hub and bearings back on the spindle.
Photo 7: Reassemble the hub
Thread the nut back onto the spindle and turn it clockwise. Spin the hub a few times as you tighten to make sure the bearings are seating properly. Tighten the nut firmly. Now back the nut off about an eighth of a turn until the hole in the spindle aligns with a space in the nut. Push in a new cotter pin (available at hardware and auto parts stores) and bend the ends of the pin to keep it from working its way out. Tap the dust cap back into place. Coat the lug threads with anti-seize compound, reinstall the wheel and tighten the lug nuts. Do the same for each wheel.
Simply follow the photo series for the complete project.
Tools & Materials List
Required Tools for this Project
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
- Hammer
- Needle-nose pliers
Jack, Wrench for lug nuts, Jack stands (2), Large screwdriver (flat tip), Adjustable wrench (large)
Required Materials for this Project
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.
- Penetrating fluid
- Brake cleaner (spray can)
- Kerosene
- Small pan
- Clean rags
- Bearing grease
