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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).

4 way tow vehicle side.

4 way trailer side.

5 Way Systems

Same as 4 way system listed above but adds a extra blue wire for brake signal or auxiliary power. Used on pop up camper trailers or utility / boat trailers.

5 way tow vehicle side.

5 way trailer side.

6 Way System, Rectangle Plug

3/4 inch by 1 inch 6 way rectangle connectors right turn signal (green), left turn signal (yellow), taillight (brown), ground (white). The red and blue wire can be used for brake control or auxiliary. Use on a small motorcycle trailer, snowmobile trailer or utility trailer. Can also be used as custom wiring on trailers with 3 light/wire systems. See "Tow Vehicle Taillight Wiring" below.

6 way tow vehicle side.

6 way trailer side.

6 Way Systems, Round Plug

Round 1 1/4" diameter metal connector allows 1 or 2 additional wiring and lighting functions such as back up lights, auxiliary 12v power or electric brakes. Note: The black (sometimes red) 12v and blue electric brakes wire may need to be reversed to suit the trailer. Check with a test light or VOM. Uses: horse trailer, travel trailers, landscaping trailer, car trailer, etc.

6 way tow vehicle side.

6 way trailer side.

7 Way Systems

Round 2" diameter connector allows additional pin for auxiliary 12 volt power or backup lights. Uses: heavy duty landscaping trailer, car trailer, boat trailers, horse trailer, travel trailer, construction trailer, etc. Check with a test light or VOM.

7 way RV flat blade tow vehicle side.

7 way RV flat blade trailer side.

Tow Vehicle Taillight Wiring

2 Wire System

• Tow vehicle has same bulb for lighting stop and turn signals.
• Some vehicles use a combination bulb (dual filament) for turn and stop.
• Use a simple 4 way flat connector to power your 2 light trailer lights or use a custom vehicle specific trailer wiring harness.

3 Wire System

• Tow vehicle has amber turn signals, separate.
• Has separate bulbs for lighting stop and turn signals (both red).
• Use a taillight converter to power your 2 light trailer lights or use a custom vehicle specific trailer wiring harness.

Tow Bar Wiring

When towing a car behind a truck or motorhome, diodes are needed for taillight wire harness. More info..

Tail Light Converters

Taillight Converters are used when the tow vehicle has separate wires for turn signals and brake lights (3 wire system).

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.

1. 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)
2. The t-connector from the tow vehicle should have a ground wire and should be secured tightly to a clean surface on the trailer.
3. 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.
4. 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.