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Tow Trailer Lighting: A Primer on Troubleshooting and Repairs

Many types of tow-behind trailers depend on the simple plug in hook-up to enable the turn signal, brake and tail lights, to operate correctly. Whether it’s a tow behind RV, boat trailer, or small utility trailer, they all use basically the same system. If your vehicle has a tow package already installed it is just a matter of converting the trailer plug to the one on your vehicle. The most common type of plug is the four prong. This is a flat plug with three female and one male on one end with the matching end being three male and one female. There are also several round plugs with a six and seven prong set up.

To make it much easier you can buy adapters to match different plugs on the tow vehicle and trailer. Most major auto parts stores carry a wide variety. These come in handy if you have more than one trailer with each having a different plug type. In some businesses this isn’t all that uncommon.

Sooner or later after the trailer has a few years on it you will inevitably have lighting problems. Usually you don’t know there’s a problem until you are ready to use the trailer. The last time you used it everything worked perfectly and now the lights are acting screwy or not working at all.

A bad ground on the trailer can cause many lighting problems. For instance, when you activate your turn signal both of the lights will blink. They may stay on and blink, or they may just stay on. The towing ball and the receptacle for it is used as a link of the grounding system itself. This is done to eliminate extra wiring and works well most of the time.

The lights themselves have to be grounded to the trailer frame to complete the circuit and enable the lights to perform properly. Check to be sure a clean surface is available for the lights to complete the circuit. A 12 volt test light with a sharp probe is the best to use for this application. It has a clip on ground wire which enables you to check the electricity going to the lights themselves.

Before you connect the trailer to the vehicle, use the probe to check the power coming from the plug on the vehicle. Attach the ground clip and turn the parking lights on the vehicle on. By inserting the probe into the prong holes you can see which hole carries the tail light current. Repeat this with the turn signals and brake lights. Having someone to push the brakes is helpful.

If everything checks out here the problem is on the trailer end. By using the test probe you will be able to completely check each individual light for proper grounding and good power connections. The probe is easier to use than an ordinary voltage meter because it leaves one hand free.

The sharp probe can be used to check for a broken wire by pushing it through the insulation at points along the wire until the light inside the probe handle doesn’t light up. In most cases replacing the entire wire is preferred but in some cases, such as in RV’s the wire cannot be exposed because of an area being inaccessible.

By clipping the ground wire on the probe to the metal frame or metal covering of the particular trailer you can check the power to the light assembly itself. By removing the bulb and activating the turn signal the probe can be inserted into the socket to check for a closed circuit.

Do not touch the inside of the socket itself. A fuse can easily be blown by touching the contact point where the wire enters and touching the socket wall at the same time. This will also tell you if a bad bulb is the problem. If the light in the probe glows and the bulb doesn’t work a new bulb is probably the answer.

Usually the white wire on the trailer end of the plug is the ground. In many cases it is attached directly to the frame. Many times this contact point will rust or become corroded causing the ground to be bad. Often this is simply a self tapping bolt with the wire wrapped around it. A thorough cleaning with steel wool or sand paper will often solve this problem.

I have observed many different ways to attach the ground wire to the frame even soldering. Sometimes it is just better to reattach the wire in a new spot. Using a flat, round crimp on terminal will usually give a better ground than just wrapping a wire around the bolt. Once again, your local auto parts supply can help you.

Sometimes though, the lighting system itself is so corroded that replacing the entire thing is the best way to go. For RV’s, you may be able to replace the old lights with new ones by ordering them from the company. Usually the wiring is good and the fixtures themselves have become corroded. It is a simple matter to unscrew the wires, one at a time and connecting them to the proper spot.

On other types of trailers such as a boat trailer, the light fixtures attach directly to the frame and replacing the wiring is a snap. Be sure the surface contacted by the fixture is clean and shiny. If you do a lot of towing, or even just on occasion, learning how the system works and how to use the test light probe will save you a lot of time and frustration.

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