Boat Trailer Lights

Lights usually are a constant headache if used on trailers that are submerged in saltwater. You freshwater guys don't know what you are missing. There will be all kinds of lights, some of the "standard" automotive lights will even be advertised as "waterproof". In actuality very few are. Some marine dealers will have simple automotive trailer lights on their shelves, & let the buyer beware attitude.

First Things First -- The light bulbs used here are a dual filament, meaning that one bulb will function for both the dimmer tail light on one filament & the brighter stop light on the other. If you don't get the proper lights functioning, CHECK THE BULBS first. They may not be burned out, but simply corroded. One common somewhat cure is to clean the electrical nubbins on the rear with fine steel wool, then cover them with Vaseline, to keep off the corrosion. More on the bulbs later in this article.

Other than the bulbs, the most common cause of light failure is a bad connection SOMEWHERE in the system. This is usually in the form of corrosion at a connection & usually at a grounded wire location.

Shown below is an e-mail plea from a reader. First off this could be more than one thing wrong. I would first look at the bulb & the contacts in the bulb socket or clean the small lead contacts on the bulb. Then I would check any connections for good continuity starting from the back & going forward. There may even be a chaffed wire that is cut into somewhere under the trailer. I have since revised this article for more clarity.

Also how many times do you still see someone unplug the trailer light pigtail from the vehicle just before they dump the trailer in the water? This may have been the order of the day when the light units were open to the water & they figured that when the bulbs were submerged, that when you hit the brakes to slide the boat off, that the cold water would break the now hot bulb. This has been long abandoned with the advent of more sealed type light units.

The Light Units Themselves -- Most trailer light units will be wired for 2 wires coming out of the light unit & will need to have at least one mounting screw that is grounded inside, which will be grounded to the trailer frame, either under the light unit mounting bolt or by a separate wire all the way forward to the plug in.

This spot will need to be cleaned so that good contact can be made. It is also recommended that some chassis grease be smeared at this point to help reduce rusting & future non-conductivity of the connection. You can tell which one by looking inside the unit & finding the metal part that the bulb locks into. This one will then more than likely have the attaching bolt go thru it to the outside.

As a better alternate, the white ground wire could be wired together from each the RH & LH light units & clear back to the connector at the hitch & then grounded at the vehicle thru the plug.

The wires will need to be connected to the wires coming from the plug-in connection unit. At the point you connect them at the light unit, it is recommended that you get some heat shrinkable tubing. Slide a 1 1/2" section over the wire, & away enough so that the soldering does not effect it. After the solder job is finished & cool, slide the tubing over the new joint. Now take a match & heat the tubing up just enough that it shrinks & makes a tight seal on both sides of the joint.

You could use wire nuts or crimp on connectors, but these are not waterproof. It also has been this persons observations that if you happen to be traveling over an extended rough road, (the Alcan Hiway for instance) that just the weight of the wire nut connections will vibrate enough to where it will break the wire at the connection if the wire is not secured to the axle, frame or part of the trailer very close to the connection.

You may have to make a choice of light units that match your trailer needs. This can mean that you need a low unit if your trailer is so designed.

All light units are mounted so that the license plate holder is mounted on the LH side of the trailer looking forward. All units will come with about 10" of color coded wires protruding for you to attach to the main trailer wiring. The RH unit will have a brown & green wire, while the LH unit will have a brown & yellow wire protruding.

Universal trailer wiring color code -- The normal trailer wire colors will be white, brown, green, & yellow.
      White --  Gr = Ground
      Brown -- TM = Tail Markers = Tail lights
      Green --   RT = Right Tail Lights = Right hand directional
      Yellow -- LT = Left Tail Lights = Left hand directional

The directional light bulbs also function as stop lights, so in testing if the directional light comes on so will that brake light when stopping.

Other colors may be red & blue, and will normally be used for auxiliary (side clearance lights on larger trailers) and electric brakes. However these are 2 that you normally do not see on boat trailers.

Ground wire -- Be sure that your wiring is grounded to the frame or bumper on the towing vehicle and then ran into the ground lug on the receptacle. Now be sure that this ground wire is again bolted to the trailer so it is also well grounded. If you start to have light problems, check either of these ground connections for good contact FIRST.

Universal plug in wiring code -- Check the wiring diagram of your receptacle unit. Years ago (like maybe 40-50) there was no universal wiring & I have even seen some of the old plugs with marking for red, black, white & ground. Using this they usually ran a blue or green as ground, rather confusing.

If the one you buy does not offer this advise, check with a different brand. It also is a good idea when you are finished, to mark on the diagram or make up a wiring plug diagram with the wire colors to which prong that you do use, & keep it with the vehicle or boat.

Plug in Receptacles -- Most common receptacles will be either a 4 or 6 wire unit. In the old days there was even a 5 & 7 wire unit. Currently the simplest ones are a flat plastic 4 wire male/female receptacle, with color coded wires in & out. These will have 3 wires insulated & 1 bare coming from the vehicle. The trailer end will be reversed using 1 insulated & 3 bare. Most all boat dealers now are pre-wiring the trailers with these 4 wire plastic connectors. They are many times very short to the trailer, so short that you will need a short, (18" to 24") section of wire coupler. This is good in that with the many types of receptacles on the different towing vehicles, you make up one to fit your vehicle, with the 4 wire on the rear & a 6 wire going to your vehicle.

Most new towing vehicles are pre-wired with a standard 6 wire RV plug in receptacle, however they are large in comparison to the standard 6 prong. I have even seen a 7 wire RV plug, with the center plug going to a extra battery. The standard 6 wire RV is basically the same, but does not use a center prong.

Just because the plug in, or receptacle you purchase has places for 6 wires does not mean you need to use all of them when 4 will normally do for standard boat trailer lighting. These 6 wire units are more readily available & better built.

I have one of each that no matter who's vehicle is used there is one that fits, (if the vehicle is wired right). I even have an extra 4 wire flat connector (to the trailer) with about 24" of wire attached if someone has no wiring or it is wired to current non coding. Using this I can splice into the trailer wiring & then plug my pre-wired pigtails & be on the road without all the hassle of trying to figure these old ones out.

The metal ones usually will be round & the female part that fits the vehicle will have a spring loaded door that covers the prongs when the 2 units are disconnected. They have the 5 or 6 prongs & will normally have some internal marking for the prongs showing where which wire goes. I have settled on the 6 prong plug unit, as this gives me the option of using the same receptacle unit for many different trailer requirements with spare connector prongs if needed. Also it is readily available from many different manufacturers & is usually interchangeable between manufacturers. It is even available in Canada.

As mentioned before, many trailers come with the flat plastic 4 wire unit. It is best in this case, to make up a short, 18" or so wire set that plugs into this plastic unit on the trailer & then on the other end use the 6 prong male plug to your vehicle. In this manner you can then get another plastic 4 wire unit & wire in a different male plug if there is a chance that your son or neighbor with a different plug may tow your boat. Wire this other unit to fit theirs ahead of time if they are different.

This is important if you may ever have the need for someone else to tow your trailer. It is especially important if you have electric brakes. As IF you have the brakes wired to the lug that would normally have been the lights or a directional, your brakes will come on at times you never intended like turning your lights on. And this could lead to HOT brakes & or a fire. My brother borrowed my stock trailer once & his pickup was not wired according to the diagrams, Everything was fine during the day, but when it got to where he had to turn the lights on coming home, things started happening. And the brakes were on and got hot enough to lock the trailer up until he figured he had to unplug the light cable from the vehicle.

If you ever have to track down a problem in the wiring, you will more than likely need a tester. The 4 wire tester shown on the left in the photo below may have some benefit at times as a preliminary tester. I do not see how it can tell you that the wire or a bulb is OK behind this tester. It will surely tell you if you have the receptacle on the towing vehicle right.

But the one on the right is the old standby. It has a 12volt bulb in the red handle, an alligator clip for the ground wire & a sharp probe to make continuity to a hot wire. In use you have to have the lights on & if it is directional related, that blinker on. You can clamp the clip onto any GOOD ground & push the sharp probe into a suspected wire until you make contact with the inner wire, (be careful as you MAY WELL POKE YOURSELF). If you have a connection, the red handle will light up.

You may even have to remove the lamp lens cover & test the contact points where the bulb mounts. Again, clean the bulb contacts if need be.

In the photo below the 4 wire plug in tester would be good to test the wiring you have in the pig-tail & from the vehicle. Since it needs power to light up the small indicator bulbs, it would not work trying to run down the trailer lights themselves.

Testers designed to perform different tests.

4 Wire Vehicle to 3 Wire Converter for Vehicles With Separate Directional Lights -- For newer vehicles (especially non domestic) with separate & usually amber directional lights, it is a pain to wire in an extra set of trailer lights just for the directionals. However there is a "little black box" that you wire between your vehicle & the receptacle that converts 4 wires coming in to 3 wires going out. The 4th wire is the ground wire. They are not overly expensive, $20 or so. It has been noted that these do not last indefinitely however, and may have to be replaced in 4-5 years.

Bulbs -- The standard combo trailer light bulb has dual bayonet lugs on the base. These short lugs will be offset, one being slightly closer to the base than the other. This will ensure that the bulb is positioned so that the dual internal filament of the bulb will always have the stop light filament (which is brighter than the tail light side) taking power from the brake sender unit. The number of this bulb will usually be #1147

Automotive -- These lights will usually have the housings made of plastic, with the rear lens cover screwed on. Many times they will be universal, meaning there is no dedicated RH or LH side. Most times you will find a LH (license) side with a clear lower lens cover to illuminate the license plate. You can also find the RH or LH with a small outside lens cover to act as a side signal light. They usually will have no provisions for keeping water out. There may not be any gaskets under the lenses & WILL NOT have any sealing grommet around the wires.

Weatherproof -- These many times are not really much different than the automotive, except they may have a gasket under the lens cover. But usually do not have any type of a grommet or seal around the wires. They must mean weatherproof from water spray on a rainy day on the freeway.

Waterproof -- These can be had in a few different styles.   Some can be the same as the weatherproof, but with a seal or grommet around the wires. Not enough waterproof in my book.

Trapped Air -- You will find this version having an air chamber on the bottom where the lights are. In essence it is the same as a one ended open can upside down. In use, the air is trapped in the upper chamber as the trailer is submerged. The height is made so that water never reaches the bulbs. A couple of drawbacks when using this style, is that you need to be sure there are no cracks in the case & that any seals are intact around the wires.   Also it is not the best when using around saltwater as just that close of proximity to the saltwater, as over time the light bulb connections will corrode. Usually the light bulb is mounted in a slide out unit that comes out from the bottom.   But I have not had any luck trying to find replacement internal parts for these.

Sealed -- These are the true water-proof lights. They are made as a sealed beam type unit. They may be more expensive to start with, but will last many years.

LED -- These appear to be the best out there. They do cost more, but as with the sealed units are sealed & are pretty maintenance free. Many have bullet type plug-in wiring at the unit. These connections may need to be looked at occasionally & cleaned, even greased with chassis grease or Vaseline to keep corrosion off.

Things you can do -- In a pinch & or on a short budget, you can be selective when purchasing automotive type light units. Then either silicone seal the lens cover gaskets to the body, or if you have ideas of ever replacing the bulbs, buy some soft neoprene or rubber type sheet gasket material, this stuff comes in different thickness, however about 3/32" would be about right. It is usually offered in 12" square sizes. Now make your own gaskets to go under the lens covers. Also you should apply some silicone caulking around the wiring exit hole.

Some trailers will have side or clearance lights that are wired into the same brown wire as the tail lights, silicone any opening here also. Do not silicone the bulb receptacle as silicone seems to contain ammonia & will corrode if in close contact.

If you have a smaller boat, you can take a wood 2"x 4" & clamp it onto the stern of the boat, with automotive lights attached to this, with wires running forward to the vehicle plug in. In this manner you can detach the light bar when you launch the boat. The lights will never get wet.

I have also seen trailers that have the side rub rails, & the rear upright is taller than actually needed. On the top of these uprights are mounted the trailer lights, high & dry.

One bit of advise, always unplug the connector before you drive the vehicle off. I have found that on a recent boat that I bought that one brake light was not coming on. Everything checked out on the trailer, but still no go. I was at the point to pulling a new wire from the plug in back to the bad light unit. But what I found was that the wire was broken inside the rubber 4 wire connector on the trailer wiring. While checking continuity of that wire by working my way forward, if I twisted the rubber just right, I could get continuity. Apparently the previous owner had forgotten to unplug from his vehicle & stretched that one wire beyond it's limit. And it was all inside the rubber unit beyond being able to see the problem.

4 wire flat to 4 wire flat extension	4 wire to standard 6 wire extension, note the extra blue wire not attached, it is so I can use it for brakes by wiring around the plug if needed later

Here is the 4 to 4 seen above with a 4 to 6 RV adapter	Standard 4 wire flat to 6 wire RV