For most consumers, trailer wiring repair can be a frustrating experience. Diagnosing the root of the problem early in the process is the key. I recommend a 12 volt stick-tester to check your wiring and plug for continuity as the first step in any electrical repair. Plug testers are available for all types of trailer plugs, but a stick tester can be used on any receptacle and can test wiring between connections. And they typically cost less than ten bucks.
First, with the trailer wiring plugged into your tow vehicle, turn on your running lights. I recommend doing this with the hitch not connected to the trailer coupler, for reasons we will explain later. Do a walk around the trailer and check to see if all lights are lit. If your tow vehicle blows a fuse during this phase, first remove your trailer tail-light lenses and check for blown bulbs (usually black or creamy white inside if blown), and also check to make sure the bulb is in correctly. The industry standard for trailer tail lights is an “1157″ bulb, which is installed by pushing into the light socket and twisting it so that both contacts rest on the tail-light prongs. If the bulb is only “half-twisted” in, then both filaments will light, as the bulb’s two contacts will touch both tail-light prongs. This will create a dead short when a turn signal or brake is used with the running lights on, and can also result in all marker lights blinking or illuminating along with the turn signals or brake lights. Next check behind the tail lights, and then behind each marker light to see if the hot leads to any of them might be crushed between the light housing and the trailer frame from improper installation. This is a common problem, prevalent on new trailers. A tail light wire could have been crushed behind the housing during installation, but the inevitable short may not surface for long periods of time, perhaps when moisture gets behind the light and completes the circuit. Most lights are connected to the trailer with ¼” bolts and will require a 7/16” wrench to loosen the nuts. Pull the housing away and make sure the wire(s) are run through the provided channel behind the light to avoid smashing them on re-install. If any wires are exposed, then replace that section with new, insulated wire. On most trailer lights, 14 to 16 gauge wire is sufficient.
With the running lights still on, turn your ignition key enough to operate turn signals and put it in “left” or “right” turn (NOT hazard flashers). Do another walk around and check each turn signal. Are any running lights flashing? If so, and you have already checked the bulbs and wiring on the previous step, then you probably have a bad ground. The ground problem could be on the tow vehicle or trailer, but sometimes you can get a sporadic ground through the hitch ball and coupler connection, although it isn’t a stable enough ground to rely on for lighting and electric brakes. Unplug your trailer and connect your light tester “ground” to the tow vehicle ground terminal. This is the “male” post on a 4 or 5-way flat plug, or at roughly “7 o’clock” as you look into the 7-way plug typically included in factory hitch packages. Turn on running lights and a turn signal and test by putting your stick tester to the terminals. On a 4 way, tail lights are the brown wire, yellow and green are left and right turn/brake respectively. On a 7-way, left turn/brake is at “9:00” and right turn/brake is at “3:00” and running lights are at “11:00” or above the left turn. If all functions are working, move on to the next step. If not, then ground your tester to the tailpipe (the hitch sometimes has too much paint on it to get a ground) or steel bumper and re-test. If the tester is lighting now and didn’t light using the vehicle plug’s ground, then you need to ground the tow vehicle plug. Check for a broken wire coming out of the plug. The ground should be a white wire. Ground this using a wire “eye” terminal directly to the frame using a self-tapping screw and re-test. If no broken connections are found, then open the plug (if possible) by removing the screws in the side of the housing and checking all wire connections to the terminals. On a 4-way, this will not be possible. Some vehicles have 7-way plugs that will not be repairable in this fashion and must be replaced. 4-way plugs sometimes lose their connections inside the molded housing and are not repairable. Replace as needed. If I am not getting a test light at the terminal, I like to stick the pointed tester into the wire leading to the rear of the plug to see if there is any signal going to the plug, while grounding to a reliable ground source (such as a tailpipe).
If some of the functions in the plug work, the ground is likely fine and you need to check wiring and fuses going to the plug. On aftermarket installations, some vehicles (those with separate turn and brake lights) incorporate a “converter box” to merge those functions so the trailer lights will operate with either turn or brake using the same wire. It is essentially a box of diodes, and these can and do burn out. Test all functions going “into” and “out” of this converter box. If you have turn and brakes (separate functions on this type of vehicle) coming from the vehicle into the converter box, but not working coming out of the box (and merged with the turn signal function) then you have a bad converter. Snip the wires and splice a new one into the wiring harness.
Many vehicles have factory tow packages, which incorporate a separate fuse block for towing functions. This system is preferable, in that a blown fuse from malfunctioning trailer wiring will not affect the tow vehicle lights, which are on a different circuit. If no signal is reaching the plug, check the owner’s manual (usually towards the rear of the manual) to locate the towing package fuse placement and pull them out one at a time. There is usually a separate fuse for running lights, left and right turn (and 12v hot, electric brakes, etc. as needed). Most tow package fuse blocks are located under the hood. All tow packages are already converted if tow vehicle has separate turn and brake functions.
Once the vehicle wiring is deemed to be operating, it is time to plug it back into your trailer and take another look at the lights. If trailer lights still flash on and off with tail lights and a turn signal on, then the trailer has a bad ground. Check the white wire from the plug and make sure it is properly attached to the frame, usually near the coupler. Note: On tilt-bed trailers, the lights are usually attached to a frame that is NOT permanently attached to the tongue of the trailer. A ground wire needs to be run either directly to the trailer light attachment bolts from the plug, or at a minimum the wire needs to be run to the “tilt-bed” part of the trailer. This is the only way the lights will be able to use the tow vehicle’s ground on this type of trailer. Although a sporadic ground is sometimes transferred to the tilt-bed, since the bed moves independant of the main frame, the ground will not be consistent.
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