Troubleshooting Trailer Wiring Issues & Electrical Bugs

There is nothing worse than attaching a trailer to your vehicle and finding that nothing across the wiring harness works. Sometimes, the tail lights fail to light up. Other times, the brake lights don’t illuminate – or worse, the brakes don’t come on line when the pedal is pushed.

While these scenarios can be maddening, they also apply directly to the safety of your trailer, the pulling vehicle, and most importantly, the protection of your passengers. As a result, we thought it might be helpful to take a look at what, where, and how trailer components fail, as well as how to troubleshoot, fix, and maintain them after-the-fact.

What General Systems/Components Apply

In general, there can be several culprits causing trailer wiring and electrical issues. These include:

• Wire Harness
  This linear wiring structure represents the electrical backbone of a trailer system. Regardless of how good the harness may look, there are always potential gremlins lurking from front-to-back. Consequently, if something goes awry at this level, usually nothing else will work as advertised either.

• Connectors
  In professional racing, there’s an old failure axiom which suggests that more times than not, “what gets you is the 50¢ part”. In this case, the rule also applies to electrical systems related to trailers.

• Light Housing
  Depending on the size and configuration of the trailer, there can be up to eight tail or position light housings involved. These components cover and protect bulbs and connectors. If a housing is damaged or overly-corroded, it is possible that the connector won’t operate properly.

• Tail or Position Light Bulb
  This is self-evident. If a light bulb is damaged, or the filament shows a broken element, the component will not illuminate.

• Brake Light Housing
  This component is similar to tail or position light housings. If the housing is damaged, a connector for a brake light will not illuminate.

• Brake Light Bulb
  Same component as tail and position lights.

In the event of electric trailer brakes…

• Electrical Brake Controller
  In some instances, trailers stop on the basis of electrical drum brakes. The controller manages and energizes the brakes as the driver’s pedal is depressed. If the controller malfunctions, the braking system will not work properly.

• Brake Drum
  Ordinarily, brake drums don’t involve themselves with electrical processes during typical trailer operations. However, in an electric braking system, the brake drum serves as a housing for electro-magnetically driven components associated with a magnet/drum/brake complex.

• Brake Shoe
  These components create braking force as the brake controller excites a magnet/spring system. In turn, those components electrically extend the shoes against the inside of the drum.
  

• Brake Magnet
  Magnets are central to the entire electrical braking system. As described above, when energized, they extend the shoes against the insides of the drum, thereby creating braking force.

Failure Hot Spots

Now that you know what the components are, and where they apply, here are areas where things can easily go bad.

• Burned-Out Bulb
  More times than not, burned out bulbs are the reason that trailer-mounted electrical lighting systems fail to illuminate.

• Dirty or Corroded Plug/Socket
  Similar to a damaged bulb, if the plug/socket complex is messed up, even a good bulb won’t work.

• Damaged Wiring
  This failure area is usually identified within the frame rails. Some trailers are wired in series, so in this event, if an electrical arc occurs and burns through the conductor, nothing will work.

• Weak/Poor Connection
  Wire-end connectors are really a weak link since even a bent, corroded or otherwise malformed wire connector can block current throughout a wire harness.

• Overloaded Harness
  Electrical energy creates heat. If the wire harness is damaged due to chafing of the outside insulation, other electrical kerfuffle, or the voltage controller malfunctions, it is possible to create an electrical arc, aka a ‘short’. Should this occur, it’s entirely possible to fry the entire harness.

• Weak Ground
  12v trailer electrics typically involve two circuit legs: a positive versus a ground (negative) circuit side. If a negative connector is corroded or loose, the entire system will fail to produce end-to-end electrical continuity and nothing will power up properly.

The Troubleshooting Process

Troubleshooting electrics on a trailer involves the same processes as any other vehicle electrical system. In other words, you have to have the right tools handy — along with a healthy heaping of patience. To start, get your tools together and organize them accordingly.

Here’s what you’ll need to think about:

A collection of hand tools, including a hi-po flashlight; screwdrivers; pliers; wire cutter; wire stripper; wire-connector crimping tool; multimeter; digital power continuity tester; bulb tester; electrical connection cleaner; anti-corrosive spray; or non-dielectric grease; circuit schematic (if available); Microfiber cloth; rubber gloves; small pad of paper, pencil, plus wheel chocks, wheel stands; sturdy jack, and anything else you might want to think about.

As a follow-in thought, if the trailer has been covered, leave it unhitched and disconnected from your vehicle. Then, do a walk around from the front of the trailer and all the way ‘round. Ensure that you don’t see anything that’s obviously amiss. If you do, mark it down for further investigation.

On the other hand, if the trailer hasn’t been covered, do a more critical walk around. Use a flashlight to identify any areas of deeper investigation like new rust, clag, mud, hanging wires, or any damage that might have been caused by weather or other outdoor-driven issues. Ensure you look under the bottom of the trailer too, since lots of time critters have build nests on the inside rail of trailers if the configuration employs C-rail fabrication.

The goal here is to take a beat, think about what you’re about to do, and save some time by eyeballing obvious problems before they surprise you.

The Troubleshooting Checklist

Once you’re hitched up, and connected to the pulling vehicle, work problems methodically.

Identify a problem.

What’s wrong? Have someone work at the back of the trailer while you push the brake pedal. Do the brake lights come on? Does only one brake light come on while the other doesn’t? Do both left and right tail signal lights flash correctly? Bear in mind that the tail lights work in concert with a vehicle’s flasher system, so if there’s something wrong there, you also might want to do a continuity test on the flasher as well.

If you have electric trailer brakes, do they engage properly? Do you hear any odd noises? Move the vehicle forward and backward slightly. Are you sure that both brakes engage evenly? In the case of brake problems, remember that the system is managed by the electric brake controller, so that component could be a culprit rather than the electro-magnetic complex.

Again, the goal is to identify ‘everything’ you can find before you start trying to fix things on the list. If everything works as advertised, simply load up and go! But, let’s assume you have one or more gremlins afoot. In that case, let’s go through some likely issue resolutions.

Check brakes & tail lights.

Open the light housing – making sure you keep the screws handy after you remove them – and visually investigate the bulb. If you see that the internal element is broken, remove the bulb and replace it with a fresh one. Then test.

If the ‘bad’ bulb doesn’t exhibit a broken element, look for burn-scoring or other physical damage at the end of the bulb or connector. Again, the simplest fix will be to replace the bulb, but don’t necessarily expect that to resolve your problem.

Let’s assume it doesn’t. Use the digital continuity probe to ensure that power is being seen by the trailer wiring harness. If you don’t get an up-check, start working across the wire harness from the bulb connector to the front of the trailer’s connector to the vehicle.

More times than not, you’ll find a wire-cut, a chafed wire that creates a short, or a simple wire conductor break. In the latter case, simply re-attach the broken wire-ends and solder them together for security. Then, cover the patch with electrical tape or a shrink-wrap sleeve. The same approach applies to either of the other two problems.

However, if you don’t find any of those obvious problems, it’s time to try the next item on the checklist: circuit continuity.

Test for circuit continuity.

If nothing powers up, then it’s time to investigate power loss. Without going into an entire article on electrical continuity, and what causes may or may not cause loss across a wire harness, there are typically two central areas of failure: the connectors or the individual wire elements within the wire harness.

If you already know how a multimeter works, connect the device to the connective points on the harness. Proper voltages showing overall? You’re good to go, and can move to another work point. If not, work the problem, failure point by failure point (including the vehicle connectors), beginning with the first fault where the meter shows loss or reduced voltage.

Check wire connectors.

As dumb as this may sound, many trailer electrical problems are caused by simple dirt or corrosion on wire-end connectors. If you’ve measured nominal voltage across the wire harness, the next most likely area of investigation are the connectors themselves.

Causes for a fix/replace work-arounds include physical damage, grit, dirt, rust, and/or corrosion. After investigating and fixing, replacing, or cleaning one or more connectors, ensure that you employ an anti-corrosive layer.

Check ground connections.

Another ‘silly’ but important area of focus are all ground points involved on the wire harness. Most modern trailer wiring harnesses employ multi-pin male/female connectors. However, a loose connector can exhibit looseness throughout, and particularly related to the negative pins in connectors. Therefore, ensure that all ground points are tight and clean.

So, there you go, a little primer on electrical troubleshooting for trailer systems. Enjoy the pull, but first, ensure that your trailer is up to snuff before you roll out, or the ride can be really bumpy.

CURT, a Lippert brand, offers a complete line of towing products and is the leading manufacturer of USA-made custom-fit trailer hitches. The company offers the broadest selection of custom-fit receiver hitches for nearly every vehicle on the road today, as well as gooseneck, 5th wheel and weight distribution hitches. Equip your rig with quality accessories from CURT, such as electrical components, ball mounts, trailer balls, cargo carriers, bike racks and more. For everything from passenger cars and vans to full-size pickups and SUVs, CURT is ‘The First Name in Towing Products®.’