6 Reasons Why There's No Power to Your Trailer Plug & How to Fix It

Your trailer might be essential for work or leisure, activities that require hauling your latest hunt, bikes, boats, or motor home. These are all heavy and valuable goods that need a trailer that doesn't only transport them securely but safely for you, your passengers, and everyone else on the road.

So there are a few things more aggravating than going through the effort of setting up your trailer plug to find no power going across it and your trailer lights not working. A dim turn signal or faulty brake lights mean your trailer is grounded until you can figure out the source of the problem, even if your tail lights work 50% of the time.

If your trailer plug is suffering from issues like this we're going to go over how you can locate the source of the problem. Luckily, there are often a few main offenders that cause issues like this, we'll discuss the main components involved in trailer wiring and how to troubleshoot the problem.

The Importance of Trailer Wiring

In 99% of cases, a trailer is going to be taller and wider than the truck you use to tow it, without adequate width and tail lights to alert other drivers to the size of your load you put yourself and other drivers in danger.

Like all components of your car, the trailer plug and wiring will experience natural wear and tear so faulty trailer lights can often just be the case of you getting good use out of them. In any case, as soon as you notice the issue it's your responsibility to address it immediately.

Common Faulty Components

Let's familiarize ourselves with which trailer wiring components may become faulty before we discuss the faults they might experience.

Light bulb

This one is self-explanatory and you should cross your fingers that it's the source of the issue, cleaning out a dirty filament or replacing a busted brake or tail bulb is as easy a fix as you could hope for.

Tail light housing

There can be up to tail light housings depending on the size and type of your trailer. Their purpose is to protect and cover the connectors and the bulbs. They can succumb to corrosion or damage, leading to the connector becoming damaged.

Brake light housing

This component is the same as the tail light housing, so any damage it receives can lead to a faulty brake light connector.

Wire harness

This wiring structure is the foundation on that your trailer's electrical system is built. They can look spotless on the outside they can develop unseen faults. Due to the comprehensive purpose of the wire harness, a faulty one can lead to all manner of unforeseen faults.

Electric Trailer Brakes

Faulty trailer lights could be the least of your worries if your trailer happens to rely on electric brakes.

Some components that can suffer from a lack of power distribution across your trailer plug are:

Brake Drum

Typically a brake drum isn't involved in any electrical processes in your vehicle, but an electrical one contains electro-magnetically driven components that won't function without electrical power.

Electrical Brake Controller

The controller powers and manages the brakes proportionate to the power applied to the brake pedal. A lack of power will disrupt this transmission and cause the brakes to malfunction.

Brake magnet

Without power, this component won't be able to extend the brake shoe against the inside of the drum, failing to create brake force.

6 Common Symptoms and Causes of no Power to Trailer Plug

Before we start to troubleshoot the issue we need to understand what is causing your trailer connector to fail. These are some common symptoms of a loss of power to your trailer and their most common causes:

Symptom .1

One function, right turn signal or trailer brakes, for example, functions while another doesn't.

Causes

Faulty ground power wire, a disconnected brake wire, a poorly connected wiring harness blown fuse, or connectors failing to form a strong enough connection.

Symptom .2

The reverse lights don't work.

Causes

Insufficient ground power or the fifth wire isn't connected to the reverse circuit.

Symptom .3

None of the tail lights work.

Causes

Your harness has a factory tow package while your vehicle doesn't, a missing relay or blown fuse, there's a poor connection to the ground wire, harness power overload, or the 12V power isn't linked to your vehicle's battery.

Symptom .4

Both turn signals activate at once.

Causes

Insufficient ground power or the brake wire isn't grounded properly.

Symptom .5

The vehicle's headlights stop the trailer lights from working.

Causes

Inadequate ground power on the trailer or truck or an overload on the harness caused by too many lights.

Symptom .6

The trailer has running lights while the ignition is turned off.

Causes

Your trailer may have LED lights powered by a 4-way plug, there's an improper connection to the truck wire, or insufficient ground power.

From the above list, we can tell that there isn't a huge amount of variation in the causes of these problems and that particular problems are often caused by a small number of issues that can be easily determined.

Furthermore, we can see that the most common culprit is a faulty ground wire. Most of these problems can be diagnosed and solved with a few simple steps. It's important to troubleshoot to the best of your ability before you set about rewiring your whole system.

How to Fix a Trailer Plug with no Power

Let's take a look at the various ways we can diagnose and troubleshoot any trailer light issues and what you'll need to fix your trailer plugs.

Equipment

To troubleshoot the issue comprehensively you'll need the following tools:

  • Sandpaper
  • Screwdriver
  • Tow vehicle tester
  • Electrical contact cleaner
  • Electrical tape
  • Jumper wire
  • Wire fasteners
  • Wire stripper
  • 12V battery
  • Additional wire
  • Continuity tester
  • Dielectric grease
  • Test light
  • Wiring kit

1. Analyze the trailer and vehicle separately

Firstly you'll need to determine if it's your trailer or vehicle that's causing the problem, inspecting them both together will prevent you from narrowing down where the problem is coming from.

Separate them and give the trailer a good walk around, inspect it closely and see if there's any dirt or rust build-up or corrosion. You may find you'll be able to find the issue quickly without having to go through the fuss of electrical troubleshooting.

2. Identify the issue

This sounds like a no-brainer, of course, you've already identified an issue or you wouldn't be here, but it never hurts to inspect every symptom as some issues can appear very similar to others.

Does only one brake light come on? Are the tail lights flashing correctly? The tail lights are activated by your vehicle's flasher system so if there's a failure there you'll need to test your flasher as well.

If it's your electric brakes that aren't working, check if they're making any unusual noises when you engage them, or if they're engaging correctly. It might be a component issue rather than a connectivity one.

The key is to have as comprehensive an idea of the problem as possible before you embark on delving into your vehicle's electrics.

3. Clean the connector plug/wiring connectors

With any issue like this, it's always best to start small. Use a specialized cleaner to clean the socket and plug, using an electrical contact cleaner with a fine wire brush to thoroughly but carefully clean the contact pins.

4. Inspect and tighten ground connections

As we covered earlier, a loose connection with the ground wire is the source of many trailer connection issues, so it's crucial to check it's not the source of your grief.

Check for any paint build-up or corrosion around the connectors, using a wire brush to gently clear any potential obstructions where you find them.

Check all the ground points on the wire harness; loose connectors between male/female connectors, especially those related to the negative pins, so ensure they are all tight and clean.

Try removing any ground screws, and sanding down the wire terminal and the chassis terminal. You may find the screw has eroded which can lead to a poor connection.

5. Test for functions

Use a 12V circuit tester for a 4-way plug, removing the fuse panel for 10 minutes then reinstalling it before testing. If the test light functions don't give you the correct power reading, test the wiring input into the converter box. If the lighting functions are operating correctly then check the trailer wiring.

Check the signals into the converter box from your vehicle or trailer.

Green and yellow wires are responsible for the turn signals while the red wire carries the signal to the brake lights. Double-check your plug's wiring diagram to confirm each color code.

If any of these light functions don't give the correct reading your issue may be caused by one of the following:

  • Loose or poor ground connections
  • Incorrect wire connections
  • Loose connectors or wiring

6. Check the brake and tail lights

Unscrew the light housings, make sure you keep the screws safe in a container, and investigate the lightbulbs. Look for a broken element, burn scores, or other damage on the connector.

It may simply be the case that you need to replace the bulb to solve the problem, this may be the case if you've only got one faulty light.

If replacing the bulb doesn't resolve the issue, you've confirmed that it's a wiring issue, in which case you move on to step 5.

You should also test all the lights by having someone stand behind your truck and watch as you test the brakes, reverse lights, and indicators in turn.

7. Accurate connections and continuity testing

Start by cleaning the ground contact, the ground is represented by a white wire. In a 4-way system, the 12v power wire should connect to the positive terminal on your car's battery, in a 5-way plug ensure that the 5th wire is connected to the reverse light signal.

During your inspection, you may find a chafed or broken wire that's causing a short circuit or a conductor break. If it's a conductor break, reattach the broken ends and solder them together, sealing them with a shrink wrap sleeve or electrical tape, use the same approach for chafed or broken wires.

If the connections are accurate you can run a continuity test which will help determine whether it's the connectors or the individual wires that are causing the problem.

Connect a multimeter to the green contact, used for the right turn and right brake light, on your trailer cord and set your multimeter to its continuity function, you'll be able to find the correct symbol for continuity on your multimeter. Then connect the meter's red wire to the contact used for the green wire.

Remove the protective cap over the lights so you can access the wire contacts beneath and touch the green contact connected to the light. You should get a reading of 0.6-0.7ohms, if you don't get a reading then you know that this is the faulty wire, and a professional will be able to rewire it for you.

If you get a reading then try the same process with the corresponding colored connectors on your multimeter until you find one that doesn't give a reading. If they all seem to be working then there might be an issue with your connectors or your tow vehicle circuits.

8. Corrosion and physical obstructions

Corrosion often looks like a white or green build-up and if left for long enough it can reach plug sockets or connectors, and result in loss of electrical continuity. This may be the issue if you've run electrical tests to no avail.

Cleaning connector pins with a fine wire brush and electrical contact cleaners will help create a stronger connection as well removing any build-up.

You can also use a dowel to clean sockets you can't reach. Use hot glue to stick a strip of 220 sandpaper to a 3/8 inch dowel. Place the dowel inside the socket, gently twist it, and move it from side to side, like a Q-Tip. Once you're satisfied, put a spot of dielectric grease into the socket and reinsert the bulb.

If this doesn't work, inspect the mounting bolts to ensure they're making clean contact with the trailer. If you find corrosion here then clear with sandpaper and try the lights again.

9. Check for overload conditions

When a circuit carries more electricity than it can handle it can cause overheating, melting, and subsequent circuit failure. Check the maximum amp rating of your harness against the draw of your tow lights.

Remove all the fuses for several minutes then use a circuit tester to check the 4-way plug connectivity. If each function is working after you remove the fuse panel then you might have a short circuit. If the lights are drawing too much power, remove them and connect the trailer. On the subject of fuses, also make sure everything is properly in place in the power distribution box and the fuse box.

If it works without bulbs then this indicates an overdraw of power. You can alleviate this issue by replacing them with low-draw LED lights.

10. Seek expert help

If you've tried all of the above and still can't find the source of the issue you may need to seek professional help.

If power isn't crossing over your trailer plug it's usually a relatively straightforward issue that's easy to diagnose, but if it's still eluding you then it's likely a more complex issue that a professional will be able to diagnose.

Due to the likely uncomplicated electrical issue, it shouldn't set you back too much and will save you a stressful couple of hours banging your head against a wall trying to find the answer.

Closing Notes

You've now got solid starter knowledge of how to diagnose the issues that may be causing a faulty connection with your trailer.

Make sure you're well prepared before you begin inspecting it and give yourself plenty of time to diagnose exactly what the issue is so you don't overdo your diagnosis and your attempts to fix it.

Make sure your trailer is 100% functional before you take it out on the road as traveling without a solid connection is sure to end in a bumpy ride.

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