How to Diagnose Trailer Wiring Problems

Your trailer wiring system is critical for when you are out on the open road towing your RV, boat trailer, or utility vehicle. This is because your trailer wiring needs to work well to ensure the lights on your trailer work. The person traveling behind you will need to be able to see your brake lights, turn signal lights, and running lights.

You need to know how to detect issues with your trailer wiring, which tools you'll need to fix them, how to diagnose these problems and how to fix them. We'll discuss common trailer wiring issues, the tests for problems and how to check if your wiring system is overloaded and what to do about it.

The Purpose and Relevance of Trailer Light Wiring

Can you picture driving down a highway at night time when your trailer's lights aren't working? People behind you, either on foot or in a car, won't notice that you are towing an extended trailer, which is dangerous. You need to ensure that your trailer wiring system is in order, so your trailer lights work.

Your wiring system can become damaged over time while your trailer is in storage, so you should check the wiring and test the functionality of the trailer lights before towing your travel trailer, RV, utility trailer, or boat trailer.

Common Trailer Wiring Issues

Your trailer lights can either be too dim or not work entirely. This could be due to a burned-out bulb, corrosion on the trailer plug, a broken wire, or a poor ground wire. These issues are easy for you to fix, and we'll discuss how to make the correct repairs to your trailer.

Ground wiring is a normal cause of problems, but other wiring issues encompass the following scenarios:

1. Problem: One aspect of the trailer lighting system isn't working, such as the brake lights or right indicator light.
2. Possible causes of the problem: The wiring harness's wires are not connected, the connection isn't strong enough, you have blown a fuse, the brake wire isn't connected, or the ground connection isn't working.
3. Problem: All the lights aren't working on your trailer.
4. Possible cause of the problem: The power wire (usually 12 V) isn't connected to the tow vehicle's battery, the wiring harness has a "factory tow package," and the tow vehicle does not, a fuse has blown, there's a missing relay, the wiring harness has a weak connection to the ground, or there's an overloading issue on the harness.
5. Problem: The lights worked to start off with, but now they don't anymore.
6. Possible causes of the problem: There may be a loose or poor ground connection, the wiring harness is overloaded due to excessive use of power, or there is a short in your trailer wiring.
7. Problem: Turning on the turn signal to the right or left activates both sides' lights.
8. Possible causes of the problem: The wire for the brake on the harness is not grounded, or there is a weak grounding.
9. Problem: When you switch on your tow vehicle's headlights, your trailer lights malfunction.
10. Possible causes of the problem: There's a weak ground on the vehicle or trailer, or the wiring harness is overloaded due to supplying too many trailer lights.
11. Problem: One or several trailer lights stay on, even when the tow vehicle's ignition is switched off.
12. Possible causes of the problem: There's a weak connection on the truck wiring, the ground connection is weak, or the trailer uses LED lights with a power supply from a 4-way plug.
13. Problem: The wire harness functions right up to when you are connecting the trailer.
14. Possible causes of the problem: There's a weak ground, or you may have wiring harness overload at the point when you connect your trailer to your tow car.
15. Problem: The trailer reversing lights don't work.
16. Possible causes of the problem: Your fifth wire is not connected to the reverse circuit on your tow vehicle, or there's a weak grounding.

In each of these scenarios, there is a range of possible sources to the problem that you can detect. If, for example, there is one function of your trailer's wiring that's not working, it could indicate that your wiring harness's wires aren't connected to the tow vehicle properly.

The below wiring source issues and how to fix these problems yourself correlate to the above examples of issues.

What's the Commonality Between These Wiring Issues?

It can be seen that a common cause of these issues when trailer lights don't work is a poor ground connection. You can rectify most wiring problems by following certain procedures; if you have to do a full wiring replacement or a very complex job, we recommend that you take your trailer and tow vehicle to a mechanic to handle the job for you.

Which tools do I need to have for trailer light issues?

• A 12V battery
• Some extra wiring
• A continuity tester
• A bit of dielectric grease
• A dowel rod
• Some electrical contact cleaner
• Some electrical tape
• A jumper wire
• New light bulbs
• A nut driver
• A power drill
• Some sandpaper
• A screwdriver
• A tow vehicle tester
• Some wire fastenings
• A wire stripping device
• A new wiring kit
• Some heat shrink tubing

If you have these handy tools at the ready, you will be prepared for any trailer light wiring problem and be ready to tackle it head-on. We'll make mention of more tools you can add to your toolbox below. Your trailer lights will be easier to fix if you are prepared.

Just as important as testing your trailer light wiring before you head out is carrying your tools with you. Your trailer lights might be intact before you head out when you test them at home, but they could start giving you problems once you're already on your way, and having your tools accessible in a toolbox dedicated to trailer wiring will be just what you need!

Fixing Common Trailer Wiring Problems

Firstly, you need to test the tow vehicle and trailer one at a time to cancel out common issues. To figure out if the issue lies with the tow vehicle or trailer, you need to assess the individual wiring systems in "bite-sized chunks," so to speak.

Testing for issues while the trailer is connected to your tow car will make it difficult to figure out what the root cause of the problem is.

Below we've provided an easy-to-use guide to help you troubleshoot your trailer's wiring system. Whether you have a 4-way plug or not, it's important to assess your ground connections or ascertain if the system's overloaded.

These minor issues have simple solutions that can be fixed using certain tools mentioned.

Troubleshooting 4 and 5-Way Wire Harness Setups

Wiring problems are sometimes tough and time-heavy to fix. If your trailer lights aren't working, this makes your rig unusable as the person driving behind you won't know that you're there, and this poses a safety risk.

Below, we'll look at diagnosing and testing your wire harness issues on a 4-way and 5-way wire harness, so you can set off on your road trip sooner than later.

Where do I start with troubleshooting a trailer wiring system?

A trailer light issue can stem from any part of the wiring in the tow car or on the trailer, so you need to know what's causing the problem and where the problem stems from.

First things first, you need to figure out if the problem is located on the tow vehicle or the trailer. When you test your trailer, it's hard to figure out if the problem is related to the wire harness because the trailer's wiring is still connected.

Testing the tow vehicle without the trailer allows you to separate your wiring system into digestible bits.

Which tools will I need to use to troubleshoot the 4 and 5-way wiring system?

There are certain tools you will need that will make troubleshooting trailer wiring issues on a 4 and 5-way wiring system a lot easier:

• A 12 Vault probe circuit tester
• Electrical tape to repair connections
• A wire stripper to ensure you have clean wire ends
• Dielectric grease
• Wiring fasteners such as butt connectors and quick splice connectors/ ring terminals
• Wiring kits that include a trim fastener, a flat-head screwdriver, a power drill, and a 12 vault battery to test trailer lights

Testing for 4-Way Plug Functionality

Get your 12 V probe circuit tester and check the functionality of your 4-way plug if that's what you have. Have a second person sit in the tow car to help you test your trailer light function.

For a power-operated converter only, before you start testing your wiring harness functionality, remove the fuse on the poor wire for half an hour, then plug it back in.

The fuse is found near the battery in what is known as a fuse holder. If the power-operated converter box performs its protection feature, the box will reset; this won't be the case if it was under overloading stress and the connections were damaged.

Don't plug your trailer into its 4-way plug until you check its functionality with a circuit tester.

If you discover that certain functions don't have the right power reading at the 4-way plug, you need to do a test on the wires moving towards the converter box from the tow vehicle side. If the functions show up in working order at the 4-way plug, you can move on to testing the trailer.

Testing whether the signals are traveling into the converter box from the tow vehicle side

If you have a 2-wire car, green and yellow ( green will be on the passenger's side and yellow will be on the driver's side), wires power the turn signals and brake light functionality. In 3-wire cars, the red wire operates brake light functionality, and the turn signals are on green and yellow wires.

If any function doesn't have the right power reading, check the following:

Plug-in harness connectors are secured and aren't plugged in in a flush fashion. There may be loose wires at the back of the connectors. There could also be fuses or relays from the tow package or trailer wire system missing.

On a hardwired trailer harness, look for a loose or weak ground connection. Ensure the wires are connected to the right wires on the tow vehicle.

What Else Can You Do to Check on Your Wiring System?

What you can also try to do is a continuity test. When you want to troubleshoot your wiring, attach a jumper wire to your connector pins and connect the continuity tester onto the wiring system's sockets.

What does a continuity test show you? It allows you to ascertain if there are broken wires. Choose a color of wire from the socket and look for the same one on the connector's front side. Secure one side of the jumper wire to the connector pin and secure the other to your continuity tester.

Probe your testing device into the socket area. If your lights on the trailer are malfunctioning, follow the wire and look for breaks. Cut it down; whenever you see a fault, then you need to solder on a brand new connection, plus add heat-shrink tubing to fix the insulation.

How to Check Ground on the Trailer Wiring

Look at your tow vehicle and asses the grounding area for any corrosion or paint residue. Clean any corrosion or paint off until you come up with an untarnished metal surface or even get rid of corroded ground screws and put new ones in.

If your harness comes with a factory ground screw, ensure the extra ring terminals are not found below the ground area. If this is the case, maneuver the ground from the harness to another spot or near the bottom.

Then, detach the ground wire and secure it to a wire that will run to the toe vehicle's "negative battery terminal." If this resolves your trailer lighting issue, you can leave it as is.

You must always check the ground system and make sure the ground wire is connected to your trailer frame. If your trailer comes with a tongue, ensure the connection runs behind your tongue on the rig.

What you can do, as well, is maneuver your ground wire to the trailer frame if this is occurring in the aluminum section.

Assessing Whether Your Trailer Light Wiring System is Overloaded

What is an overloaded wiring system? This happens when your circuit has more electricity traveling through it than it can handle, this can result in the system overheating or even melting.

Check on the "maximum amperage rating" of your wire harness and check it against the trailer light draw. Sometimes you can reset the system by taking out the fuse for a few minutes. You can utilize a circuit tester to assess the 4-way plug functionality, but don't plug it into your trailer before testing.

Testing your light bulbs for efficiency

Your system might be experiencing a short if each light works after the reset. If your trailer lights pull more current than the harness is meant to take, take out the bulbs in the extra clearance light system and connect your trailer.

If the wiring harness functions without bulbs, it means there is too much pull from the number of lights on your trailer. Take out your clearance lights and insert LED lightbulbs, so less power is pulled.

The benefits of LED lights in your trailer

LEDs burn cooler and don't make use of flimsy wire filaments that stretch and weaken with time. LED light bulbs last longer as they handle road vibration well. They also give off a consistent, good light.

An LED trailer light is brighter, which helps drivers behind you see you better during the day. Ensure your LED trailer lights are waterproof, so water doesn't enter the casing. These lights also use less energy than a regular light bulb, putting less draw on your battery, which has a positive impact on battery life.

LED lights light up an area quicker. For example, when you brake, the LEDs on the trailer react instantly and give off a brighter, concentrated light. An incandescent light takes 0.25 seconds to reach 90% brightness. A University of Michigan study showed that people traveling at 65 mph behind a vehicle with LED lights had an improved reaction time and shortened braking distance by 16 feet.

What Other Trailer Light Wiring Issues Might You Come Across?

Your trailer is often exposed to the weather, which can cause corrosion in multiple areas. Ensure you examine connection areas for corrosion and check on your trailer plug too. You must replace a corroded plug or clean it with an electrical contact cleaner.

You'll need to do this once you've checked the tow car's lights and trailer lights. If they are dim or not working at all, it could be corrosion. You can spray the plug with an electrical contact cleaner or use a fine wire brush to clean your contact pins.

If your running lights are the only ones working, it could mean you have a faulty control switch.

Checking for corrosion

If your trailer is stored outdoors, there can be a concentration of corrosion on certain spots of your wire harness or connections. Ensure you look for corrosion; it's usually green or white in color. You will need to replace the trailer plug or clean it with a battery terminal cleaner.

Before you do this, check if your trailer lights are still weak or not working. You can spray your trailer plug with electrical contact cleaner as well as use a fine wire brush to clean the pins. This helps ensure a better connection between your wires.

Alternative ways to clean the corroded areas of your trailer wiring system

If your wiring socket is corroded, your lights may not work. You can get rid of corrosive materials with 220-grit sandpaper, but if your fingers are too large for smaller crevices, then glue some sandpaper onto a 3/8 inch dowel and use that.

Clean the area by spinning the dowel and moving it from side to side. Once you're done, add some dielectric grease to the contact points and fit a new light bulb. If this doesn't fix the issue, make sure your mounting bolts are connected to a clean trailer frame.

Make sure the non-aluminum mount spot is clean and free from paint residue if your lights are grounded through mounting hardware. If the surface area is made of aluminum, then connect the wiring from the ground and connect to the frame.

Ensure your light bulbs are in working order. Unscrew them and screw them back in. The running lights, turn signal lights, and brake light bulbs can be broken or blown and will need to be replaced.

If your trailer wiring issue can't be resolved with our handy solutions to fixing wiring issues, you may need the help of a mechanic.

How to Find a Short in Trailer Wiring

What does a short look like in your trailer light system? All the lights in this example are LEDs. The running lights can stop working, and you can blow a fuse in the tow vehicle engine. You must examine the lights for obvious problems. Then, replace the fuse, and it blows again. The brake lights and turn signals work, just not the running lights.

So, how do you find a short when it isn't obvious like your light has water damage? If you keep putting fuses in, and they blow, what does that mean?

Start by examining the places where the wires go through the trailer frame, check if they aren't broken or frayed, and ensure they are connected to the main wire harness. Sometimes when the fuse blows, there can be a bare male end pulled from the light casing, and it's hitting the frame internally. Check if this isn't the case and rectify it if it is.

You can also disconnect the backlights and check once more for a short to see what other factors to eliminate. Reason for causing the short. You can also use a voltammeter to check continuity on your tail lights to the ground.

How to Test Trailer Hitch Wiring on a 7-Pin Trailer Plug?

A 4-pin trailer plug harness only offers turn signals, brake lights and running lights, while a 7-pin trailer plug also offers a charge line, reversing lights, and trailer brake lights.

The 7-pin plug is seen on bigger trailers that have trailer brakes on them as well as batteries that need to be charged.

The 6 pins have different functions. Pin 1 offers a charge line to charge batteries, pin 2 is the right-hand turn signal and the right brake, pin 3 is the trailer brake, pin 4 is the ground, and pin 5 is the left-hand turn signal, and the left brake light. Pin 6 operates the running lights, and the middle pin is the reverse light.

To test the trailer harness function while it's attached to a tow vehicle, use your circuit tester.

Ground the circuit tester to your vehicle's frame, then open the 7-pin trailer plug, find the top notch; it might be angled sideways, and touch the tip of pin 2 to test the right-hand turn signal. If the circuit tester picks up a good signal, the tester's bulb will light up.

You can test all the other lights in the same way. This helps you troubleshoot hitch wiring more quickly and easily.

How To Test Why the Trailer Lights System Isn't Working on Your Boat Trailer or Utility Trailer

There are certain similar steps that need to be taken if the trailer lights aren't working on your boat trailer or utility trailer, similar to that of a 4-way and 5-way wiring system.

Using a tow car tester

Firstly, plug in a tow car tester by putting it into the connector of your vehicle to troubleshoot your trailer wiring system. Check the plug setup is configured properly. Disconnect the wire harness and plug your tester into your tow vehicle. This will detect any trailer lights wiring issues.

Cleaning corroded residue off your trailer plug

Clean the trailer plug with an electrical contact cleaner. Clean your ground contact and make the ground wire connection to your trailer's metal frame strong and neat. Then, examine the ground wire. As mentioned in another scenario, the ground wire is the common culprit in these trailer light faults.

Take out the ground screw and sand down the wire terminal and trailer chassis area using some sandpaper. If your ground screw appears damaged or it has corrosion, replace your screw.

Check the status of your light bulbs

Check on your light bulbs and replace them if need be. If only one light is out (the running lights or turn signal lights), you may only need to replace the light bulb.

Get rid of corrosion with fine sandpaper and a 3/8-inch dowel to get into tight spaces. If your light isn't working still, there may be corrosion of the socket at the various connection points. Add some dielectric grease to the contacts and insert your light bulb. If the light still isn't working, check your mounting bolts and make sure they have a clean connection with your trailer frame.

Perform a continuity test

Look at your trailer light wiring by carrying out a continuity test. Do this via connecting a jumper wire to your connector pin area and then placing a continuity tester connecting to the sockets. A continuity tester has a light bulb at its tip, and it has a battery. The bulb will illuminate when it detects an optimal circuit.

Using a jumper wire to test the functionality of trailer wiring

By placing alligator clips at your wires' ends, continuity connections are made faster and easier. If the lights on one side don't work, there could be a break in your wiring. To test if you have a broken wire, look at the wire entering the socket and then source that same wire on the connector in the front.

Clip your jumper wire onto the connector pin and clip the other end onto your continuity tester. Probe into the socket using your tester. If the light is triggered, follow the wire and look for breaks.

If you find any breaks, cut your wire, solder on a new connection, and fix the insulation of your wiring using heat-shrink tubing.

Replacing the entire wiring system

You may have to replace the entire wiring system if there appears to be bad corrosion. A new wire harness costs around $20. A new wiring harness comes together with the connector, trailer lights and lenses, and an instruction manual.

It can be installed in about two hours, but if wiring is new to you, we recommend that you take your boat trailer or utility trailer to a mechanic who will do it all for you.

FAQs

What Would Cause Trailer Lights Not to Work?

A lot of trailer light wiring problems are connected to a poor ground connection; this is identified as the white wire that comes out of the trailer plug. If you have a poor grounding, the lights may work at times, or sometimes not at all. Make sure the wiring going to the plug is intact and that the ground connections to the trailer frame are adequate.

How Do You Check for Bad Ground on a Trailer?

There are certain spots you can examine on your trailer frame for poor ground connections. Start by looking at the trailer plug connection to the tow vehicle. To do this, follow the white wire coming from the trailer plug and ensure it's properly grounded onto the frame of the vehicle or the chassis. This must be connected to a clean metal area.

Why Do My Brake Lights Work but Not My Running Lights?

The most well-known reason your tail lights aren't working but your brake lights are is because of a bad or incorrect type of light bulb being installed. The reason may also be a blown fuse, incorrect wiring, or it could be a socket or plug that is corroded. A faulty control switch may also be the culprit.

Why Am I Not Getting Power to My Trailer Plug?

If your trailer plug is clean, and you are checking it after cleaning it, and the power still isn't coming, check your ground connections. Your ground wires should be attached to clean metal surfaces. You can also test the pins on the trailer plug at the point where the wire harness plugs into the tow vehicle using a circuit tester.

Final Thoughts

The trailer lights need to work on the trailer you are towing, and this leans very heavily on the trailer light wiring system functioning so that the trailer lights work at the back of your trailer. Trailer lights draw on energy from the wiring harness.

There are certain common problems like loose or damaged wires, a poor ground wire connection, corrosion on the trailer plug, the trailer light wiring system has been wired up incorrectly, there may be broken relays or fuses, or a blown out light bulb, the trailer frame isn't clean at certain connection points of your trailer light wiring system.

We have also discussed some common examples of wiring problems people face when they tow their RVs, utility trailers, or boats and how you can try to resolve them yourself with certain techniques we discussed above.

If your problem looks really severe and you have tried testing for issues and fixing them using our discussed methods, the entire trailer light wiring system may need to be rewired by your trusted mechanic. If you're experienced, you can try rewiring the whole system. Most of the time, you can fix wiring issues yourself if you have the right tools and procedures to follow.

Resources

https://www.boatus.com/expert-advice/expert-advice-archive/2019/february/troubleshooting-trailer-lights

https://www.etrailer.com/question-36130.html

https://mechanicbase.com/cars/tail-lights-does-not-work-but-brake-lights-do/.:~:text=The%20most%20common%20reason%20why,could%20also%20be%20to%20blame

https://www.etrailer.com/question-267158.html.:~:text=If%20they%20are%20clean%20or,circuit%20tester%20like%20Item%20%23%2040376

https://www.trailersuperstore.com/troubleshooting-trailer-wiring-issues/

https://www.familyhandyman.com/project/fix-bad-boat-and-utility-trailer-wiring/

https://www.etrailer.com/faq-4-5-way-troubleshooting.aspx

https://www.truckspring.com/trailer-parts/trailer-wiring/test-troubleshoot-trailer-lights.aspx

https://www.boatus.com/expert-advice/expert-advice-archive/2012/september/the-trouble-with-trailer-lights.:~:text=Unlike%20traditional%2C%20incandescent%20lights%20that,much%20more%20effectively%20than%20bulbs

https://www.in-depthoutdoors.com/community/forums/topic/ftlgeneral.897608/

https://www.youtube.com/watch?v=yEOrQ8nj3I0

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