5 Reasons Why Your Truck's Trailer Plug Isn't Working

Suppose you have a nifty SUV or truck for your boat trailer to act as a tow vehicle for a utility trailer or RV, and you're aiming to head off on vacation soon. You hitch your trailer to your tow vehicle, but the lights on your trailer don't work when you push your pedals or use the turn signals.

Disaster, right? Wrong! Your trailer needs to be connected to your tow vehicle with a trailer wiring system and the correct plug, and it's easier than you think to do so.

Below, we'll go into trailer plug types and reasons why your truck plug may not be working. We'll also look into the trailer light wiring system affecting brake and turn signal lights, how to use a circuit tester, and the tools you need to make sure your lights are all in order.

We'll also explore examples of plugs that don't work and pickup trucks whose plugs have been known not to work, and how to rectify these problems.

What is the Purpose of a Trailer Plug?

Tow vehicles are connected to the trailer they are towing by a wire harness that has a plug on it. This "electrical connector" has a power supply to your trailer frame indicators and brake lights at the back of your trailer, so a car traveling behind you has a view of your movement signals.

Trailer Lights Troubleshooting

If your trailer light system is giving you hassles, your trailer won't be legal on the road. There are several guidelines we recommend you follow on troubleshooting the trailer lights that will help you identify the problem and solve it.

Identifying the cause of your trailer lights malfunctioning

An issue with your trailer lights can be identified in the tow vehicle's electrical system or the trailer wiring system. The first thing you need to do is a visual inspection of your rig's setup, then you will need to test the wiring.

5 Reasons Why Your Trailer Plug Isn't Working

  1. Grounding issues

Check the ground connection for possible problems. The area the ground wire is attached to should be the trailer's bare metal frame. The ground wiring can become too loose with time.

How to fix it

Ensure each of your tail lights is grounded correctly - if not, you could experience trailer light problems.

You can ground it correctly in two separate ways. The first method to fix grounding is to separate the wiring out of each tail light casing; they're connected to a metal frame. Ensure the ground wiring isn't loose and tighten the wiring at the connection points.

The second method involves the tail light casings. They must be connected to a metal area like the trailer frame, never to wood or even plastic. You could also use a circuit tester to check for current flow.

Ensure they are connected at the right point and if the current flow is weak, check the connections and if the wiring isn't loose and needs to be fixed in a tighter manner.

  1. Blown fuse

Check your tow vehicle's fuse box for issues that can affect your trailer light working poorly or not working at all. This is when you need to check your fuses.

How to fix it

Trailer light issues can be checked by looking at the fuse box on your tow vehicle for a blown fuse. If you have a power converter/T-connector, first make sure you disconnect the trailer and do a circuit test. This will show you if a signal is traveling into and from the converter (Modulite box) on the correct wires.

If there is no signal traveling into the box, the issue is coming from the tow vehicle (such as a blown fuse or incorrect connections). If the signal is going into the box area and doesn't come out or travel along the wrong wires, check where the grounding point is.

Check all your connections and light fittings to see why your trailer lights are malfunctioning.

  1. Your light bulb is blown or the fitting doesn't wire up properly

If only one of your trailer lights is working, this could indicate a blown lightbulb or incorrectly wired connections near the bulb.

How to fix it

Replace your lightbulb if only one of your trailer lights isn't working. Get a screwdriver and take out the screws in the faceplate’s corners that covers the trailer light. Unscrew your blown lightbulb and fit a new one in with the same voltage.

Test your trailer lights using a second person hitting the brakes or turn signals and see if your light works now.

If the light doesn’t work, check the wiring connections near the bulb for looseness or poor connections and rectify them.

  1. Corrosion

Your trailer plug can get corroded, which means moisture can get into the electrical system. A corroded trailer plug doesn’t work properly and needs to be cleaned. This affects your lighting. Take a look at your trailer plug and wires and look for corrosion

How to fix it

Vinegar or even soda water can help get rid of corrosion, also ensure you energize the plug with an electrical contact cleaner like WD-40, and dry them off with a pressurized air can.

If your trailer plug is too badly corroded, you could splice in a new plug that you must buy if the wiring isn’t corroded.

  1. "Hot wiring problems"

Only certain trailer lights are working, indicating "hot wiring problems" or broken wires.

How to fix it

Use your circuit tester to ascertain if the current is traveling towards the light assembly at all. Your brake controller in your trailer affects the trailer brake lights, which need to work.

First, check the wiring harness that fits into the fixture with the issue, then look at the tow vehicle, trying to find which point shows a circuit break. It is wise to clean the brass terminals with steel wool or a fine wire brush, so you can establish a good connection point.

A Few Tools You Will Need to Fix the Entire Wiring System

You should aim not to have to rewire the entire electrical system after you have investigated the route cause of the trailer wiring issue. We recommend you get hold of the following tools and keep them handy together in your "towing toolbox" that you can store in your garage and take on the open road with you when you set out on your next trip.

  • 12V battery
  • Extra wiring
  • Continuity tester
  • Some dielectric grease
  • Dowel rod
  • Electrical contact cleaner
  • Electrical tape
  • Jumper wire
  • New light bulb
  • Nut driver
  • Power drill
  • Roll of sandpaper
  • Screwdriver
  • Tow vehicle tester
  • Wire fastenings
  • Wire stripping device
  • Wiring kit
  • Heat shrink tubing

Armed with these tools in your arsenal, you will be prepared for any wiring problems.

Examples of Trailer Plugs Not Working Properly

Here are some working examples of trailer plugs and trailer wiring not working properly, what causes the fault in the trailer lights, and how to solve the issue.

7-pin trailer plug not working

If your 7-pin trailer plug isn't working, there could be various causes.

What does a 7-pin trailer plug do?

The 7-pin trailer plug is about 2 inches in diameter and allows for an extra pin supplying an auxiliary 12-Volt power system or reserve lights. This 7-pin plug suits heavy towing of RVs, cargo trailers, trailers made out of aluminum, dump trailers, utility trailers, toy haulers, and open-air and enclosed car hauling trailers.

Water damage on a 7-pin trailer plug

Trailer plugs on RVs, being round or flat, are exposed to the outdoor weather. Water coming into contact with the plug will create corrosion. In turn, this causes flickering tail lights or tail lights that aren't working altogether.

Corrosion looks like a green or white substance on the plug or simply rust. Wipe away the corroded material with sandpaper or electrical contact cleaner spray. However, if this doesn't work and it's too corroded, you may need to buy a new 7-pin plug. A new trailer plug costs in the region of $10.

If there's no corrosion, what do I check for next?

Next, remove the cover of the plug with a screwdriver, and check the connection point between the colored wires and screws securing the wires. Make sure all the wires are screwed down securely.

If you leave your trailer outside, cover the plug with a plastic bag and zip tie or buy an extra socket and use that with the plug while storing it.

Dodge Ram trailer plug not working

The trailer plug on your Dodge Ram isn't working means the trailer lights aren't working properly either. There are a lot of reasons that your trailer lights may not work.

A poor ground wire is the most significant reason why a 2015 Dodge Ram's trailer lights won't work. Moving away from poor power connections, you could have a bad 7-pin plug that doesn't get enough power or a bad bulb that needs to be replaced.

Another possible reason for a Dodge Ram trailer plug not working is a fuse short circuit, where you need to replace the fuse. Also, check the plug isn't corroded, in which case you must clean it with electrical contact cleaner. Replace the 7-pin plug if it's damaged and if the plug doesn't seem to draw enough power, check the fuses and fix those.

Ford F-150 trailer plug not working

A common problem with Ford F-150s is that the running lights don't work, but the brake lights and turn signal lights do.

A Ford F-150 can have relays linked to the tow package wiring, not merely the fuses. Take a look at your owner's manual to locate the fuse areas related to the tow package wiring. The best way to check if a relay is faulty is simply to replace it and see if the problem is fixed.

If all the fuses and relays are in working order, your problem lies between the trailer plug at the back of the truck and the point it terminates at the front.

Use a circuit tester to test the truck's side trailer connector. If you discover there's no power to the running light pin, check the back side of the connecter at the wiring attachment area. Look for loose wires and clean any corrosion.

Ford Ranger trailer plug not working

Ford Rangers come with a 4-prong flat wire harness as a standard. Some people have found it sends a signal for brake lights, turning, and hazards, but not for marker lights.

The running lights should work well if they're connected to the correct spot. The 4 wires produce the following: right turn, left turn, brake lights and running/parking/license lights. Test to see which lights have power or not.

Use a tester light to figure out which post has power when turning on your truck’s lights. Connect the running lights to this post. If you don’t experience a “hot” wire in your lights in this position, then wiring to your trailer plug is wrong.

Slide underneath your tow vehicle to see which wire is in the loom and then rewire it appropriately.

You will see a trailer “tap” T that will plug into your loom that could help. We recommend grounding your trailer harness to your trailer frame. This will help eliminate strange light malfunctions.

FAQs

Why am I not getting power to my trailer plug?

You need to clean your trailer plug first. If there is no power after cleaning it, look at your ground connections. Ensure your grounds are connected to your trailer frame, being metal. Test the pins of the plug on the connector at the point where the wire harness plugs into the truck using a circuit tester.

Why does my trailer plug not work?

Lots of trailers have poor ground connections, the white wire that comes out of the plug. If the ground is faulty, the light can work sporadically or not at all. If the wiring to the plug is done correctly, ensure the ground connection secured to the trailer frame is adequate.

Is there a fuse for a trailer plug?

If the problem you have regards turn signals, look for the trailer RT or LT fuse and replace it if it needs to be done. This will solve an issue related to brake lights as this uses the same fuse.

Why do my trailer lights work with one vehicle but not another?

There may be a weak ground on your trailer affecting the lights working on it. Try to run a ground wire from each light casing to the main ground of the trailer.

Final Thoughts

There are multiple reasons why your trailer wiring and trailer plug may not be working, from ground wire connections to blown fuses or a burned-out bulb, corrosion, hot wiring issues or the plug itself being the wrong size and you need to replace it.

We looked at real-world scenarios where a trailer plug on a truck is not working, why this is and how to solve them.

We hope we have shed some _light _on the correct trailer wiring affecting trailer lighting. You need to do trailer lights troubleshooting to identify the problem and solve it.

We hope our tips and examples give you some insight so you can easily solve your trailer plug problems and tow your trailer safely to your desired destination.

Resources

https://auto.howstuffworks.com/auto-parts/towing/equipment/protective-towing/trailer-wiring.htm.:~:text=The%20T%2Dharness%20has%20two,the%20newly%20installed%20T%2Dharness

https://www.rvservicecentre.com.au/blog/article/caravan-tail-lights-not-working-7-pin-trailer-plug-maintenance-guide.:~:text=The%20first%20step%20in%20diagnosing,spray%2C%20might%20solve%20your%20problem

https://www.etrailer.com/question-120056.html

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

https://bullyusa.com/trailer-lights-troubleshooting.html

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

https://www.wikihow.com/Test-Trailer-Lights

https://www.therangerstation.com/forums/index.php?threads/trailer-lights-wiring-issue-w-ranger.98012/

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