How to Check if Your Trailer Plug is Working

Are you wondering how to check if your trailer plug is working? If so, you've come to the right place. Whatever trailer type you have, exposure to dirt, grime, rain, snow, and even sun can cause trailer lights to develop faults.

Driving around with faulty brake lights can be incredibly dangerous. Not only do you risk having an accident, but you could also be subject to a fine too. But how do you test trailer lights? That's what we aim to cover in this guide, so if you'd like to know more, just keep reading!

Testing Trailer Lights

Your trailer's lights must be working properly to ensure other drivers can see you braking and signaling left or right. If the trailer's lights seem to be faulty, there are a few ways of diagnosing and fixing the problem by yourself.

The first step is to have someone help you to check that the lights are functioning. If they're not, there are tools designed to check the contacts and wires within the trailer's circuitry. Take a look at the steps below for testing your trailer connector.

How To Test Trailer Connector

Testing the lights

First, test the trailer lights and have someone check if they're functioning. Start up the truck or tow vehicle while the trailer is connected and plug the trailer wire into the connector.

Next, press the brakes, both blinker lights, and the hazard lights while your helper is standing behind the vehicle to check if the lights are operating correctly.

The trailer lights should match the rear towing vehicle lights. If some of the lights aren't working, note down the faulty ones.

Replacing the bulb

If one light isn't working, it could be due to a blown bulb. To fix this, remove the faceplate screws that are over the trailer light, using a screwdriver. Unscrew the faulty light bulb and switch it for a bulb that has the same level of voltage.

Then, test the trailer lights a second time by pressing the brake in your towing vehicle. If the lights still don't work, it means there's an issue with the wires.

Disconnect the trailer

Next, disconnect the chains that attach the trailer and the tow vehicle, and lift the latch that can be found at the front of the trailer. Turn the crank clockwise and lift it to push your trailer away from the towing vehicle.

Unplug the black cord that's connected to the towing vehicle - this will allow you to test each connection individually. Make sure to engage the front wheel when disconnecting it as it could fall forward.

It's also important to detach the trailer and the towing vehicle so no problems are concealed with the grounding wire.

Plug a light tester into the connector

Next, make sure the teeth on the light tester are lined up with the plug on the bumper of the tow vehicle, then plug the tester into the connector. If the tester turns yellow or red, that means there's an issue with the connector rather than the trailer lights.

Ensure that the fuse is not blown by checking that the tow vehicle lights are working:

  • Wipe the connector contacts with a rag and contact cleaner to solve potential issues with the plug.
  • If you can't diagnose the problem, you may need to take it to a professional so the wiring can be examined.

Look for broken wires

Some of the trailer wirings might be obscured because it runs into the trailer frame. If you can't see any damage to the wires or you suspect that the interior wires are frayed or broken, you'll need to take your trailer to a professional. Some things to remember:

  • The brown wire is for the tail lights.
  • The white wire is the ground wire for the trailer.
  • The yellow wire is for the left brake light and the left turning signal.
  • The green wire is for the right brake light and turning signal.

Continuity Testing With A Multimeter

Attach the multimeter

Change the multimeter to the continuity mode. Your multimeter manual will tell you what the continuity icon looks like.

Clip the red wire from the multimeter and connect it to the contact that's attached to the green wire inside the trailer connector plug. Make sure that the wires are long enough so you can reach the rear of your trailer.

Unscrew the faulty light cap

If the light cap is still on, you'll need to unscrew it so you can reach the wire contacts inside the light. Using a screwdriver, remove the screws in all corners of the cap. Then remove the cap to find the wire contacts and the bulb inside. Put the cap aside so it doesn't get lost.

Connect the multimeter and the green contact

Touch the contact and the other multimeter wire together under the light to do a continuity test. The continuity should be around .6-.7 ohms.

If you don't get a reading when you touch together the black wire and the trailer contact, it means that particular wire is broken. A professional can rewire the lights for you.

Repeat with the other wires

To test the rest of the wiring system, disconnect the multimeter and the green contact of the trailer's plug, then reattach the multimeter to whichever contact you need to test.

Next, touch the black wire of the multimeter and the same color contact together under the rear trailer light. Keep testing each wire for continuity until you come across one that isn't working.

If the wiring system seems to be in good working order, you may need to fix or clean the plug wire contacts. Or, there could be an issue with your towing vehicle continuity.

Fixing And Cleaning The Wire Contacts

Sand the contacts

Gently scrape the contacts of the trailer wire with 150 grit sandpaper to get rid of any buildup that could be stopping the connection. Repeat this process on the vehicle's connector contacts. This process only takes around 10-30 seconds, just be sure not to scrape too hard as you could damage the contacts.

Apply grease and contact cleaner

Spray contact cleaner onto the plug contacts and each trailer light to remove debris and dirt that could be affecting the connection. Next, apply a generous amount of dielectric grease onto the trailer's plug contacts and the lights to boost circulation.

Greasing and cleaning the contacts may improve the dimming issues you have with the trailer lights.

Connect the trailer to the tow vehicle

Lower your trailer onto the towing vehicle and connect the wire back into the vehicle connector, then turn the vehicle on and test each trailer light again.

If they still don't work you might need to take the trailer to a professional to find the issue with the wiring or the circuitry. Diagnosing the problem means you can get your trailer repaired quicker.

How To Test If Trailer Plug Is Working With A Multimeter

Testing for grounding

The first thing you'll need to do is disconnect the trailer plugs. You'll find that the lighting is connected to three pinholes for each positive connector. There's also an extra opening for a negative connector.

Most faulty trailer lights are caused by a weak ground connection. To test the ground connection, take the two probes out of the multimeter - the red one is for the positive connection and the black probe is for the negative.

Be sure to adjust to ohms settings on your multimeter and you may need to put the probes together to make sure they work. Connect the black probe and the negative plug terminal and the red probe to the grounding. For sufficient grounding, the multimeter should read around 0.3 ohms.

Testing your trailer plugs

If you've found that the grounding is adequate, you'll need to test the trailer's plug to make sure it's receiving voltage. Take a look at the connector and get to know the different wires for each light.

Some of them may feature control labels on them, but most will just have a color code - for example, the white wire is for the ground connection. On most trailers, the turn signals and the brake lights function together, meaning there are four wires - ground, park light, and running.

The other two are for the turning signals and brakes. To test the trailer's plugs, turn the multimeter up to volt DC settings. Next, attach the black probe to the negative terminal and the other probe to the positive pin. Then turn the light on that's controlled by that pin.

Next, connect the red probe to the left signal control and turn it on. If you use a 12-volt battery for your tow vehicle, your multimeter should have a reading of 12 volts. If this happens, there aren't any faults with the trailer's plugs.

Testing your lighting connector

The next test you'll need to do is the lighting connector to find the issue with the wiring system. To do this, you'll need to test the system's resistivity. To test the resistivity, change the settings on your multimeter to ohms.

Ensure the red and black wires are properly connected to the multimeter. Then, unplug the trailer connector, and place the black probe onto the ground connection and the red probe onto each point pin.

Things To Keep In Mind When Testing Your Trailer Lights With A Battery

When you're testing your trailer lights with a battery, there are a few things you'll need to keep in mind:

  • Make sure the light bulbs are correctly fitted and working.
  • Make sure your battery is fully charged.
  • Ensure that the connectors are correctly attached.
  • Make sure there's no damage to the wiring.
  • Ensure that the ground is attached properly.
  • Ensure that the fuses are in good condition.
  • Make sure the brake lights aren't faulty.
  • Ensure that the trailer is correctly hitched to your tow vehicle.
  • Ensure that the reverse trailer lights are working.
  • Ensure the turn signals are working properly.

Common Trailer Light Problems

There are a few problems that people commonly experience with their trailer lights. The most obvious one is that the lights aren't working at all. This could be because of a bad connection, blown fuse, or a broken light.

Another common issue is that the tail lights may not be bright enough. This could be caused by a wiring issue or a faulty light bulb.

Other issues include lights that are flickering or switching on and off. This may be due to a bad connection or an issue with the wiring.

How To Troubleshoot Trailer Lights

To troubleshoot your trailer lights there are a few things you can do. Firstly, check the fuse which can be found in the trailer wiring. If it's blown, replace it with another fuse with the same rating.

Next, check if there's any damage to the wiring. If any of the wires are damaged, you can either replace or repair them. Lastly, check the light bulbs in your trailer lights. If you find that the bulbs are blown, you'll need to replace them.

How To Test 4-Way Trailer Plug

If you're wondering how to test 4 pin trailer plug on a truck, you'll be pleased to know that it's a relatively simple process. Firstly, ensure that the electrical system of the trailer is turned off. Next, find the plug test points.

There are usually four testing points on a 4-way trailer plug - two on the bottom and two on the top. With a multimeter, measure the voltage between each set of testing points. There shouldn't be any voltage between ay test points.

If you find that there is a voltage between the test points, it means the plug isn't wired correctly and it shouldn't be used.

How To Test A 7-Pin Trailer Plug

If you're wondering how to test 7-pin trailer plug on truck, good news! It's a quick procedure and it's fairly simple to do! The most commonly used method is to use a test light. This is a device that you plug into the connector which has a light that lights up once the circuit is finished.

You can also use a multimeter to test your trailer lights. This is a device that measures resistance, current, and voltage.

FAQs

Can I repair my trailer lights myself?

This depends on how severe the problem is. If it's simply a case of replacing a bulb, this is something you can usually do yourself at home.

But, if it's a more complex problem, it may be better to take it to a professional for repairs. Trying to fix it yourself may cause more damage, especially if you're not 100% sure what you're doing.

How do you test the trailer wiring with a battery?

To test the trailer wires using a battery, connect the positive terminal of the battery to the positive trailer wire, and the negative terminal of the battery to the negative trailer wire.

Doing this creates a circuit that allows the electricity to flow around the system. If your trailer lights come on it means the wiring is working properly. But if they don't, it means there's an issue with the wires.

Can you test trailer lights without a vehicle?

Testing your trailer lights without a vehicle may not be as simple as doing it with a vehicle. However, it can be done, you'll just need to turn on your trailer tail light using a vehicle battery.

To do this, simply detach the trailer plugs and use the wiring on the pin to assist you in finding the pinholes you'd like to test. You'll also need some wires to attach the plugs to the battery.

Connect the negative pinhole to the negative battery terminal and the positive pinhole to the positive battery terminal - the lights that are attached to the pinhole should come on. Repeat this process with the other pinholes.

Final Thoughts

Most of the time, you can fix or test your trailer lights yourself by following the steps above. However, there may be times when you'll need to have it professionally repaired.

This is highly recommended if you've tried the basic tests and still can't diagnose the problem, because attempting to fix it yourself could result in more damage.

Sources

https://poweringautos.com/how-to-test-trailer-lights-with-a-battery/

https://housetechlab.com/how-to-test-trailer-lights-with-a-multimeter/

https://www.wikihow.com/Test-Trailer-Lights?amp=1

Link To or Reference This Page

We spend a lot of time collecting, cleaning, merging, and formatting the data that is shown on the site to be as useful to you as possible.

If you found the data or information on this page useful in your research, please use the tool below to properly cite or reference Tow Ratings as the source. We appreciate your support!

  • "How to Check if Your Trailer Plug is Working". Tow Ratings. Accessed on March 28, 2024. http://towratings.net/blog/how-to-check-if-your-trailer-plug-is-working/.

  • "How to Check if Your Trailer Plug is Working". Tow Ratings, http://towratings.net/blog/how-to-check-if-your-trailer-plug-is-working/. Accessed 28 March, 2024

  • How to Check if Your Trailer Plug is Working. Tow Ratings. Retrieved from http://towratings.net/blog/how-to-check-if-your-trailer-plug-is-working/.