How to Test Trailer Lights with a Multimeter

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It does not matter what kind of trailer you tow as you can only drive well if you have well-functioning lights on your trailer with this is a vital component that enhances legal and safe towering. Typically, the exposure of your trailer to dirt, snow, rain, sun, as well as grit, can result in damage to your trailer lights since this starts to foster the development of some mechanical faults. Consequently, you might find yourself in a not very pleasant situation of being stuck in a traffic jam with absolutely no light running.

Moreover, this is also a tricky scenario as it increases the risk of accidents as well as attracting a fine from the local traffic offers due to your ignorance. Therefore, you should always confirm to check whether your trailer lights are working correctly and not attempting to venture on the road before you confirm that everything has been resolved. 

Testing your trailer lights

For you to fix your trailer lights, a multimeter is an essential equipment since this tool helps you troubleshoot the electrical systems as well as detecting the source of break down.

However, before you dispose of your trailer lights for not working as expected, you need first to test it to verify that it, without a doubt, is damaged and needs to be replaced. It is at this point that a multimeter comes in hand, but how do you run a test on your trailer lights using a multimeter? Well, you need not worry as in this article, you will be taken through the steps you need to follow when there are several steps you are required to follow to test trailer lights with a multimeter. 

Step 1 – Do the necessary check on your trailer lighting

Before going ahead to run the test using your multimeter in your trailer lights, you first need to check a few things that must never be overlooked. First, you should inspect whether your trailer might be having a burnout bulb that needs to be fixed to make sure that the bulb is still functioning correctly. 

Additionally, in case of any issue, examine whether your trailer has a connection problem, and to do this, you would probably need to disconnect your trailer from the tow vehicle wirings. Subsequently, make a connection of trailer light directly to your vehicle and inspect if it is running, and the same applies to the left as well as right light plus brake. With these results in mind, you will be better placed to establish what is wrong with your connections.

Step 2 – Test for grounding

You need first to disconnect your trailer plugs, and then check whether your lighting system is well connected to the three vital pinholes for the positive connectors. There is also a tiny opening preserved for the negative connector. Additionally, it is also crucial you take note of any trailer light breakdown, which is usually the result of the weak ground connection.

For you to successfully test the ground connection, you have to use a multimeter. A multimeter has two probes, and these are; 

  • The red probe is for the positive connection
  • The black probe is for the negative connection

Set your multimeter settings to ohms, and since you may need to position the probes together to ensure they function effectively. To do this, place the red probe on your grounding and the black probe to the negative terminal, which typically has a minus sign of the plug. For suitable grounding, your multimeter should display a reading of around 0.3 ohms.

Step 3 – Examine your trailer plugs

After testing for grounding and establishing that is not the issue, you should now go ahead and run the test on your trailer plug to make sure that it is properly getting the required voltage. Furthermore, make sure you understand your connecter to understand all the wires well to avoid any confusion which might result in either an accident or working incorrectly. However, some multimeters are usually labeled, while others have different color codes.

To run your test on trailer plugs, you need to set your multimeter on the DC volt settings and then connect to the black negative terminal. After that, connect the other pin to the positive terminal and turn on the light controlled by that particular pin. 

For example, if you connect your red probe to the control of the left signal, you should then switch on its light. Subsequently, your multimeter will display a reading of around 12 volts, and if it keeps showing this after repeated tests, it then means that the trailer plugs have no issue and are working properly. 

Step 4 – Test your lighting connectors

The lighting of the connectors will allow you to determine the problem with the wiring system. To be able to achieve this, you have to test the resistivity of the system by setting your multimeter on the ohm position. Make sure your black and red probes are correctly connected to your multimeter. Consequently, remove the connectors of your trailer and put the red probe on the point pins and black probe on the connection ground.

If your wiring system is good, the readings displayed on your multimeter should be approximately 3 ohms. However, for wires, it needs more than one control; for example, the ones sourcing the turning as well as bake lights. Moreover, it would be best if you also took note that these types of wires have a series of connections, so your multimeter might be displaying the reading value with the least resistance. 

Therefore, to avoid challenges, you should isolate these wires by removing bulbs and testing each one of them on their own. To examine the right signal, you have to do away with the brake lights, so that the multimeter reads only the right lights, the one that is being tested. Consequently, repeat this same procedure on the other lights as you record the readings being displayed.


Having read through this article, you now have a great idea of how to make use of your multimeter to test trailer lights. Consequently, you never again should worry about how to best tell whether your trailer lights are defective or not, thanks to a multimeter.

Therefore, whenever you encounter any issue with the trailer lights, get your multimeter, and do the tests to establish what the problem might be. By doing this, you will then be able to pinpoint what needs to be repaired or replaced.

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