Learning, Multimeter,

How to Test Trailer Lights with a Multimeter (Guide)

Having your trailer lights working well is crucial. Driving without them puts your life and the lives of others in danger. However, even if you take care of them, they frequently stop working.

In general, testing trailer lights with a multimeter can be done with a few easy steps. The first step is Identifying the pinholes of the light. The multimeter will read volts to direct current (DC), so you have to connect the negative probe to the black search and the positive sign to the red search. Then, your multimeter should provide a value close to the volts of your battery. 

Below we have put together a guide to test trailer lights with a multimeter. It will help you prevent future mistakes and do the work more precisely.

Why a Trailer Light Doesn’t Work

checking trailer lights

A faulty ground connection causes many trailer wiring issues. It is often the white wire coming out of the trailer connector. Lights may function infrequently or not at all if the ground is poor.

Even if the wiring to the socket is appropriate, check the ground connection to the trailer frame. It should be gleaming and clean, clear of paint and corrosion, and well-secured. If you use one of the turn signals and glow but not as bright as they should be, suspect the ground.

How to Test Trailer Lights with a Multimeter

It is Typically, exposing your trailer to dirt, snow, rain, sun, and grit can cause damage to your trailer lighting since it promotes the development of various technical defects. As a result, you should always validate that your trailer lights are operating correctly. 

multimeter set on table

#1. Diagnostic Test

Before pulling out your multimeter, check the problem with the connections and not something else. How do you make that happen? 

  • Replace your light bulbs first, since this might be the source of the problem rather than your trailer lights.
  • If it still does not function, there might be a problem with the wiring.
  • Remove the cables that connect the primary car to the trailer. 
  • Attach the lights directly to the trailer to test this.
  • If the lights still do not work, you will need to use your multimeter.

#2. Ground Connection Test

testing for ground using multimeter

The ground connection must now be verified using the multimeter.

  • Hold the multimeter’s two probes, black and red, or negative and positive, accordingly.
  • For grounding tests, the multimeter should be set to Ohms or resistance.
  • Connect the probes to confirm that they operate.
  • Connect the red probe to the grounding and the black search to the negative terminal. The multimeter should detect about 0.3 ohms.

#3. Trailer plugs Test

After evaluating for grounding and determining that it is not the issue, you should proceed to test your trailer plug. It is to ensure that it is receiving the requisite voltage. Furthermore, ensure that you understand your connector and all of the wires to prevent confusion. It might lead to an accident or erroneous operation. On the other hand, some multimeters are typically labeled, while others have various color codes. (1)

To do your trailer plug test.

  • Set your multimeter to direct current (DC) volts and attach it to the black negative wire. 
  • Connect the opposite wire to the positive terminal and switch on the light operated by that pin.
  • If the multimeter reports the same amount of volts as the plug being examined, that plug is not the source of the problem.

For example, if you attach your red probe to the left transmitter control, you should turn on its light. As a result, your multimeter will indicate a reading of roughly 12 volts. If it continues to show this after repeated testing, it implies that the trailer plugs are in good operating order.

man testing trailer lights using multimeter

#4. Voltage Test

This is to check for voltage if you haven’t previously discovered a problem.

  • Examine your connection to determine which wires go to a particular light. Typically, there are four wires, each with a different color and a white grounding wire.
  • Set the voltage setting on the multimeter to measure the voltage. Make sure it’s configured to measure both DC and AC. A straight line is used to symbolize DC.
  • Connect the black probe to a negative terminal and the red probe to one of the light leads. Then switch on the light.
  • Take note of the reading. Your multimeter should indicate the value corresponding to the voltage of the battery you are using. So, if the battery is 12 volts, the readout should be 12 volts.

trailer lights and wires

#5. Lighting connector Test

You must measure resistance to evaluate the lighting connection. Follow the steps indicated below to do this:

  • Make sure the multimeter is set to measure resistance (ohms)
  • Connect the multimeter leads to the multimeter.
  • Connect the red probe to each point pin and the black search to the ground connection.
  • Take note of the reading. If the value is 3 ohms, your wiring system is working well. (2)

However, more than one control is required for wirings, such as those supplying the turning and baking lights. Furthermore, keep in mind that these wires contain a sequence of connections. Your multimeter may report the reading value with the low resistance.

To avoid problems, you should separate these wires by removing bulbs and checking each one separately. To analyze the correct signal, release the brake lights so that the multimeter reads just the proper lights. Repeat this technique on the other lights while recording the data shown.

Wrapping Up

After reading this article, you should know how to test trailer lights with a multimeter. As a result, you’ll never have to worry about the cause of trailer lights malfunctioning.

Other learning guides on multimeter are below. Until our next article!

(1) color codes – https://www.computerhope.com/htmcolor.htm
(2) wiring system – https://www.youtube.com/watch?v=ulL9VBjETpk

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About Sam Orlovsky

AvatarCertifications: B.E.E.
Education: University Of Denver - Electric Engineering
Lives In: Denver Colorado

Electrical engineering is my passion, and I’ve been in the industry for over 20 years. This gives me a unique ability to give you expert home improvement and DIY recommendations. I’m not only an electrician, but I also like machinery and anything to do with carpentry. One of my career paths started as a general handyman, so I also have a lot of experience with home improvement I love to share.

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