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How Do You Test Trailer Lights With a Voltage Tester?

Home » Blog » How Do You Test Trailer Lights With a Voltage Tester?

Apr 24, 2024 | News

A basic voltage tester can quickly check the working status of your trailer lights before a trip or while on the road. The entire testing process takes only a few minutes to complete. At David’s Heavy Duty Tool Sales, we can help you find a reliable trailer tester to frequently perform such checks on your tractor-trailers. Here is more information about testing trailer lights and finding the right equipment:

Multimeters vs. Voltage Testers

Both voltage testers and multimeters can be used to test trailer lights. A multimeter is a multipurpose tool capable of measuring elements like temperature, relative humidity, frequency, and voltage. Multimeters are able to measure electrical flow in units aside from volts, such as amps and current. A voltage tester performs the same function as a multimeter at a slightly lower price point. Its simple data display may also be easier to interpret for those unfamiliar with using a trailer tester.

Prepping For a Test

A simple trailer light test requires users to understand the functionality of their equipment. The equipment being tested includes the trailer plug socket on your tow vehicle and the connecting 7-pin plug from the trailer. The electrical wiring in the socket receptacles is set up in the following clockwise configuration: positive terminal, right turn/stop signal, electric brakes, negative terminal (ground), left turn/stop signal, and running lights.

The center receptacle powers the reverse or accessory lights. The corresponding pin setup in the plug is the mirror image of the socket. You can test the pins in the plug itself. Some plugs run to an attached junction box affixed to the trailer’s arm. If your equipment has a junction box, you can remove the lid and run the test on the wiring within the box. The standard color-coding of the wiring is as follows:

  • Red: Positive

  • Green: Right T/S

  • Blue: Brakes

  • White: Ground

  • Yellow: Left T/S

  • Brown: Running Lights

  • Black: Reverse/Accessory Lighting

Work With a Partner

You’ll need someone at the wheel of the tractor-trailer to turn the engine on, tap the brakes, and turn on various signals. If you are still in the garage, ask a colleague or technician for assistance. If you’re alone on the road, ask whether a gas station clerk or fellow driver can lend you a hand.

Attach The Truck To The Trailer

The tow vehicle and the trailer should be attached during testing. This is needed because when the truck is running, it supplies power to the trailer. Leave the tow vehicle’s parking brake on to prevent it from rolling while you’re conducting the test.

Set Up The Voltage Tester

Confirm the voltage tester is set to measure direct current (DC), as the tow vehicle’s battery powers the trailer lights. Connect the tester’s alligator clip to a grounded element like the trailer hitch or frame. Touch the tester tip to the positive terminal in the trailer plug socket of the tow vehicle. If the tester lights up, this is an indication of a good ground connection. If it doesn’t light up, clear the ground connection of any debris or corrosion that may be disrupting the current’s flow.

Testing the Trailer Lights

Testing for flow involves touching the tester tip to the desired element’s pin in the 7-pin plug or the wire terminals in the adjoining junction box. Be sure the plug is connected to the socket in the tow vehicle if testing the box. Test each of the trailer light components as follows:

Running Lights

Ask your partner to turn on the vehicle’s engine and headlights. Touch the tester tip to the running lights pin or brown wire in the junction box. A solid light indicates that the running lights are functioning.

Brake Lights

Have your partner leave the engine running, turn the headlights off, and apply their foot to the brake pedal. With the tester tip, touch both the left and right t/s pins in the plug or the yellow and green wires in the box. The trailer’s brake lights are working properly if the tester shows a solid light when both pins or wires are touched.

Turn Signals

Ask your partner to turn the right turn signal on. Touch the tester tip to the right t/s pin or wire. If the turn signal is working, the tester should show a blinking light. Repeat the same process with the left turn signal and its corresponding pin or wire.

Reverse Lights

Complete the reverse light test by having your partner apply the brakes and shift the tow vehicle into reverse. Locate the center pin in the plug for the reverse/accessory lights, or identify the black wire in the box. A solid light display shows that these lights work and will inform other drivers when your truck backs up.

Finding the Right Trailer Tester

A quick check of your lighting system with a trailer tester confirms your tractor-trailer is in good condition, protecting the safety of all drivers on the road. Identifying lighting problems before getting on the road using a voltage tester helps you and your team avoid potentially costly liability concerns. Carrying a tester in your truck allows you to test your lights on the go. At David’s Heavy Duty Tools Sales, we carry a wide range of testers and other equipment to help keep your truck running efficiently. Contact us today to learn more about our inventory.