Trailer lights that do not work are one of the most common problems we see, and one of the easiest ways to get stopped by an officer. They are also a safety problem. If your brake lights and turn signals are out, the driver behind you has no warning.
The good news is that most trailer light problems come from a short list of causes. This guide walks you through them in the order that finds the problem fastest.
Before you start
You need a second person, or a light tester that plugs into the 7-way socket. Park on level ground, set the brakes and chock the wheels. Connect the trailer cable to the tractor and turn the key on. Then have your helper run the lights while you check the back of the trailer: running lights, left and right turn signals, brake lights and hazards.
Write down what works and what does not. The pattern tells you a lot. If every light is out, think power or ground. If only one side is out, think one circuit. If one bulb is out, think that bulb.
Step 1: Check the ground first
A bad ground is the number one cause of weak, dim or flickering trailer lights. Lights that work on their own but act strange when you turn on another light are a classic sign.
- Look at the ground wire at the plug and where it connects to the trailer frame
- Clean any rust or paint where the ground touches the metal
- Make sure the connection is tight and not corroded
Many trailer problems that look like a wiring disaster turn out to be a loose or rusty ground.
Step 2: Inspect the 7-way plug and socket
Look inside both the tractor socket and the trailer plug. Green or white powder on the pins is corrosion. Bent pins, melted plastic, and broken springs inside the socket are also common.
- Clean the pins with a contact cleaner and a small brush
- Replace a plug or socket that is cracked, loose or burned
- Make sure the socket cover closes when the plug is not in use
- Add dielectric grease to the pins after cleaning so moisture stays out
Use the wiring chart for your plug. Different trailers and tractors can use different layouts, so do not guess which pin does what.
Step 3: Follow the cable
The coiled cable between the tractor and trailer gets pulled, stepped on and pinched. Check it from end to end.
- Look for cuts, cracked insulation and flat spots
- Check where the cable enters the plug, since broken wires often hide there
- Check the cable for stretching near the coil ends
- Flex the cable gently while a helper watches the lights. Lights that flicker as you move the cable show you where the break is
Step 4: Check the bulbs and lamps
If only one light is out, look at it directly. Replace the bulb or, on LED lamps, check the connection and the lamp itself.
- Look for water inside the lamp or a loose connector
- Check the pigtail and the wire running to that lamp
- LED lamps usually need a clean ground just as much as bulbs do
One more thing: a wet lamp can work for a day and fail the next. If a lamp has water inside, replace it.
Step 5: Look at the trailer wiring
If the plug and the cable test good but lights are still out, the problem may be in the trailer harness. Wires run along the frame, through crossmembers and past axles. They rub, corrode and get cut by road debris.
- Check junction boxes for water or loose connections
- Look for wire that has rubbed through near the frame and axles
- Look for old repairs made with tape instead of proper connectors
Trailer wiring is easier to fix when you find the break early. A long run of bad wire is a job for a shop.
Step 6: Check the tractor side
If nothing works on the trailer but the lights are fine on a different trailer, the problem is on the tractor side. That could be a fuse, a relay or the wiring to the socket. Check the fuses first, then look at the wiring back to the socket.
What about the ABS light?
Many trailers have an ABS warning lamp. The ABS system needs a steady power supply from the tractor through the plug. If the ABS light stays on, a poor connection at the plug, a bad ground or a power problem is often the cause. Do not ignore it. An ABS fault can mean a wheel sensor or the trailer module, and it needs a proper scan to find.
When to bring the trailer to a shop
- You cleaned the ground and the plug and the lights are still out
- Lights work only when you wiggle the cable or the plug
- Fuses keep blowing
- The ABS light stays on
- You find melted wiring or burned connectors
Do not pull a loaded trailer with no brake lights. If the problem is on the road, we try to repair it on the spot. If it cannot be fixed roadside, we tow it to our own shop. We do not offer standalone towing.
Trailer light repair in Dallas
Bawa’s Truck & Trailer Repair is at 4700 Irving Blvd, Ste 200, Dallas, TX 75247. Our trailer repair and electrical diagnostics pages list what we handle. Call (214) 400-4228. We are open Monday to Saturday, 8 AM to 6 PM, and we tell you the price before the work starts.