If the lights on your trailer stop working, finding and replacing the right fuse is often the quick fix to get you back on the road. Trailer light fuses can blow for a number of reasons – from faulty wiring to moisture getting in – but regardless of the cause, a blown fuse will leave you without critical lighting like turn signals, brake lights and running lights. Knowing where to check for the fuse and having spares on hand can make a roadside repair quick and straightforward.
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Where are trailer light fuses located?
Trailer light fuses are most often found in one of three general locations:
On the trailer itself
Many trailers have a small plastic fuse box mounted somewhere on the exterior. Common spots are near the hitch, on the tongue of the trailer, or along the side. Inside this box you’ll find one or more fuses that protect the various lighting circuits. Depending on the complexity of the trailer lighting system, there may be separate fuses for turn signals, brake lights, running lights, etc. Simple utility trailers may have a single fuse for all functions.
In the tow vehicle fuse box
For trailers that plug directly into a wiring harness on the tow vehicle, the fuses for the trailer lights will be located in the fuse box on the tow vehicle. Usually there will be specific slots labeled as trailer fuses, but not always. Check your owner’s manual to see if any fuses also supply power to the trailer plug. These will be the ones to check.
Inline fuse holder on the trailer wiring harness
Some trailers have an exposed wiring harness running from the vehicle plug to the lights on the trailer. This harness may have an inline fuse holder mounted somewhere along its length. It should be secured to the trailer frame somewhere convenient, often near the tongue. Open it up to check or replace the fuse.
Identifying blown fuses
The first step in diagnosing trailer light problems is locating the fuse box and checking for any blown fuses. Here’s what to look for:
Check for broken or missing fuse
Obviously a missing or broken fuse is an easy issue to identify. Either the metal strips inside will be damaged or the fuse may be gone altogether. If you’re troubleshooting intermittent problems, check for any black burn marks on the fuse as this can indicate a partial meltdown.
Remove fuse and inspect the filament
Less obvious is a blown fuse that still appears intact. Remove each fuse in question and hold it up to the light. Look through the translucent plastic for the thin metal filament inside. If the fuse is good, the filament should be continuous. Any gap or break means the fuse is blown.
Use a fuse tester
For a more definitive test, use a fuse tester. This is a simple device that lights up or beeps when a fuse is good. Hold the ends to the metal tabs on the fuse. No light means it’s blown. Car fuse testers are inexpensive and handy for checking trailer and vehicle fuses quickly.
Try swapping with a known good fuse
If you don’t have a fuse tester, a simple swap can indicate if you’ve found the culprit. Pull out the suspect fuse and replace it with one you know to be good from another slot in the fuse box. If that fixes the lights, you’ve proven the original fuse was bad.
What to do when you find a blown fuse
Replace it with an identical new fuse
Once you’ve located a blown fuse, replace it with a new one of the same amp rating. Trailer fuses are typically 10-15 amps. Make sure the fuse fits tightly in the clips. Loose fuses can blow again quickly. If you don’t have an exact replacement, use a lower amperage as a temporary fix to limp home, but replace it with proper fuse as soon as possible.
Check for damage before reconnecting lights
Before putting a fresh fuse in, try to determine what caused the blown fuse in the first place. Look for pinched or bare wires that may be shorting. Examine all lights and connectors for damage. If lights weren’t working before, check bulbs and sockets. You want to fix any underlying problem before hooking everything back up.
Have vehicle and trailer systems inspected
If you can’t find an obvious issue, it’s a good idea to have the trailer wiring and tow vehicle electrical systems checked more thoroughly. Repeated blown fuses often mean there is a hidden problem. Better to have it inspected when convenient rather than get stranded out on the road later.
Preventing electrical problems
While fuses blowing can occasionally just be bad luck, there are things you can do to minimize electrical issues:
Use dielectric grease
Apply dielectric grease to all electrical connections. This prevents corrosion from moisture and road grime.
Check grounds
Clean and tighten ground connections on the trailer and tow vehicle. Corrosion here is the cause of many problems.
Inspect wiring routinely
Look for any worn or damaged wires and connectors. Replace or repair them promptly.
Avoid submersion
Don’t let wiring spend prolonged time submerged in water. Crossing streams briefly is ok but long immersion can cause issues.
Plug in when not in use
Always connect the trailer plug to the tow vehicle, even when not towing. This activates the fuses and lights so faults are noticeable instead of hidden.
Add a fuse box
If your trailer lacks a fuse box, adding an aftermarket one can be a big improvement. It allows easy diagnostics and protects the tow vehicle electrical system too.
Carry spare fuses
Keep a variety of common trailer fuse ratings on hand so you’re never stranded without the right replacement. 10 amp, 15 amp and 20 amp fuses cover most trailer needs.
Conclusion
While blown fuses that disable your trailer lights are annoying, they’re easy to fix once you know where to look. Keeping spare fuses on hand makes it a quick roadside repair if needed. But it’s also important to determine what caused the blown fuse rather than just replacing it over and over. Solutions like adding dielectric grease, replacing worn wiring, and routine inspections can prevent many electrical gremlins down the road.