Aagh! My trailer lights aren’t working!

It always seems that problems arise when it is most inconvenient.  Anyone who has ever owned a trailer has more than likely experienced lighting issues. 

Whether it is due to faulty wiring or years of use and abuse, at some point in time you are bound to lose a lighting function such as: turn signal failure, brake signal failure, or reversed signals, etc.  No matter where you live in the United States you are required by law to have a functional lighting system on your trailer, and "It worked when I left?" isn't always an acceptable excuse for the State Trooper.  Towing laws by state.

First things first, don't panic and run your trailer to the nearest repair facility just for them to hand you a bill for 70 dollars and the cost of an 1157 incandescent bulb.  There are times when it can be necessary to take your lighting problem to a trailer repair facility, but before you make that jump here are a few pointers to consider when diagnosing an electrical problem.            

There are a few things that you can check without the necessity of electrical diagnostic tools.  Let's say that one of your turn signals are not functioning. First check to see that the signal on the tow vehicle is flashing.  Chances are if it isn't flashing that you have a fuse that is blown in the vehicles in-cab fuse box.  If you have trouble locating your fuse box see your owner's manual for fuse box location(s). If the tow vehicle's signals are flashing, you should check to see if your trailer's incandescent bulb is burnt out.  If your trailer has LED trailer lights it is highly unlikely for them to be burnt out due to the incredible long life of these low amperage diodes. Most trailers have an incandescent bulb such as an 1157 which has two filaments to operate the different signals (stop, turn, and tail) if either filament is split, replace bulb and recheck to see if your turn signals are working. Still not working and your vehicle is equipped with a factory harness?  The last thing that you can check is the engine compartment fuse box.  Most fuse boxes have a legend printed on the cover of the fuse box; check owner's manual if one doesn't exist.  Check the fuse labeled "Trailer Stop lamp/Turn Signal" or "Trailer tow left hand stop/turn" key words to look for are Trailer or Tow.  See flow chart below illustrating this process. Click to enlarge.

If all of these methods fail to solve your problem you will need some sort of electrical tester such as probe tester or multifunction tester.  When it comes down to this point it is more than likely the trailer or the vehicle's electrical harness.  Start by checking the output on the vehicle end by grounding your electrical tester and with your head lights on, vehicle running, and failed signal on; test the corresponding wire.  On vehicles that have an after-market 3-to-2 wire converter it is also a good idea to test the input and output of the converter box.  Although colors can vary from manufacturer to manufacturer the industry standard for aftermarket converter modules are; Left turn-yellow, Right turn-green, Tail marker-brown, Stop-green/yellow, and Ground-white.  If you have an input signal and no output, this means that the converter module has a shorted output or has failed.  When this occurs, the short either needs to be corrected or the module will need to be replaced.  Next connect the trailer plug end to the vehicles plug end and test the continuity through the plugs to rule out a shoddy connection inside. This wire probing instruction shows detailed photos of this process. Click to enlarge.

Now on to the trailer, the easiest way to find out if it is a problem with your trailer is to plug it into another vehicle that has been verified to have a harness that is in good working condition. This would be a good time to meet that neighbor that never mows his lawn, or invite your no-good brother-in-law over for dinner. However, if this is not a possibility it's a good idea to check all ground locations and look for tears in the wiring insulation along your entire trailer to rule out possible shorts. A lot of trailers have most of the wiring inside the frame, making it very difficult to check for shorted wiring.  At this point after you have exhausted every diagnostic tip that I could possibly think of it would be a good idea for you to take your trailer to a repair facility.

Travis Mai - Hitch Guy
Technical Product Support Rep. - CURT Mfg.