Utility trailer lighting problems

[road_rascal]

I'm borrowing my father in law's 4x8 utility trailer to haul a new ATV to my cabin. All the lights work on the trailer (turn signal/running) but the right rear brake light doesn't work. I cleaned the ground on the tongue, tore apart the rear light housing and cleaned the bulb contacts, and tried a new bulb. My vehicle connections are working (hooked them up to a friends trailer) so there's something goofy with the trailer wiring. I'm going to buy a new light housing tomorrow (Blazer brand) and see if that works. For me to check the trailer wiring (to see if there's a break somewhere) I'm going to have to flip the trailer over and trace the wires since they're run though the frame. I watched a bunch of YouTube videos on trailer wiring and I'm 100% sure my (the tow car) connections are good. Any suggestions? I'm the type of driver who hates having having rear taillights not working. Thanks.

 

[mk378]

The brake and the turn signal are the same circuit. There are only three live wires, the standard color code is:
Yellow -- Left turn / brake
Green -- Right turn / brake
Brown -- Tail and running lights

And then the white wire grounds the trailer lights to the tow vehicle ground. On a well-wired trailer, there is a network of white wires to each bulb socket. On a typical one, the trailer frame is grounded at the tongue and the light sockets are expected to also ground to the frame.

By far the largest problem with trailer wiring is the grounds. You will likely find that the right turn signal blinks at a much lower intensity than the left one. This means that the right socket has lost its path to ground.

 

[Snagglefoot]

Originally Posted by mk378
The brake and the turn signal are the same circuit. There are only three live wires, the standard color code is:
Yellow -- Left turn / brake
Green -- Right turn / brake
Brown -- Tail and running lights

And then the white wire grounds the trailer lights to the tow vehicle ground. On a well-wired trailer, there is a network of white wires to each bulb socket. On a typical one, the trailer frame is grounded at the tongue and the light sockets are expected to also ground to the frame.

By far the largest problem with trailer wiring is the grounds. You will likely find that the right turn signal blinks at a much lower intensity than the left one. This means that the right socket has lost its path to ground.


I second the need for the white ground wires. Really helps. If the lights you bought are not coded, yellow, green brown, let us know and I'll did up the codes I once posted on Chinese made light fixtures.

 

[road_rascal]

Thanks for the replies. The left and right lights only have 2 wires going to them and are color coded. I'll pull the housings off and see what colors the wires are (plus they're marked on the housings). It'll be a day or two since the trailer is parked at my work. Thanks again.

 

[sweezy722]

If the brake light does not work on that side then the turn signal does not work.
With the brakes mashed - use a cheap tester to see if you have 12 volts going to the light socket.
If you do not have 12 volts here you start checking backwards toward the vehicle.
Check for 12 volts in the wire. Keep going backwards.
It could be a
1. rusty trailer ground - at the light - white wire connected from light to trailer
2. bad light
3. broken wire
4. dirty trailer connector plug
5. wire connector loose inside

 

[Donald]

If the right turn signal works but not the right brake light then you know that filament works.

With the running lights on are they both the same brightness.

When you do testing with a voltmeter the bulb may need to be in the socket to put a load on the power source.

 

[road_rascal]

Today at the ATV dealer I had a service guy hook up the trailer to some sort of portable tester and had the same result- all lights work except for the right rear brake. This stumped him too as he said a ground problem would cause the light to not work at all. My vehicle harness tested ok. I even replaced the light housing with a new one and that didn't work either. I'll have my father in law deal with it since it's his trailer.

 

[Donald]

I would try a one wire of battery jumper cables between the grounds of the two rear lights.