Trailer wiring troubleshooting

[Cam A]

My left rear turn signal does not work, nor the left brake light, but the running light does. LED replacements are planned, but I need to sort the wiring and probably grounds first. The socket has two posts on top, and the bulb grounds to the socket. (See fuzzy picture) When I put the black probe of the multi-metre on the socket and the red probe on the top left tang (multimetre set to range of 20V), I have a reading of 8.92 alternating to 8.88 with the turn signal click of the tug. The top right tang has a reading of zero. When I change the black probe from the socket to the trailer bumper (not hooked up to tug) I get a reading of .75 for the top right. The running light stays on; the turn signal (smaller filament) does not light. What does the top left tang supply and what should the reading be? Same question for the top right? Ideas? My search for trailer wiring troubleshooting has not been very successful; any links provided are welcome.

[Paul from NWOnt]

I admit that I only skimmed the description, without trying to picture it in my head, but it seriously sounds like a ground issue. You'll have to hunt down where the ground is on the trailer, and disassemble/clean/reassemble it, then make sure you also have a good ground on the tow vehicle, and wiring harness. In my experiences with highway salt trucks, I've found that 96 percent of light troubles come from connections and faulty grounds.

Good luck, just take your time and it'll be a piece of cake.

By the way, the brighter of the two filaments is the turn/brake light, and the dimmer is the running light.

[Herb P]

My left rear turn signal does not work, nor the left brake light, but the running light does. LED replacements are planned, but I need to sort the wiring and probably grounds first. The socket has two posts on top, and the bulb grounds to the socket. (See fuzzy picture) When I put the black probe of the multi-metre on the socket and the red probe on the top left tang (multimetre set to range of 20V), I have a reading of 8.92 alternating to 8.88 with the turn signal click of the tug. The top right tang has a reading of zero. When I change the black probe from the socket to the trailer bumper (not hooked up to tug) I get a reading of .75 for the top right. The running light stays on; the turn signal (smaller filament) does not light. What does the top left tang supply and what should the reading be? Same question for the top right? Ideas? My search for trailer wiring troubleshooting has not been very successful; any links provided are welcome.

Those readings are a little low but probably not the source of your problem. I would also take a reading at the tug end... The 4 pin connector is "GTLR" (ground, tail, left, right) where 'Ground' is the pin that's different from the other 3. If you also get about 8 or 9 volts there, then that's pretty low or there's something wrong with your meter. You might try cleaning the corrosion out of the connectors... You might also check further upstream right to the battery and see what you get there....

Regardless the above, the light bulbs have 3 connections. Ground, upper filament, and lower filament. One of the filaments is your brake/turn signal, the other is your tail. The filament that has fewer windings is the brighter one (brake/turn). It's possible/likely, that your trailer frame is not a ground point so unless you know the wiring, that is probably a meaningless measurement. You should be reading something close to your battery voltage at the tail lights modulo a little loss for distance and small wire gauge. 7 or so volts is fairly lossy.

I would start at the hitch on the tug, take measurements there, and follow the problem whichever way your measurements indicate.

[Byron Kinnaman]

My left rear turn signal does not work, nor the left brake light, but the running light does. LED replacements are planned, but I need to sort the wiring and probably grounds first. The socket has two posts on top, and the bulb grounds to the socket. (See fuzzy picture) When I put the black probe of the multi-metre on the socket and the red probe on the top left tang (multimetre set to range of 20V), I have a reading of 8.92 alternating to 8.88 with the turn signal click of the tug. The top right tang has a reading of zero. When I change the black probe from the socket to the trailer bumper (not hooked up to tug) I get a reading of .75 for the top right. The running light stays on; the turn signal (smaller filament) does not light. What does the top left tang supply and what should the reading be? Same question for the top right? Ideas? My search for trailer wiring troubleshooting has not been very successful; any links provided are welcome.

As far as the measurement to the trialer bumper, the negative side of DC power might not be connected to the frame of the trailer. It's not necessary for a trailer frame connection unless you have a converter and AC outlets.

The readings between the tangs and the base should be close to 12 volts when the light is supposed to be on. If the right side is working and left is not then I would try to find the wire connected to the plate of the fixture. That's the negative DC power (some like to call it "ground") and it should be attached to the plate of light fixture. Since that's common to both filiments that's the most likely suspect.

[Joe MacDonald]

Cam; is your meter set on vdc or volts dc., or v with a dot dash dot line?
are the probes plugged in in the right holes?

if they are, put the black probe on the socket, and the red on the left tang, it should go to plus 12v unless your blinker is really, really fast.
If you put your red probe on the right side, it should show plus 12v when your park lights are on.

to check how good your ground wiring is,(i'm thinking a bad ground is your problem)

put the meter on ohms, (or the little horseshoe shape),make sure the leads are in the right holes, put one lead on the socket and one on the frame on a clean metal part (scratch the frame if necessary) you should have a reading between 0 and .02 or .04 ohms, if it is higher you have a bad ground somewhere. Start at the incoming wire from your tug and follow it back. to the taillight.

Joe

[james kent]

Have you tried the simple thing first. Interchange bulbs and see if the problem follows. if so then one of the filaments may be shot. The second thing I'd check is grounding. It seems to be a common source of evil for these eggs.

[John Perry]

Most, if not all 12V replacement lights we use are not designed for fiberglass trailers therefore they do not have a ground wire. They are designed to be grounded to the metal body with the mounting screws.
This may be the reason that some new to fiberglass trailers have a difficult time with lights.
For your lights to work properly you need one common ground circuit that each trailer light connects to from tug to the 12 V lights on the trailer. You cannot rely on the trailer coupler to complete a ground circuit as on your boat or stick built trailer.
I have had a Burro, 2 Scamps and a Trails West and none have had the lighting system grounded to the frame.

Hope this helps,
John

[Cam A]

Thanks for all your replies. I decided eliminate any issues with ancient stock tail lights and go the LED route; picked up a pair of sealed 31 LED tail lights from the legendary Princess Auto ($24). Cleaned everything up, back to the wiring when it quits raining. Recommendations on attaching new LED tail lights - pop-rivets, self-taping bolts (like the stock license plate bolts), or nut & bolts? Hoping to complete lighting to allow trip to 2007 Prairie Egg Gathering in Macklin, Saskatchewan...

[Brian B-P]

Lights designed for trucks, on the other hand, all seem to have full wiring pigtails so no body "ground" is required. Truck lights are routinely designed to be installed in a rubber gasket (so they must have separate wires) and I haven't see a body-grounded automotive lamp for years, so it looks like common RV stuff is trailing behind as usual. Going to LED lamps often means switching to truck hardware, and thus proper wiring.

Like John's trailers, my Boler does not use the trailer frame as part of the lighting circuits, and the factory wiring diagrams for Boler and (original) Trillium also show negative ("ground") wires to all circuits; that makes for a strange connection to the tail lamps on the B1700, since the lamp uses its metal base for the socket's common ("ground") connection.

[Cam A]

Main issue was bad tug wiring. U-Haul installed a beauty flat 4 plug; it has mini-LED's that light when the wire is being used - proof positive of current. Now for the side markers...