trailer electrical question

I have a 16 foot trailer and cannot solve a lighting problem. All lights work and the right rear turn signal (green or yellow depending on light fixture) works but the headlight bulb (brown wire) does not illuminate the bulb. Tried new lights. I had a voltage problem due to bad connections but fixed that, but to no avail, and then ran a new “brown” wire and ground from the male end of the connector on the truck to the trailer light. I have 12+ volts at the wire before the connection to the light but when I connect and measure the voltage at the light, when the light is connected, I have 0 volts. Using alligator clips I ran a ground and light wire to a bare bulb and nothing. What am I missing here? Thanks, Tom

I think I understand what you are saying,you have twelve volts from the vehicle socket but when you put a load on it,ie the light bulb,you have zero volts??
if that is the case,you are reading feed back voltage from another source and when load is applied you lose it.
so,you either have a bad fuse to that circuit,or a bad ground,it is possible that the wiring at the vehicle still isnt up to snuff.

feedback voltage can be read with a volt meter but wont light a bulb as it is cancelled out by the load.
it can come from from another lamp circuit that is bleeding voltage through its circuit,or,possibly a bad ground.

if you dont have a bad fuse find the rear connector from the main harness,( I dont know what your vehicle is so I am shooting from the hip here ),probe that harnesses connector for voltage and grounds,using your clips and a bulb,make sure it can handle that load,if it will,your problem lays in the rear wiring.
if it still wont handle the load of the bulb,your problem lays forward of the rear harnesses connector,good luck !!

You still have a bad connection, OR test the brown wire coming out of the truck plug. Put a light right into the plug and test it right there. 12 volts is easy just like water in a garden hose. You have a source and a wire/hose, water in water out if not there is a break somewhere.

Iowaboy1 always has great advice.

Do you have access to a buddy, neighbor or family member with a vehicle with a known working trailer wiring harness or another trailer to check to see if it’s in the vehicle or the trailer? Seems like more work, but it’s a quick way to eliminate one or the other after all the poking and prodding starts leading to profanity problems.

Most problems seem to be ground or fuse related.

Trailer harness fuses under the hood are usually pretty easy to check and find. Have had a lot just pop for craps and giggles on gm trucks.

Trailer grounds on trailers used in the winter in mn are prone to corrosion from all the salt and grime on the roads. As well as the vehicle harnesses.

Good luck with it and let us know what it ends up being.

Iowaboy1, bigcrappie and basseyes thank you all for your great insights. Its appreciated.

2004 Tundra

Feeback voltage – never thought of that. When I do apply a load (connect the light to the brown wire) I read 0 volts at the splice before the light and inside the light. Unspliced I read 12+ v before the light.

So it might be on the truck side. I will check. I will also check the grounds but if the turn signal in the light works, all the other lights work and I can get 12v in the wire right before the splice to the light right before the light using the ground inside the light to check the brown wire, could it be a bad ground?

Would not one fuse energize all the lights on the trailer circuit? If so the rest of the lights work fine so it wouldn’t be a bad fuse; or so it seems.

When I tried a bulb with clips right from the truck-side connector (where I read 12+v) it would not light (that could be operator error as well so will re-check).

Using a buddy’s truck to check is a great idea.

Thanks for your input. I will report.

you say you have power before the splice and not after it?
I really really hate to admit this,many years ago in a hurry I forgot to strip the wires covering off and crimped the connector on it,about drove myself nuts figuring that one out but it didnt take long once I realized I had power before and not after the connector.
I have seen that stunt pulled several times since then so I am not alone at least.

the lights should be fused separately,tail lights will be fused on their own circuit,turn signals will have their own fuse etc.

Iowaboy1 wrote:

you say you have power before the splice and not after it?
I really really hate to admit this,many years ago in a hurry I forgot to strip the wires covering off and crimped the connector on it,about drove myself nuts figuring that one out but it didnt take long once I realized I had power before and not after the connector.
I have seen that stunt pulled several times since then so I am not alone at least.

the lights should be fused separately,tail lights will be fused on their own circuit,turn signals will have their own fuse etc.

I have done some really stupid stuff over the years with trailers. When all else fails a simple vehicle swap to the trailer or a trailer swap to the vehicle can at least eliminate which one is the problem when all the testing just leads to more hair pulling and cursing. Once one or the other is eliminated, it usually makes following the gremlins easier by 50%.

GM vehicles I’ve had have had a problem with popping the left or right turn signal fuse that once caused some weird issues I swore was a grounding issue.

Feel the frustration. As simple as trailer wiring is, it can make a guy go bat crap crazy.

First step buy a 12 volt test light with a sharp point and ground clamp. This will tell you if you really have 12 volts (1 strand of a multi-wire trailer wire) can show 12 volts on a meter but will not carry load of a bulb. If all the other running light on the trailer work, your problem is in the brown wire. With test light work back from non working light till you find true 12 volts. Most likely a splice to one of the marker lights or the split to the other tail light. Good luck.

I looked at all the light connections and “re-did the grounds”. After that the tail light worked but the blinker did not. Exactly the opposite of the earlier dilemma. Go figure. I then cleaned out the channel on the top left side side of the trailer where the wires and connections run in conduit aft with breaks for the black wire to drop down to the running lights to look at the wires, grounds and connections, and pulled and pushed on the connectors. Now all lights function as they should. So I suspect that that there is a short that back-feeds one wire to the other wire/s. I will run a new set of wires and that will hopefully solve the issue. Thanks for your input. It’s appreciated.

Glad you got it figured out.