So I’m sure someone has asked this before, but I can’t find it by searching…
My back left trailer light/running light won’t work. Bulb is fine. The left turn signal and brake light work. Just not when the actual lights turn on. I checked and rechecked the fuses, and those seem fine. I just replaced the wiring, so I guess that could be the culprit though I hope not. The right light works fine. Ideas?
There should be two wires running to each light a green or yellow one, and a brown one. Green/Yellow wires are the turn signal-brake light connection and the brown wire is the running lights. If you have a voltage tester or light, see if you’re getting any juice on the brown wire on the left side.
From what you describe, 99% of it is working great after your wiring job. I’m guessing this is something simple.
SR
It is the brown wire on the left side. Typically the run back to the clearance lamps on the side, then to the back right lamp, then to the running lamps on the bar in the center, then to the left lamp. Do you have clearance on the side and the 3 light marker lamps on the rear, center ?
Now IM heading out with a volt meter!! I should be able to figure out my trailer. Its an old utility trailer with 2 lights and they aint working right.
How do you know the bulb is fine, did you try it on the other side?
A typical bulb has 2 filaments, 1 for the brake/turn signal, the other for the taillight/running light. If you have power to the socket, it’s the bulb.
HRG
So I’m sure someone has asked this before, but I can’t find it by searching…
My back left trailer light/running light won’t work. Bulb is fine. The left turn signal and brake light work. Just not when the actual lights turn on. I checked and rechecked the fuses, and those seem fine. I just replaced the wiring, so I guess that could be the culprit though I hope not. The right light works fine. Ideas?
When you replaced the wiring did you run a ground wire from the trailer plug to each and every light? The ground wire is the white wire on your trailer plug. The issues you are having with the left hand light sounds like a faulty ground with that light.
“When you replaced the wiring did you run a ground wire from the trailer plug to each and every light? The ground wire is the white wire on your trailer plug. The issues you are having with the left hand light sounds like a faulty ground with that light.”
If the problem was a bad ground on the left hand light assembly, then the brake/turn signal wouldn’t work either. He said they do so that tells us the ground is OK.
If the problem was a bad ground on the left hand light assembly, then the brake/turn signal wouldn’t work either. He said they do so that tells us the ground is OK.
No, the ground is NOT OK.
. The left turn signal and brake light work. Just not when the actual lights turn on.
So then the the brake/turn in fact do NOT work correctly.
It is the ground that is faulty. I repaired so many trailer lights in my career, I can do it in my sleep.
In a case like what has been described, the electricity is feeding back thru both filaments which makes it appear that the brake and turn work when in fact they will not work when the running lights are turned on. (which prevents the feedback)
Hmmm….I understood him to say that the turn signal worked but the running lights didn’t. Maybe he can check back in with a better description of the problem.
Yes, some clearification of the problem would be helpful.
My back left trailer light/running light won’t work. Bulb is fine. The left turn signal and brake light work.
So does that mean:
Left turn signal works and left brake light works when running lights are “off”.
When running lights are turned on, no running lights working AND left brake/turn signal now do NOT work?
Or is it some other combination? Please describe the situation as completely as possible.
In addition, the above question already posted: How do you know the bulb is good?
Grouse
Guys,
Wow! Thanks so much for the tips. This is awesome. Sorry for the delay in circling back. I haven’t had a chance to try out the tips yet but hope to do so this evening.
I had no idea about the 2 separate filaments in a bulb, so I’ll try that option (a new bulb) first.
So to recap from my end. I replaced the wiring this spring and it’s – generally – worked fine since then. In fact the lights etc. worked fine last week. But a couple of days ago I check and all the brake lights and turn signals worked fine. But when I turned the lights on in my truck, all the running lights, including the back right light, worked. The left light did not work. But when the lights were on still and I pushed on the brake or turned on the turn signal, those did work in the back left light. So maybe it is a filament issue as one of you suggested.
If that doesn’t work, I will explore the ground issue too.
After installing the new lights, I did have bulbs burn out a couple of times and I don’t know if it’s cuz of cheap bulbs/lights, or operator error. I have replaced them and they did work fine the last week or two – until this issue.
Thanks again, guys. This is super helpful.
Hmmmmm
Still not working. It’s not a bulb issue as I tested the bulb in other side and it worked.
To recap … Everything works. Except the back left light. The left signal and brake light work at all times – with headlights on or off.
Fuses all appear to be ok. But rechecking those now.
Hmmmmm
Still not working. It’s not a bulb issue as I tested the bulb in other side and it worked.
To recap … Everything works. Except the back left light. The left signal and brake light work at all times – with headlights on or off.
Fuses all appear to be ok. But rechecking those now.
My guess is …you have running light wire that has worn through due to vibration,I cant believe it would be a ground if the other lights work !Test for power at front of trailer,then test at the light hook up for power. If you have power at front.. it good, if you have power at the back.. you good … Then light is BAD
If no power at rear ,you have bad wire in trailer frame .
<div class=”d4p-bbt-quote-title”>shockers wrote:</div>
HmmmmmStill not working. It’s not a bulb issue as I tested the bulb in other side and it worked.
To recap … Everything works. Except the back left light. The left signal and brake light work at all times – with headlights on or off.
Fuses all appear to be ok. But rechecking those now.
My guess is …you have running light wire that has worn through due to vibration,I cant believe it would be a ground if the other lights work !Test for power at front of trailer,then test at the light hook up for power. If you have power at front.. it good, if you have power at the back.. you good … Then light is BAD
If no power at rear ,you have bad wire in trailer frame .
+1. Completely agree. Check for current at locations as above. The meter tells the tale.
Grouse
again.. check that brown wire back there… 
Thanks, Guys. Whew. Gotta be a darn electrician nowadays to keep boat trailer lights going!
I’ll follow your advice and keep workin at it.
Not that this is your trailer problem, but as long as folks are sharing information…
Last year I borrowed a friends trailer in which the lights were thought to work good, but I found they had issues. Turned out that the ground from the connector was made to the tongue of the trailer, but given it had a tongue that was jointed, the grounding to the back of the trailer was intermittent. This had me quite bewildered as my problem changed as I worked on it.
Eventually, I realized that the pivot of the tongue would allow or disallow grounding depending on whether I wiggled the trailer and I then added a ground wire to the tongue to the frame. Problem solved.
Not that this is your trailer problem, but as long as folks are sharing information…
Last year I borrowed a friends trailer in which the lights were thought to work good, but I found they had issues. Turned out that the ground from the connector was made to the tongue of the trailer, but given it had a tongue that was jointed, the grounding to the back of the trailer was intermittent. This had me quite bewildered as my problem changed as I worked on it.
Eventually, I realized that the pivot of the tongue would allow or disallow grounding depending on whether I wiggled the trailer and I then added a ground wire to the tongue to the frame. Problem solved.
Good piece of advice and I can see where many would miss this potential issue of a swing tongue producing an intermittent ground.
I would also add another thing I discovered when switching trailers over to LED lights.
LED lights do NOT tolerate a weak ground! Unlike incandescent lights where they will just be dimmer with a poor ground, LEDs don’t like lower current flow, so they just won’t work at all.
With LEDs the solution is just to ground every light separately to the frame and the LED light fixtures I’ve installed actually have a grounding wire pre-installed on each fixture.
So I just use my Dremal tool to remove a little spot of paint about the size of a dime and get down to bare metal, drill a hole, and ground with a stainless sheet metal screw right at each fixture. Then I cover the screw and the bare metal area with liquid electrical tape to keep corrosion out.
BTW, it you’re tired of fighting trailer light battles, I can tell you from experience that LED lights are TERRIFIC. I installed them on 3 trailers and have helped others install them too.
I used to be troubleshooting lights on trailers every year. I haven’t had to touch any of the LED systems after I’ve installed them. Ever. They just work. The difference in reliability is absolutely incredible.
Grouse
I’m beginning to think I should just start over with LED! Whew. I spent an hour last night trying to narrow my problem and still coming up empty.
The saga continues.
Tried trailer on another vehicle and same issue. So I’ve eliminated the truck (and I also tested the plug on truck with volt meter).
I thought it might be a ground issue – and it still might – but so far haven’t narrowed it down.
so you are saying you have power at the brown wire in the back left corner ? if you do, then it can only be 1 of 3 things, poor ground, bad bulb or bad socket.
Yes, we have to find out if you have power at the brown wire back by the tail light fixture. Do you have a multi-meter to test this?
Do you have a ground wire right there at this light? Or is there one central ground up by the tongue of the trailer? Either way, I’m having trouble seeing how just the ground could be the issue because the other lights are working.
Since your other light (right side) is working, I’d be tempted to go to Fleet Farm and buy a new tail light fixture if yours is one of the standard sized ones. I suspect your bulb socket might be bad (assuming you have power to the brown wire as per above).
Save your receipt and if this doesn’t fix the problem, return the new tail light and buy a set of LED lights. Pull all new wire and install according to instructions. That will be the last time you have to touch lights for a long, long time.
Grouse
Shockers you get it figured out yet? I’m here in Roch and will give you a hand if needed.
99.9% of the time, trailer light problems are a bad ground. I would start there and you will most likely find your problem.
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