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Author Topic:   The Case of the Baffling Trailer Light
69boo307 posted 07-22-2009 12:16 PM ET (US)   Profile for 69boo307   Send Email to 69boo307  
The Shoreline trailer for my Striper has fully functioning but very rusted light fixtures. As a precautionary measure I decided to upgrade it with some sealed LED lighting. My approach was to replace one [lamp] at a time and verify that everything works before proceeding to the next [lamp]. I started with the right amber marker lamp.

I removed the old [lamp] and attached the shiny new one, connected the wire and the ground, and it doesn't work. None of the lights come on now. After messing with it for a while I thought maybe I have a defective [LED lamp]; I removed [the LED lamp] and hooked the old, previously functioning, light back up [incandescent lamp]. Same result, none of them work. Keep in mind this was working just 30 minutes earlier. I remove the old light again, and break out the multimeter.

At the socket under my hitch on the truck I tested the voltage; with the running lights turned on I have 12V as expected. Back at the wire to the marker light, no voltage. I trace it back to the main wire off of which the marker light wire is spliced: no voltage there either. What the heck?

Next I turn OFF the truck lights and hit the brake pedal, all of the trailer lights, including the marker light that is still connected, light up. Turn on the truck lights, nothing.

The frustrating thing is that I did nothing to the trailer wiring except remove the old marker light and hook up the new one. I used the exact same wire that the old light was hooked up to. How do I suddenly no longer have voltage at the wire? Either it has to be grounded or have a bad connection somewhere right? If so I can't find it, and I didn't change any of the wiring from it's working state before I started.

My next thought was maybe I blew a fuse in the truck, but all of the truck lights are still working fine, I have voltage at the trailer light socket, and I can't find a blown fuse. I'm still wondering though if it couldn't be a fuse somewhere?

Any ideas?


Bella con23 posted 07-22-2009 01:03 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
What kind of truck Brian?
It's possible that the trailer connection has it's own fuse like it does in my Nissan.
69boo307 posted 07-22-2009 03:00 PM ET (US)     Profile for 69boo307  Send Email to 69boo307     
It's an '04 Chevy Colorado. I was trying to determine if the trailer lighting has a dedicated fuse, but so far it doesn't appear that way. It has the OEM towing package, so the trailer socket and all was set up from the factory.
Buckda posted 07-22-2009 03:33 PM ET (US)     Profile for Buckda  Send Email to Buckda     
My 07 Ford F-150 has dedicated and fuse-protected circuits for the factory tow package. Look carefully at your fuse diagram.

I recommend that you replace all your lamps with the LED upgrade before testing and troubleshooting.

LED's require a lot less current than incandescent lamps, so be prepared to deal with corrosion blocking current flow - which may explain why your brake lights work, but the running lights do not. Most likely culprit there is the fuse in your truck, however.

Inspect all the original wiring on the trailer very carefully for damage to the protective coating/insulation. Any nicks from rocks or wear or pinching, etc can result in a short that will blow the fuse in your vehicle and result in lamps not working. I was bedeviled by this phenomenon late last year and early this spring. Finally last month I just pulled the boat off the trailer and re-wired the whole thing. Problem solved. Close inspection of original wiring revealed several problem spots where the insulation had been compromised and other spots where a problem was only a short time away.

Good luck.

Buckda posted 07-22-2009 03:54 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Brian -

The reason your brake lights work is that the brake light electricity is running on the "other" wire that runs to your trailer lamps.

After re-reading your post, if you have 12V at the truck plug, then your problem lies entirely within the wiring for your trailer - and at that - only along the wire that carries current for your running lights.

Wiring is not expensive - less than $20 should do it. Re-wire the trailer with new and save the old wiring for projects.

69boo307 posted 07-22-2009 09:21 PM ET (US)     Profile for 69boo307  Send Email to 69boo307     
I'm afraid I may have to do that :(. What doesn't make sense is why would it suddenly stop working. I messed with it some more tonight, re-installed the LED marker light that I was working on. Same result, they all come on if I hit the brake lights while the truck lights are off, none come on if I just turn on the truck headlights, even if I then hit the brake lights.

I've been poking around with the multimeter and I've been having trouble determining what's what. It seems to me that there is some strange behavior going on with the ground, which would again point to a bad wire somewhere. I suppose when I hit the brakes without the headlights turned on, I am using a wire that does not have a short in it. With the headlights turned on, all the wires appear to then be shorted, perhaps through a single point.

PeteB88 posted 07-22-2009 11:03 PM ET (US)     Profile for PeteB88  Send Email to PeteB88     
I have come to the conclusion that once trailer lights become problematic, if I get a different (used) trailer or I have intermittent problems I just install new lights and wires. Kits are less than $30 bucks, LEDs a bit more. I've had great luck with Optronics, think Reese is junk (took the kit back). One benefit of rewiring and installing new lights is you get to know your trailer real well - you spot other problems, potential failure modes and are able to do some preventive maintenance. Good luck.
69boo307 posted 07-23-2009 09:40 AM ET (US)     Profile for 69boo307  Send Email to 69boo307     
I bought the Optronics lights as well. My one gripe so far is that the side marker lights have this little bullet connector that plugs into the back of the light module, and the connector they use is extremely inadequate. After 2 removals it just broke off. I had to go buy a little box of the connectors with heat-shrink sleeves on them and re-do the wires that came with the kit.
wbullwin posted 07-23-2009 12:11 PM ET (US)     Profile for wbullwin  Send Email to wbullwin     
I just replaced my rear lights with the Optronics and they work fine. If I read your procedure that you did correctly, you say if you hit the truck brake pedal, ALL the lights come on. Stop and running lights? If that is the case, you have a bad ground somewhere. It is really weird how a bad ground will make lights work/not work and make the lights that shouldn't light up, light up, etc.
Chevy does have a separate fuse for trailer lights which I found out the hard way after wasting an hour trying to find out what was wrong with my TRAILER bulbs. But as stated, if you got power at the truck socket, it is not the fuse. Go back and check all the lights for ground. I think you have broken the ground somewhere.
swist posted 07-23-2009 03:50 PM ET (US)     Profile for swist  Send Email to swist     
Yes indeed. Whenever I see "weird" and "trailer lights" in the same topic, I say to myself "bad ground" before I even read it.

And yes indeed, most trailer electrical components are pretty low quality. I think with all the water immersion and other abuse, they've pretty much given up on supplying robust systems. As someone said, it's just cheaper and easier to replace the whole mess every few years, or at the first sign of trouble.

pglein posted 07-23-2009 05:22 PM ET (US)     Profile for pglein  Send Email to pglein     
It's a little confusing becuase you aren't explaining exactly which light you're replacing. Was it a tail light, or a side marker? Perhaps before jimh [changed the word "light" to "lamp"], this was clear, but now it is not.

The reason I ask is becuase you mention attaching the "wire and the ground". Tail lights usually have two wires, and a ground. If those were hooked up incorrectly, it could conceivably cause this problem.

Buckda posted 07-23-2009 05:43 PM ET (US)     Profile for Buckda  Send Email to Buckda     
pglein -

Good thoughts.

Incidentally, JimH did edit it - but it still states that he started with the right amber marker lamp.

...My approach was to replace one [lamp] at a time and verify that everything works before proceeding to the next [lamp]. I started with the right amber marker lamp.

I added the emphasis.


jimh posted 07-23-2009 08:11 PM ET (US)     Profile for jimh  Send Email to jimh     
Light comes from lamps. If this causes confusion in comprehension, please review the meaning of these two words.
jimh posted 07-23-2009 08:17 PM ET (US)     Profile for jimh  Send Email to jimh     
In low voltage DC circuits involving incandescent lamps, there are often odd behaviors which result from two characteristics:

--an incandescent lamp has a negative resistance characteristic, that is, as the current flow through the lamp increases, its resistance goes up, increasing the voltage drop across the lamp. This can cause very confusing behavior when a lamp which is not illuminated becomes an element in the circuit for another lamp;

--DC circuits typically rely on a common negative return. If there is an interruption in the negative return portion of one branch of the circuit, other branches of the circuit can act as a negative return in unexpected ways.

I drew several diagrams showing how current flow reacted to these conditions in a prior discussion at

and these may be helpful in understanding the behavior of low voltage circuits with incandescent lamps.

jimh posted 07-23-2009 11:11 PM ET (US)     Profile for jimh  Send Email to jimh     
When incandescent lamps are replaced with light emitting diodes, two distinct differences are introduced:

--the light emitting diodes allow current flow in only one direction, thus preventing reverse current from flowing on paths which might have existed with incandescent lamps;

--the diode does not have a negative resistance characteristic, and instead has a more or less constant voltage drop across it. This is very different from an incandescent lamp.

These two differences could explain unusual behavior of a circuit when light emitting diodes replace incandescent lamps. This would be particularly true if there were a problem with the negative return wiring in the circuit where the off-resistance of a cold incandescent lamp was acting as a circuit element.

69boo307 posted 07-24-2009 10:19 AM ET (US)     Profile for 69boo307  Send Email to 69boo307     
So far I have only replaced the right side marker LAMP, but I intend on replacing all of them.

"If I read your procedure that you did correctly, you say if you hit the truck brake pedal, ALL the lights come on. Stop and running lights? If that is the case, you have a bad ground somewhere."

yes, that is what is currently happening.

At this point I think I'm going to take everyone's advice to replace all of the wiring along with the lamps (they are called 'lights' in these parts, we don't have headlamps, we have headlights! :) ), rather than scratch my head over it any longer. I can probably run new wires in a couple of hours, vs. several more days of banging my head against it.
I'll follow up on the results.

thanks for the responses, this has been very educational!

69boo307 posted 07-24-2009 11:35 PM ET (US)     Profile for 69boo307  Send Email to 69boo307     
A $15 wiring kit and 2 hours of work later, I now have fully functioning LED trailer lights! I replaced all the wiring and it worked the first time I turned it on.
Thanks for all who replied!
jimh posted 07-25-2009 08:09 AM ET (US)     Profile for jimh  Send Email to jimh     
You probably cured a bad negative return circuit when you installed the new wiring. An astonishingly high percentage of problems with trailer lamp wiring is due to sneak circuits substituting for the negative return. Thanks for the follow up.
Plotman posted 07-27-2009 02:11 PM ET (US)     Profile for Plotman  Send Email to Plotman     
Jim- what is a sneak circuit?
pglein posted 07-27-2009 02:50 PM ET (US)     Profile for pglein  Send Email to pglein     
If I had to guess what he means, he is referring to a place where the positive side of the circuit is grounding out to the chassis, and, instead of providing current to the lamps downstream of it, is serving as the ground for the lamps upstream. If the lamps downstream happened to be broken, corroded, or simply "burned out", this would not cause the fuse to blow, and it would also explain why they did not function. It would be entirely possible for this short to be the cause of the blown bulbs as well.

It is very common for the ground (white) wire between the vehicle and trailer to become compromised in one way or another. This will often go unnoticed, since a ground connection can be established through the coupler and ball. However, this connection is not reliable, and will behave strangely (I've seen trailer lights that only come on when the vehicle is slowing, becuase it is putting pressure on the coupler).

But jimh is obviously much more of an expert on this subject than I, so if I have misinterpreted his statement, I am quite prepared to be corrected.

jimh posted 07-28-2009 09:26 AM ET (US)     Profile for jimh  Send Email to jimh     
Sneak circuit = an electrical circuit in which current flows in a manner not intended or not readily perceived by a human who expects electrical current to follow the path intended for it, not the path it took.
Chuck Tribolet posted 07-28-2009 11:02 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Pglein: A short to ground won't blow bulbs. Fuses yes,
bulbs no.


btb posted 07-31-2009 09:43 PM ET (US)     Profile for btb  Send Email to btb     
Now I'm really confused - what's a "bulb"??? :-)
number9 posted 08-01-2009 08:37 AM ET (US)     Profile for number9  Send Email to number9     
"Pglein: A short to ground won't blow bulbs. Fuses yes,
bulbs no."

Qualifier = Correctly selected and rated fuse to protect the lamp and not just to protect the wiring as I've read others to suggest using.

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