Trailer wiring problem

My old trailer was falling apart so I tore it down to the metal and I'm putting it back together.  I replaced all the wiring and can't figure out an issue.  The left turn signal won't come on.  I got a 4 prong plug for the trailer that has a set of LEDs on it that are blinking when the turn signal is on.  I take it to mean that there's juice at that end of my wiring job.  At the left tail light, it does come on faintly with the headlights (like it should).  I didn't have a helper to hit the brake pedal so I'm not sure if that's working correctly.

Before this latest round of troubleshooting, I replaced the whole light since it was kinda damaged.  It was doing the same thing so I blamed the damage.  Now that the new one is on there I think the problem is in the wiring.  

I reused the ground wire on the trailer since it was rusted on really well.  The right blinker works fine.

Any ideas?

I can't help you with the wiring, but (I used to be a trucker, and we had to check all our lights, and I used to) back up to something big enough to show a reflection in the side mirrors, when the lights are switched on. If the vehicle can't be moved, a board, mirror, or something else easy to see can be placed behind it, to allow a check.

Great point Carla!  If it was dark out I probably could've seen the reflection off the pavement as well.  I was just being lazy and in a hurry...

Mike,

Did you use a light kit or did you hook all the connections up yourself?  Kits are not always perfect from the factory.  Might check all the connects on that circuit.  Another possibility is a faulty ground.   Weak grounds can do weird things, even in simple wiring.  When you say it was rusty and you went down to metal, is there any spot on the ground that is maybe needing to be wire wheel and cleaned up?  If you reused an old wire you may be missing corrosion somewhere under the insulation.  I would check connection, ground, and then replace the old wire.

Hope you find your source.  Electrical problems can be a nagging pain.

I'm with Jack -- corroding ground connections to the trailer frame are a constant source of trouble. I actually run a separate ground wire all around so the damn thing works when I need it.

You may need a distinct earth from the vehicle earth circuit.
I learned that trick from an Auto electrician who explained LED's are very susceptable to bad earths.

Thanks team, I tied into the old ground and also to the ground of the 4 prong plug.  I'm not sure if it grounds from the trailer frame through the ball/hitch to the vehicle or through the white wire of the 4 prong.

I did wire it from scratch (no kit) but the existing ground was buried under corrosion.  

It was dark now so I checked the brakes.  And of course, everything works fine now including the left blinker.  Yay!?!?!  So either the 4 prong connector did a better job of connecting, or the hitch to ball made a better connection (if that's even a thing) or the wires are warmed up now and will work perfectly for years to come...

Yup Mike that's it! The wires are warmed up now!
Yes, grounding sometimes is done thru the ball.
More common with those 4 prong plugs is a poor connection, wiggling them about can often fix the problem.

I did put some dielectric grease on the prongs and wiggled/twisted them till they giggled.

I didn't have the latch closed on the hitch so that connection will only get better...

Mike Haasl wrote:My old trailer was falling apart so I tore it down to the metal and I'm putting it back together.  I replaced all the wiring and can't figure out an issue.  The left turn signal won't come on.  I got a 4 prong plug for the trailer that has a set of LEDs on it that are blinking when the turn signal is on.  I take it to mean that there's juice at that end of my wiring job.  At the left tail light, it does come on faintly with the headlights (like it should).  I didn't have a helper to hit the brake pedal so I'm not sure if that's working correctly.

Before this latest round of troubleshooting, I replaced the whole light since it was kinda damaged.  It was doing the same thing so I blamed the damage.  Now that the new one is on there I think the problem is in the wiring.  

I reused the ground wire on the trailer since it was rusted on really well.  The right blinker works fine.

Any ideas?

I run a marine repair facility, and we see this constantly.  You will need any kind of voltmeter, test light or whatever to do this right.

First thing to check is for current in the wires @ the light.  Wires can break inside the insulation.  
If wires have power, check the light itself.   We see defective LED lights often.  Sometimes you can fix them, sometimes not.

If the wires are NOT powered, then check for power at the vehicle trailer plug.   If it tests good thee, the problem is in the wiring harness.
If no power at the vehicle plug, check to see if your vehicle has separate fuses for the R and L trailer blinkers, and make sure they are good.  Ford trucks often have these fuses under the hood.

IF all of that checks out good, you most likely have a ground issue.  Rusty trailers or trailers that have been recently painted often won't carry conductivity from the tongue all the way to the back.   There are several ways to fix this, but the easiest is to screw the pigtail ground (white) wire to the tongue, and then run a wire from that screw to somewhere in the back of the trailer and screw it to the frame there.  This should ground the whole trailer, which helps ALL of the lights, including marker and center lights, if you have them.

Of course, you can run a ground wire to each light.  Most newer LED wire kits have 3 wires going to each light, the white one being the ground.

I hope this helps.

Thanks Tony!  It seems to still be working so I'm keeping my fingers crossed for a while.  If it stops working, now I know how to troubleshoot.

EDIT:
Good to hear.
Grounding through the ball is not correct, but at least it's working for you!     You should check the ground pin in the vehicle side of the plug for continuity to the vehicle frame.

Try scrubbing the pin with a wire brush with a few shots of WD40.  The corrosion that gets on those pins is pretty tenacious.

If that doesn't help, often the ground wire to the plug of the vehicle will break or get corrosion on it.  This is common.  It's typically screwed to the frame or the bumper somewhere.  Often, the ring connector rusts or breaks off.    Follow the wire and if it's broken or rusty, just replace the connector, clean the area, wipe on some dielectric grease and put it back and your problem should go away for a very long time.