Very strange trailer light malfunction (I'm a newbie so it may be simple to you guys)

[Kamel]

If you'd like to see a very quick version of this post scroll to the bottom to the "Just a quick recap:" part.

I have personally never seen this before and have no clue what to make of it.

Best I can tell, the trailer itself is working fine with all the wiring etc. but perhaps I'm wrong. I'd love to put it on a different truck to verify but unfortunately I can't find one and I'm pressed for time.

I borrowed a trailer from a friend to make a 250mi trip. The left tail light was out on it, but only the running light (mind you this is the first time I have used the light connection on the truck since I've had it). Looking at the encasement, the side red translucent plastic part was missing and the light looked like it had some relatively mild water damage/rust. I replaced the lamp and the problem was not fixed.

After returning and testing, I discovered the tail light functions perfectly fine when connected without the running lights on. When the running lights/parking lights are on, however, the left brake light does not work at all, not the running light and it will not light up when the brake is applied.

After some fiddling around, I discovered that there are 3 electrical plugs in the compartment with the jack. When I remove the last one, it seems to disable the running light functionality. I am afraid that it still may not be enough to keep from getting pulled over though.

Any thoughts on the problem? I am very electronically inclined, so I should be able to troubleshoot and fix this somehow. I am guessing a relay or fuse is out but I have no idea.

I did, however, check for a good connection with a multimeter between each of the 4 wires connecting the trailer to the truck and all 4 wires have a good connection.

Is there a way I can test the trailer connection using a multimeter? Is this just a simple fuse? Any suggestions would be MOST helpful as I should be leaving here in about 10 hours and my wife and I are moving out of despiration hoping for better luck getting jobs in KY where my family is from. We really can not afford a ticket right now


Just a quick recap:
*First time using trailer connection on vehicle
*Vehicle did include tow package from the factory and the components should all be factory original.
*Trailer had a bulb blown but replaced both bulbs in encasement
*When running lights are on, the running light does not work nor does the brake light work, the left tail light essentially does not work at all.
*When the lights of the truck are not on, the tail light functions fully as intended
*There is a missing red plastic piece on the side of the tail light which has probably let some water in causing the bulb to blow. The trailer (including encasement) was dry when I got the trailer however. I have also since gotten tape to seal the tail light back up once I fix the problem.

 

[Michiganbear]

this really sounds like a bad ground on the trailer (not the connector to the truck, but from the trailer's harness to its frame).

bad trailer grounds will cause all sorts of strange things.

 

[corkey]

can you check the trailer wires using a battery ?and a few small pieces of wire ?
also use the same wires and a spare bulb to check the output of the plug ,

with a voltmeter you could check for voltage at the trailer plug, just turn on the signals, and measure , if it has intermittant voltage that coincides with the flasher, it works,, put the ground , or black wire to the exposed pin on the trailer plug and than use the red to see if there is voltage in the other pins,,

 

[fordnut71]

ya it does sound like a bad ground. just check the plug on the truck to make sure that its working right 1st. then hook up the trailer plug an check it from there.also check that the lights are grounded to metal, sometimes it will corrode behind the light an bracket causing a bad ground.
also im assuming this is a flat4 plug being used.



an yes i know what happenes when you assume (ass u me) something.

 

[solitary ogre]

on my bronco ll, explorer and F350 i ran a #12 wire right off the ground terminal of the battery to the 7 prong plug at the back bumper. i then ran seperate #12 wires from the 7 prong plug on the trailer, 1 to the electric brakes and break-away battery. the 2nd to the back for the lights. on the plug terminals i use that gray grease you use on aluminum wires to eliminate oxidation on the brass (i only clean the terminals every 4 or 5 years now).
if you have electric brakes make sure you hook up the saftey wire from the break-away switch and that the battery will hold a charge without being connected to the vehcile or you could get a nice coupon costing you $$$.
enjoy your drive and good luck with your new jobs.

keep smilin............t

 

[mikeinri]

Several things to try for troubleshooting.

The first thing to try is another vehicle. That is definitely the easiest way to determine if it's the truck or the trailer.

As mentioned, be sure the connectors on the male and female plugs are clean. You're using the factory truck connectors, and you've tested the truck plug with a meter, so I think it's safe to say the problem's probably in the trailer.

Remove the ground wire on the trailer (typically a white wire, bolted to the trailer frame near the trailer tongue). Use a wire brush, etc. to clean the area on the frame where the wire makes contact. Clean the wire too, and reinstall, making sure you have a sufficient contact area when the bolt is tightened.

If those two things don't fix it, and the trailer wires appear to be in good shape, personally (espcially with the damage you've described), I'd move to replace the light fixtures. These are cheap and easy, especially if you only need to do one side.

You could try to disassemble the bad fixture and clean all the metal parts with a wire brush, but quite often, these things are cheaply made with plastic parts and don't lend themselves to coming apart and going back together nicely.

Years ago, I had an old boat trailer with all sorts of light problems (mainly because the fixtures were corroded as you've described), I replaced everything on the trailer (wires and fixtures), and all the problems went away. I think it cost me $25 and an hour of time.

Beyond that...

You can do a simple test with a voltmeter (but I don't think it's worth your time in this case). Hook up the voltmeter to check for resistance. Connect one lead to the ground, the other to each of the inputs on the trailer plug. You want to see low resistance.

You could also run a wire from the leads in each light fixture back to the plug at the front of the trailer to check for resistance, but at that point, you're chasing your tail, and you're probably better off replacing the light(s).

Hope this helps.

Mike

 

[Kamel]

Hey guys, I want to say first and foremost thanks so much for your excellent prompt responses. It helps a lot as I had no idea where to even start.

I went to the local autozone and gave the issue one last attempt before just going without running lights. Admittedly, I did not check the forum again before leaving so I didn't get to try the tips posted yet. I did make it ticket free here, but I have to make the return trip, so I am definitely going to give this issue another shot.

I will try and mess with it tomorrow, just posting a quick reply letting you know how much I appreciate your efforts to help. This forum has truly been a light in a very dark world lately so to speak and for this I am forever grateful but really can offer nothing in return .

 

[Kamel]

Just an update, I tried some of the techniques listed here and was unable to determine the problem before returning the trailer. I would have much preferred to return the trailer with the lights fixed as payment for letting be borrow it heh.

Thanks for your great advice though, I will be armed with this knowledge next time a lighting issue comes up.