Trailer Electrical Conundrum

dbart

dbart

Well-known member
Joined
May 5, 2021
Messages
3,696
Reaction score
4,696
I have a 2013 Wells Cargo 16 ft box trailer and 2024 GMC 2500 AT4 6.6 Gas to tow it. Lately I have been getting the Trailer disconnected, left blinker not working, trailer brakes not working notices on my dash.
I checked the Brake light and actually swapped left to right, and it isn't the light.
I bought a tester to go into my truck to see if it was my truck causing the issue (didn't think so but it was a cheap buy on Amazon for the tester, $7.99)
Plugged it in and it seemed like there was an issue. The 12v Aux was lit up and 3 of the other lights were barely lit and blinking!
Tried my blinkers and nothing worked, tried the hazards, again nothing worked on the tester. Then plugged it in to my brother's truck and all worked just fine on the tester.
Cool, but not cool...it's my truck!
Drop off to the dealer, showed them with my tester what is going on and they looked at it.
They hooked up their tester, showed me video and all was working fine!
They say it is my trailer.
UGH!!! @#%#%^@#%!@
Get it home, and my neighbor Doug comes over, I ask him if I can use his truck to test, No Problem.
Plug in my tester, works just fine, WHAT IN THE ACTUAL EFF???
Then I pull his truck to my trailer and hook up the electrical, just like my truck...same issues, no left blinker, not brake lights, no hazzards!
Ok...it seems it is my trailer, but why the eff won't my tester work on my truck?

So, now what?
I actually ordered a new 7pin plug and wiring harness for my trailer!
Any electrical guru's on here can give me insight to the Voodoo going on here????

Tester I was using:

7Pin/Wiring harness I just ordered:
 
azyxzer said:
Ground problems cause a lot of that stuff. You might want to use a jumper wire to bare metal on the truck and the trailer and see if that clears the problem.
Click to expand...
Not to sound totally dumb, but I'm pretty sure I am at this stuff....How would I do that?
Just so you know, I hate anything to do with electrical or electricity.
 
azyxzer said:
Ground problems cause a lot of that stuff. You might want to use a jumper wire to bare metal on the truck and the trailer and see if that clears the problem.
Click to expand...
This! Do you have a dedicated ground connected in your 7-pin on both the truck side and trailer side? If you are relying on ground via contact with the ball and hitch it can cause lots of these issues.
 
dbart said:
Not to sound totally dumb, but I'm pretty sure I am at this stuff....How would I do that?
Just so you know, I hate anything to do with electrical or electricity.
Click to expand...
You would sand / grind a small spot on your trailer tongue, drill a hole through it and bolt or screw a wire with a circular connector on it to the trailer.
You would then find a chassis ground on the truck. The end of the wire going to the truck should be removable, some folks use an alligator clip or spend the time loosening a bolt that hold the wire to the truck.
You may want to take a gander at the receiver on your truck, your hitch may not be grounded due to rust and dirt.
When your trailer isn’t properly grounded to the tow vehicle you’ll get all the things you’re experiencing.
 
As mentioned, maybe a founding issue.

I would make some small jumper cables and then go pin to pin from your truck to the trailer harness and see what gives fault. You could go a step further and go pin to pin from your truck to the junction box and bypass the harness from the Jbox to the trailer connector. After going through my car the way I did, I've found that going through the whole thing is the best bet. If you go and find one thing. You'll think you fixed it only to get down the road the same issue come up in a different part. Take the time now and go through it.
 
Is it your 7 pin on the bumper or 7 pin in the bed that is giving the problem? Do you only have one or both?

Go to harbor freight, get a meter, and do exactly this:

Probably too much info 2-54 starts the trailering system
 
You'll note in the video above that the 7-pin factory connector has a plug on the back of it, not wired directly... the simplest problem would be a bad connection there. That said, likely chassis ground that he also addresses
 
1776355462016.png
Check the trailer side white wire. It's probably either broken, or heavily corroded, or the bolt holding it down is loose. Either clean/repair it, or just find it in the harness from the plug, cut it off, strip it back and put a new ring terminal on it and bolt it to a good chassis ground (the tongue jack bolts are generally a decent choice, but a lot of trailers have a junction box for the wiring, so inside of there is best). Make sure it's clean metal where the ring terminal is touching and tied off well to prevent chafing/pulling.

And use good connectors. These work surprisingly well:


You can see when it melts to the wire's insulation. Makes for both a good seal, and a strain relief.

Also, I like to use one of these:

1776355861928.png
In the tongue lock to hold the 7-way wire to prevent damage/strain.
 
It's either a Ground issue or you ran something over that compromised the exposed wiring under the trailer. The latter was my recent experience and I just rewired the entire flatbed one day, the right way with everything loomed/taped up and haven't had an issue since. New lights, 7 way plug... no more BS.
 
I'll take a pic of the front of my trailer...There is a ground, but I know it is very old!
I even want to replace the safety chains too.
Thanks, everyone for the insight and info!
I don't really want to think about rewiring my trailer because the upper running lights and rear blinker's...that wiring is all hidden inside the box of the trailer.
Maybe just redo the ground and go from there...then the new harness at the front???
 
Take the 7-pin connector apart and look at all the connections and make sure none of them are loose / flopping around - or corroded...I had something similar happen to me and one of the inner connections was barely on and as a result would contact other pins and cause issues. In short, make sure these are all tight, not corroded:
1776372055861.png
 
Last edited:
wesinls said:
Take the 7-pin connector apart and look at all the connections and make sure none of them are loose / flopping around - or corroded...I had something similar happen to me and one of the inner connections was barely on and as a result would contact other pins and cause issues. In short, make sure these are all tight, not corroded:
View attachment 180313
Click to expand...
I switched to this style on all of mine:


Everything goes to a junction box to keep things dry.
 
You must log in or register to reply here.
Back
Top