Truck blinks both trailer lights?

larry from md. · Oct 11, 2011

Need to fix my trailer lights and the problem is in the truck.When you turn on either turn signal both trailer lights blink.Both posts in the factory lights coupler blink juice when either turn signal is on.The truck lights work ok.Its a chevy 2003 1/2 ton.

AZJIM · Oct 11, 2011

It sounds like a bad ground, it's using the opposite lite to make the ground.
Way back when i was young, i worked most of the nite trying to figure it out, an old man next door came over with a piece of 1/8in welding made a temp. grd.
so i learned something that nite. jim

twopop · Oct 11, 2011

check your four way switch.[emergency flasher]

Allan in NE · Oct 11, 2011

Do they alternate or do they both come on at the same?

Allan

MarkB_MI · Oct 11, 2011

The trailer doesn't have a good ground connection to your tow vehicle. The turn signal is getting its ground through the lights on the other side.

In other words, if you turn on the RH blinker:
Current goes from the tow vehicle to the RH blinker
RH blinker to trailer ground
Trailer ground to LH signal and tail lights on the trailer
LH signal and tail lights back to vehicle signal and tail lights.

You don't notice the vehicle lights flashing because the current isn't enough to illuminate all of them.

Anonymous-0 · Oct 11, 2011

Zackerly, bad ground on trailer. Check all grounds on lights, too.

Oldmax · Oct 11, 2011

Bad ground! check ground to truck most of the time is bad. Took me about 2 days to figure that one out on utlity trailer back in my younger days.

Chip812 · Oct 11, 2011

I'm going to disagree with the consensus and go with a short instead of a bad ground... Look at the connection of the pig-tail to the trailer wiring harness...
8)

gene bender · Oct 11, 2011

If there is a short the fuse will blow thats why fuses are used.

Michael Soldan · Oct 11, 2011

90% of all trailer light problems are because of a bad ground. As others have said,it is feeding back through causing both lights to blink when in fact there is no ground for it to follow back to the truck..run a jumper cable from the truck to a bare metal spot on the trailer and they will work!..and perfectly!

John_PA · Oct 11, 2011

...rocking the boat...

need more info.

what type of plug socket on the truck? 6 pin/7 pin rv/4 pin?

what type of plug on the trailer 6 pin/7 pin rv/4 pin? Is there an adaptor?

Was it factory wired?

Is it aftermarket?

were the connections made randomly by splicing into a harness, or is it a "plug-n-play" connection?

Can't diagnose from the description, especially not knowing what type of connections we are talking about.

davediehl@hotmail.com · Oct 11, 2011

Bad ground, IF you have your parking lights illuminated AND you turn on the right signal, the patking light on the left side will go dim or out when the right signal is on. IF this happens, its definitly a poor ground. If the parking lights remain fully illuminated, then you have 2 wires conducting energy across the yellow and green wires. Brown is running lights and white wire is ground if its wired to standard.

John_PA · Oct 11, 2011

(quoted from post at 19:42:43 10/11/11) Both posts in the factory lights coupler blink juice when either turn signal is on.The truck lights work ok.

The truck plug has 12 v to yellow and green when you turn on either turn signal, and you say it is a factory coupler? everyone keeps saying ground, ground ground ground... how can anyone say ground if you are testing it without the trailer plugged in? you didn't mention if you tested it that way, or if it even is a 4 wire plug.

regardless, most set ups stick with green on right, yellow on left, [b:9d577a3bca][i:9d577a3bca][u:9d577a3bca]BUT[/u:9d577a3bca][/i:9d577a3bca][/b:9d577a3bca] when you hit the brakes, it puts 12 volts to [b:9d577a3bca][i:9d577a3bca][u:9d577a3bca]BOTH[/u:9d577a3bca][/i:9d577a3bca][/b:9d577a3bca] wires to turn them both on. So, if someone wired the yellow and green together with the truck brake lights, everytime to give input to yellow, green will also light up and vice versa...

so...

just something to think about...

would need a lot more information before anyone could just toss a diagnosis out there.

oj · Oct 11, 2011

Just a thought, but we had a problem with the turn signal switch it's self before now... made the high level (3rd) brake light work, but the rear brake lights not work but turn signals worked... explain that one too me... might be worth a look, those multi function switches do wierd things everyso often

tomNE · Oct 11, 2011

ground!

Chip812 · Oct 11, 2011

(quoted from post at 19:46:30 10/11/11) If there is a short the fuse will blow thats why fuses are used.

Not a short to ground, a short to the other light...
8)

old · Oct 11, 2011

Very common problem and 99% of the time it is because you have lost a ground some place. Could be at the trailer or the truck or both. I deal with it about once a year every year with about 5 trailers both mine and my partners and 99% of the time it is a simple ground problem and it will drive you nuts

Brian Jasper co. Ia · Oct 11, 2011

Check the ground circuit for the trailer. When the ground is open, the lights on the trailer then take the next path of least resistance. That path can be through the running lights or brake lights on the truck.

larry from md. · Oct 11, 2011

It is the plug that comes on all the gm trucks now i think 7 posts and no brakes on trailer. With the trailer unhooked both of the posts on the truck blink 12 volts when either turn signal is turned on.However the truck lights work fine.The trailer works on my sisters truck fine.Brake lights work fine.Theres no short across the 2 wires where i can see them ,they go right into a factory wireing harness.The wires do not go to the wires that feed the rear lights on the truck like my older trucks did.

John_PA · Oct 11, 2011

That's what I thought you were saying...

In that case, it is obviously not a bad ground.

This is off the wall, I know, but you might want to check the bulbs for your brake lights. the brake lights on the truck might have a bulb installed wrong, or the wrong bulb, which is linking the brakes to the turn signals on the wiring harness. That is where I would start. The other thing I would do is to trace the rear harness and see if it has had any repairs done to it, anything that looks retaped, etc...

Was the truck ever in a rear collision, or did it ever have a different bed installed on it?

Are the pins the flat type, with a round center pin? Maybe there is something going on inside the housing, like a loose wire on a screw terminal. I will have to look, but I think ground is the bottom left, bottom right is trailer brakes, right is right green, left is left yellow, then can't remember if running lights is right or left on top. Regardless, you will have to trace the wiring back from the plug to the lights. something is obviously not right.

not a bad ground. LOL

550Doug · Oct 12, 2011

Just had identical problem. Solved it yesterday.
I put a new 7-pin trailer connector on my Buick last week and the trailer lights would both blink at the same time, no matter which signal light was on. The cars lights blinked properly but trailer lights didn't. Trailer works fine on the truck.
So yesterday I took a closer look at the connector and discovered a dangling ground wire out of the new connector that I had missed hooking up.
Works fine now!!

massey333 · Oct 12, 2011

Simple test,unhook your brake controler and try it.Some controlers would short out and cause that problem.You don't have any thing to lose.

larry from md. · Oct 12, 2011

No controler no brakes on trailer.

Indiana Ken · Oct 12, 2011

You say, .... "both posts in the factory coupler blink juice when either turn signal is on..."

Well the good news is; in that case, your trailer is working as it should.

It would seem the problem is within the truck plug itself and the two turn signal wires are common, i.e. connected together. The puzzling part is why it does not back feed into the truck and cause both turn signals to light. Perhaps the truck plug has diodes to prevent backfeeding - to test this check the two turn signal wires before they enter the plug.

The reason I suggest such a goody idea is that I used diodes to convert my 1993 Ford which used separate filments for the brake and turn signals to work with my old trailer which shared brake and turn signal filments.

Owen Aaland · Oct 13, 2011

The medium duty trucks use relays to feed the trailer lights. If the light duty trucks used the same type of system that would be why it doesn't back feed into the other lights on the truck.

Truck blinks both trailer lights?

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