Weak left turn signal shown on 7 pin tester

[Jdubb1234]

Hello. I recently bought a 2500 to tow a 5th wheel. I’ve been checking the lights, trying to figure out why the left blinker/brake light does not work on the rv. Fuse in the truck is good. I just got a 7 pin tester. The right blinker has a nice bright signal. Left blinker has nothing. There are two outlets on the truck, same result on both. I did try cleaning them. Any ideas on what could cause this?

 

[NETim]

I'd still be suspicious of the trailer plug connector on the truck. What did you clean it with?

 

[Jdubb1234]

I have to get some legit cleaner tomorrow. It was a metal protectant lubricant. But same result on both outlets on the truck makes me think it’s something else.

Just checked it again and the left blinker does not light up at all on the tester. I saw a faint light and assumed it was the left light but it was actually the brake the weakly blinks. Does the same when the right blinker is flashing. I’ll edit the original post.

 

[NETim]

Fuse 53 in the TIPM is for the trailer tow LH blinker/brake light according to the info I've found. I'd start there. Reseat the fuse. Check voltage on both sides etc etc.

 

[GTyankee]

May i suggest that you use a bit of spray from either a
CRC QD Lectra Clean
or
LPS contact cleaner
spray it in the 7 way female & male connectors
Light bulb sockets, ect.

 

[crash68]

Pull the plug off the back of the 7 pin on the bumper, you'll find the jumper plug that feeds the bed plug there also. Check the voltage there.
If you check voltage at the fuses, you need to have that lighting circuit energized.

 

[BossHogg]

Another test you can run is; the brake lights/turn signal power to the trailer is managed by a relay. A simple test to remove the relay as a suspect is simply to swap the left turn and righ turn trailer circuit relays.

 

[NETim]

Another thought, does the LH pin on the flat 4 pin connector work? I would assume that the pins between the two connectors are wired in parallel.

 

[zrock]

i would start with checking the connector under the truck, it may be greened out and not getting a connection. Also check your ground for that side if they are running on 2 separate circuits for left and right.

 

[Tulecreeper]

I have to get some legit cleaner tomorrow. It was a metal protectant lubricant. But same result on both outlets on the truck makes me think it’s something else.

Just checked it again and the left blinker does not light up at all on the tester. I saw a faint light and assumed it was the left light but it was actually the brake the weakly blinks. Does the same when the right blinker is flashing. I’ll edit the original post.

Your brake lights are your turn signals. It's just that only one side is activated at a time with the signal lever. So, if you're getting a weak blink from the left light when you have your left signal on it tells me you have a bad connection/ground. Probably a ground as that is the most common problem.

When you say it does the same thing when you activate the right signal, do you mean that the left side blinks weakly then, also?

 

[Jdubb1234]

when I run the right blinker…I get a strong signal for the blinker weak for the brakes. Left blinkers is only the weak brake signal.

 

[Jdubb1234]

 

[Jdubb1234]

Found these items in and around the fuse box. Definite signal at the fuse. I am trying to access the bed plugs to check the signal there. The wire coming out of fuse 53 looks ok.

 

[Tulecreeper]

That 20A fuse you have hooked to your tester is burned out. And why has that harness with the multiple colored wires been cut?

 

[crash68]

I am trying to access the bed plugs to check the signal there.

The bed plug wiring comes from the back of the bumper 7-pin, pull the plug out of the back of the 7-pin

 

[Jdubb1234]

it’s a new to me truck…the “tester” was in the fuse box. The cut wires…I have no idea. Everything seems to work, could it be a plow setup from a previous owner? I didn’t think to remove the fuse box and look under it at the used dealer. ‍

 

[NETim]

Is the original 20A fuse for the trailer blinker blown? Is appears to me that someone at some point tried to troubleshoot this issue and made up that test jumper. The 20A fuse in the test jumper appears to be blown.

If both fuses are blown (open according to an ohmmeter) then you've probably got a pinched wire somewhere making contact with the chassis which takes out the fuse.

Not sure if this would be a valid test but maybe remove the LH trailer relay, replace the fuse and see if it holds. That would kinda cut the system in half. I suspect the fuse won't pop with the relay out, which points you downstream to wiring back to the trailer connector.

If it is indeed a wiring issue, it's visual inspection time on the wire harness.

 

[crash68]

The cut wires…I have no idea. Everything seems to work, could it be a plow setup from a previous owner?

Since this is a HD trucks, those wires could be the Aux Switches wiring as truck would be prepped for them as it can be added after the factory build. The Aux Switches is an option that has switches in the center stack you can use to run aftermarket accessories

 

[2003F350]

What year is this truck? I ask because I had an '03 that had this issue - brakes and one turn signal worked fine, one turn signal was inop on the trailer, though everything worked fine on the truck. Chased the problem for a long time, the dealer finally put a Ford computer in it that basically bypassed the existing turn signal circuit and combined it with the brake circuit. Fixed all the issues.

About a year later I was working on something and had to pull the fuse box under the hood...and found an additional relay/fuse combo, and as I looked into it I learned that some wires were chaffed through. So the previous owner had the issue pop up, and had the relay/fuse combo installed to fix the truck side but didn't worry about the trailer plug. I never bothered to mess with putting things back to factory because the Ford computer fixed all the issues, but I guess I'm pointing out that you could be fighting something similar under your fuse box.

 

[Jdubb1234]

The bed plug wiring comes from the back of the bumper 7-pin, pull the plug out of the back of the 7-pin