Weak left turn signal shown on 7 pin tester

[Jdubb1234]

Does this just pull straight out? I located it under the bed but was not able to remove it.

 

[Jdubb1234]

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.

Its a 2018. I did look under the fuse box but could not see the problem. I will take a better look.

 

[Jdubb1234]

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

I hope this is all it was.

 

[Jdubb1234]

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.

Both fuses for the left and right trailer blinkers are in tact. In fact, I swapped the 2 20a trailer blinker fuses while trouble shooting the issue.

I am not familiar with relays.

 

[Marshall]

Follow the wire under the frame all the way with a good light and look for a bad spot that is pinched, loom tore, about front of the box seems like a bad spot on some.
First make sure you have proper power at the top end fuse box.
I presume they work fine on the truck
Trailer wiring I have worked on is usually rusted out crap on older trailers.
A good ground is just as important as the 12v feed.

I just looked at the pictures you posted , some butcher has been in there screwing things up looks like , you have a bunch of chopped wires if I see what I think I see.

 

[NETim]

Both fuses for the left and right trailer blinkers are in tact. In fact, I swapped the 2 20a trailer blinker fuses while trouble shooting the issue.

I am not familiar with relays.

I have to think that since the truck isn't popping the fuse for the trailer wiring (at this point) yet you have no juice at the connector (both the 7 pin and 4 pin trailer connectors) it's now a matter of finding out where the break in the circuit is.

Is 12V getting out of the TIPM to the trailer connector? Is 12V at the back of the trailer connector? Wiring broken somewhere in between?

Could be a ground issue here too. Are you using the trailer connector ground pin as your ground when you test the circuit?

 

[crash68]

Does this just pull straight out? I located it under the bed but was not able to remove it.

The factory usually has retention clip(s) on plugs If there isn't you possibly found the problem as something is corroded holding it together.

 

[Tominator223]

Bad connector. Or ground

 

[Jdubb1234]

I have to think that since the truck isn't popping the fuse for the trailer wiring (at this point) yet you have no juice at the connector (both the 7 pin and 4 pin trailer connectors) it's now a matter of finding out where the break in the circuit is.

Is 12V getting out of the TIPM to the trailer connector? Is 12V at the back of the trailer connector? Wiring broken somewhere in between?

Could be a ground issue here too. Are you using the trailer connector ground pin as your ground when you test the circuit?

There’s 12v on the bumper but not the bed. I was not using the ground pin as the ground but I’ll try that.

 

[NETim]

Oh man! Sorry! I missed the fact that there are two connectors on your truck.

I need to get a better understanding of the symptoms. You say you have 12v on the bumper connector but not the bed connector. But this is NOT the 12v that activates the LH turn lamp? Neither connector will flash the LH turn signal?

 

[Jdubb1234]

Oh man! Sorry! I missed the fact that there are two connectors on your truck.

I need to get a better understanding of the symptoms. You say you have 12v on the bumper connector but not the bed connector. But this is NOT the 12v that activates the LH turn lamp? Neither connector will flash the LH turn signal?

No worries. I appreciate the help. I think I just need to spend some time tracing the wire from the fuse to the outlet and check each wire harness.

For some reason the 12v is always on the bumper outlet. Never on the bed outlet. On both outlets the left blinker/ brake does not work. Fuse is good.

I’m not sure what the previous owner was trying to do with the jumper tester or the cut wires under the fuse box.

 

[2003F350]

No worries. I appreciate the help. I think I just need to spend some time tracing the wire from the fuse to the outlet and check each wire harness.

For some reason the 12v is always on the bumper outlet. Never on the bed outlet. On both outlets the left blinker/ brake does not work. Fuse is good.

I’m not sure what the previous owner was trying to do with the jumper tester or the cut wires under the fuse box.

The cut wires under the fuse box tell me that they discovered a problem and tried to hack solve it, instead of fixing it correctly, like what happened on that old truck I had.