Tow behind lift tail light issues

[JJZ 109]

I've got a 2005 JLG T-350 towable lift. Truck is a 2016 F250 with all factory trailer light plugs. Everything is good on the truck end. When I turn on either signal light, both signal lights and the 3rd brake light on the lift flash.
The grounds on each tail light of the lift had seen better days, so they were repaired. The issue persists. A lot of the wiring in the lift is hidden. My first thought is to just buy a roll of trailer wire and redo it front to back and locate it in more accessible locations. Unless of course anyone has some some insight on a less time consuming fix. Thank you.

 

[TMGT]

If it has a 3rd brake light and doesn't have a logic box to seperate the tail light circuits then it's back feeding through the brake light circuit. It would also have a logic box that's bad or bad wiring.

 

[GLyford]

Could this be a symptom of the ford/dodge wiring cross? I forget which two wires they do different in a 7-wire outlet, but I seem to recall one of them was the brakes.

In situations where trailer wiring is just different and not malfunctioning, I have heard of people getting a short extension and using that to swap wires so that neither trailer nor tow vehicle have to be changed. Also makes it easier to replace the end furthest out, which seems to get damaged the most often (I do that with mine, but it is only 4-wire, so no brake wires to mess up...but I'm sure I'll end up with a trailer with brakes eventually). But it sort of sounds like your problem may be more involved than that.

For ground issues, I connect the ground to the trailer at each fixture, but I also run a ground wire along the harness and have them all tied direct all the way back to the plug...have had a lot fewer issues that way.

Good luck.

 

[zzvyb6]

I also have a T-350 and all lighting works as intended from my standard chevy tahoe trailer plug.

When I have trailer lighting issues, (actually its from my friends using my trailers), I take a 12v battery and some alligator clips and test each trailer plug connector separately. A "no Ground" fault is almost always the cause because very few hook-ups connect the white ground connector and depend on the trailer ball for the ground.

My T-350 has a surge brake type tongue, so the ball ground is probably worthless. JLG website has a free manual with wiring diagrams. Make sure you look at the USA one because there are special ones for a lot of different countries.

Somebody here could make up a switch box that you can fire up to test trailer lights so you can do it by yourself and separate vehicle problems from trailer problems.

 

[JJZ 109]

Could this be a symptom of the ford/dodge wiring cross? I forget which two wires they do different in a 7-wire outlet, but I seem to recall one of them was the brakes.

In situations where trailer wiring is just different and not malfunctioning, I have heard of people getting a short extension and using that to swap wires so that neither trailer nor tow vehicle have to be changed. Also makes it easier to replace the end furthest out, which seems to get damaged the most often (I do that with mine, but it is only 4-wire, so no brake wires to mess up...but I'm sure I'll end up with a trailer with brakes eventually). But it sort of sounds like your problem may be more involved than that.

For ground issues, I connect the ground to the trailer at each fixture, but I also run a ground wire along the harness and have them all tied direct all the way back to the plug...have had a lot fewer issues that way.

Good luck.

Not likely the Ford/Dodge thing. It's been working and I've only ever had Ford trucks. Also, there are no electric brakes on this unit so it's only a 4 pin. I should have mentioned that.

I
Somebody here could make up a switch box that you can fire up to test trailer lights so you can do it by yourself and separate vehicle problems from trailer problems.

I have both 4 & 7 pin testers for the vehicle end. All appears good there. The truck is only about 9 months old so I hope all is good on that end. It's Ford's problem to fix if that were the case.
Each of the tail lights has the white wire grounded to the cage that surrounds them. I took them apart, cleaned and used dielectric grease to be sure.

 

[JJZ 109]

If it has a 3rd brake light and doesn't have a logic box to seperate the tail light circuits then it's back feeding through the brake light circuit. It would also have a logic box that's bad or bad wiring.

That is making sense to me. In the past, the 3rd brake light would often flash with whichever signal light was being used. Now all three rear lights flash regardless of which side is turned on.

 

[coldsteelva]

There is a short between L and R tail somewhere along the wires. The 3rd brake light is likely powered off either the L or R (so ignore that), but when you apply L or R signal power, the power is transmitting to both L and R (CHMSL included). If you can't access the harness, re-stringing the wire may be easier

 

[JJZ 109]

There is a short between L and R tail somewhere along the wires. The 3rd brake light is likely powered off either the L or R (so ignore that), but when you apply L or R signal power, the power is transmitting to both L and R (CHMSL included). If you can't access the harness, re-stringing the wire may be easier

Seems likely. I eliminated the 3rd brake light from the equation for test purposes. The lights are 4 inch round Harbor Freight with 10 LEDs in each. Now, when I turn on the left turn signal both lights still flash brightly. When I turn on the right turn signal, the right flashes as it should and the left flashes dimly with only the 4 center LEDs. The other 6 LEDs on the left side do nothing.

 

[TMGT]

How did you eliminate the 3rd brake light? I've seem some systems integrate the logic box into the light house, somtimes it's in a junction box and somtimes it just somewhere inline with the harness as they aren't very large.

Have you checked everywhere that the wireing is accessible such as the connector or junction box etc?

I've had similar sounding problems with bad logic boxes when they are on the truck end(any truck that uses seprate brake and turn signals needs one to operate trailer lights), your Ford doesn't use one but it is required on the trailer to have a 3rd brake light without back feeding the taillights.

Is this a new piece of equipment or did the problem just start?

 

[JJZ 109]

How did you eliminate the 3rd brake light? I've seem some systems integrate the logic box into the light house, somtimes it's in a junction box and somtimes it just somewhere inline with the harness as they aren't very large.

Have you checked everywhere that the wireing is accessible such as the connector or junction box etc?

I've had similar sounding problems with bad logic boxes when they are on the truck end(any truck that uses seprate brake and turn signals needs one to operate trailer lights), your Ford doesn't use one but it is required on the trailer to have a 3rd brake light without back feeding the taillights.

Is this a new piece of equipment or did the problem just start?

The box is directly below the light itself. Wires from the unit into the box on one end, out to the light on the other and the light was the end of the line. I just undid the splices.
The lift is a 2005. Always stored outside. Pretty heavy duty shrink wrap around the logic box. I cut it away to view all the connections. Can't really tell if the box is faulty from a visual inspection.