Here is a real stumper for some of you wiring geniuses here. Currently when I hook up my 7 pin to my tow vehicle I'm only getting left turn signal, left brake light, left hazards, etc.

Now when I hook up a tester to my tow vehicle every light shows up properly when braking, turn signal, etc

Here is where I'm really confused. So just to test, I tried another vehicle and hooked it up to my hi lo and everything was working as it should. How can this be? Especially if the tester on my own tow vehicle shows everything should be working? I'm so confused. Any thoughts on this would be appreciated

Possibly, some damage/corrosion or whatever to your tow vehicle's pins on the non-functioning side? Or, some problem in the wiring to that side probably near the plug. It's possible your tester is making good contact with the malfunctioning circuits but the trailer plug can't do the same. And, since the trailer lights work with a different tow vehicle, you know the trailer circuits are fine.

My first thought, as I started reading your post was that the grounding was defective on the malfunctioning side, but I think that would be in your trailer circuits. When you said it worked with a different tow vehicle, that made things more complicated.

- Jack

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Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH, Progressive Dynamics Converter, Fan-Tastic Fan, LiFePO4 battery 12V DC Electrical System, SoftStartRV mounted on A/C
2024 F150 Platinum FX4 3.5L PowerBoost SCrew

Maybe the contacts on the TV are worn and not making good contact with the trailer. Testing with a meter may show power but not good contact. Can you hook up to a friend's trailer?

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My Great Wife Joyce
2018 Jayco Jay Flight 21QB
Formerly owned 1705T and 2310H
2012 F150 4X4 SuperCrew EcoBoost w/Leer Cap
Reese WD Strait-Line Hitch
Amateur Radio K3EXU

Thanks for all the helpful responses so far. As for my TV it is a 2004 Ford explorer towing a 1981 Hi Lo. I have never successfully had the wiring working. The receiver is brand new, so I'm not thinking it's a connection problem.

I simply cut the wires on my 4 pin and put them into a 7 pin receiver. But again, how can everything look ok on the tester but not on my camper?

I suspect you have a faulty connection in your 7-pin receptacle. The tester needs very little current to function, if it's anything like mine. An actual light circuit, on the other hand, needs more than an Amp, maybe as much as 10-15 Amps. If the connection can't transport that current, the light won't light.

My tester draws so little current that the onboard brake controller in my truck doesn't "see it" and reports that the trailer is disconnected.

- Jack

I had very weird symptoms on my 2004 GMC Sierra and it turned out to be the flasher module in the truck. The lights were almost unexplainable. $25 module and everything perfect. It did seem to vary the behavior based on the load on the wiring, the jet ski trailer with just 2 LED lights was fine but the camper was all messed up.

I went to pick up a U-Haul trailer for a friend of mine and the right side of the trailer lights wouldn't work. My truck had a blown fuse for the r/s of the trailer.

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"Ruben"
2404T, 2003 2500HD 4X4 GMC Sierra SLT Crew Cab w/Astro camper shell and 2003 Chevy Tahoe LE 4X2
Far West Texas

Thanks again for all the help my fellow hi lo'ers. I tried everything. So I gave in, called e-trailer and ordered everything I need. Should be here next week. They have instructions and videos on how to install. I'll give an update on the install next week.