Did Faulty Trailer Wiring Burn This FJ Cruiser To The Ground-- Twice?

Patrick of Dallas, TX had been towing with his 2012 Toyota FJ Cruiser for months, until a run with a rental trailer turned his truck into a ball of fire. Can we figure out what happened?

Patrick took to FJCruiserForums to share his plight, video and a few pictures as well as a detailed account of the incident and some advice to other owners hoping to hook up trailer wiring to an FJ. But most importantly, he described his hypothesis as to why his off-roader was consumed by fire and reduced to rubble.

Yes, it's in portrait mode. Let's move on.

Basically, he'd had his trailer wiring installed by a professional and the truck had been towing his camper for a couple incident-free years. The camper used a seven-pin connector. On the day he pulled a rented utility trailer from Home Depot, requiring him to hook up his four-pin connector for the first time, he was flagged down by another motorist who was alarmed to observe the FJ was billowing smoke.

Patrick had been towing the trailer for inside twenty minutes.

He called 911 and the fire department was dispatched quickly, undoubtedly using some of the car-fire extinguishing techniques we've discussed before. The fire was put out, the Dallas Fire Department satisfied with the scene's safety, and a tow truck was sent to pick up the scorched FJ.

But by the time the wrecker could arrive on scene, about an hour later, smoke appeared again... and soon 4X4 was completely engulfed in flames and obliterated. DFD returned to finish the job, assuredly with a little embarrassment.

Patrick has some ideas about what might have happened, but he admits his assessment is speculative and declines to specify the wiring installer "in case he's wrong," which is a classy move. What the hell do you think happened here?

In his own words:

"I had been trailering my pop-up camper for almost two years and using the 7 pin round connector. I had never had the opportunity to use the 4 pin flat connectoruntil that fateful day when I rented a small 8 foot utility trailer from Home Depot, which had no electric brakes. So this would be the first time I used the flat connector. So for a long time I thought the flat connectorand wiring MUST be different, and whatever was involved in the difference is what caused the fire. But, not so fast! Although I can't prove it, I've also always felt the utility trailer must have had a ground fault, and since that was the first time I had pulled anything but the camper then it's possible that the same result could have happened if the camper had ever had a ground fault.

When I rented the small utility trailer I checked the brake lights at the time, they were working. It's still possible to have a ground fault and the trailer lights still work. So my assumption is the trailer had a ground fault of some type. Then my next assumption is that my trailer harness wiringwas not fused correctly, or not at all, or there was a perfect storm of some other wiring problem."

...

"This is the product that my professional installer had me purchase for them to install:

http://www.draw-tite.com/content/dow......

I didn't realize it at the time, but this means they were pushing me into the "easier" way – that is, bridge into the tail light harnesseswith adapters, instead of using the pre-wired cable furnished with the vehicle, which would have required more work with switches and fuses.

Anyway, notice on the installation there is a red lead that stubs out of the harness box – this lead is supposed to connect to the battery via the 10 amp yellow fuse lead. So now I'm going to make the leap assumption that possibly (again, not proven) that the fuse was not used, or was incorrectly oversized. Possibly the "easy" route was to wire the red wire directly to the black wire that routes to the battery.

So, if that's what happened, and in the case of the trailer ground fault, then an unlimited amount of current (something like 600 amps or so) could have been on that red wire, which caught on fire, and caught the other leads on fire."

Hat tip to Kamil Kaluski!