I had a 16ga wire burn through in my 4Runner. There was about 8 inches of insulation burned off and several other wires had insulation burned away or melted together.

At 12v, would anyone have an idea of about how much current was pulled on that wire? It looks like this afterward - the wire in question is the big one in the middle with all the burned up strands exposed.


I'm trying to figure out why the fuse didn't prevent this damage - I thought the truck was going to burst into flames there was so much burning wire smell. I haven't checked everything yet, but I've read this circuit is covered with a 20A fuse. It seems to me that's a lot more than 20 amps of damage.

 

Hard to tell how much current was flowing -- a lot depends on how long it was flowing since, to quite a degree, damage in an insulated wire is cumulative and so the damaged caused by a moderate overload over many seconds is similar to the damage caused by a much larger overload over a few seconds.

While it's possible that the fuse failed to open as intended, it's probably more likely that a short occurred between something else and this wire downstream of the fuse.

 

16ga wire is nominally rated for 13A max, so a 20A fuse is too large.
Use a 10A or 15A fuse instead.
Perhaps a fuse larger than 20A was incorrectly installed.

 

My same thoughts as Cruts.

 

I started smelling the burning insulation about 10 minutes before I got home and shut the truck off. This damage is on the back harness that goes into the hatch - wiper, defroster, tag light, center brake light. I haven't found a service schematic yet, but I've read that that section is on it's own 20A fuse.
I seems to me that regardless of the short starting somewhere else, if there is a fuse between this feed wire and the battery it should have popped and kept it from burning up this stuff.

Would you think a 20A fuse is too large for this? I found a chart today which says 16ga wire can handle a max of 22 amps. Also, as I said, wire looks like it's had a lot more than 20A through it.

 

16ga wire is nominally rated for 13A max, so a 20A fuse is too large.
Use a 10A or 15A fuse instead.
Perhaps a fuse larger than 20A was incorrectly installed.

I haven't yet confirmed what's on it. A thread on a 4Runner forum said it's 20A. I'm looking for the FSM to see exactly what is supposed there and then I'll compare with what's in the fuse block.

 

Tacomaworld(dot)com. I've gone there with issues with my Toyota Tacoma. I'm guessing they have people who know about 4runners as well.

Otherwise go to your Toyota dealer and ask for a wiring diagram of that circuit. Then you'll know what size fuse is supposed to protect that circuit.

Back in the late 70's I was working for MacDonald Douglas. Working on a DC-9 a wire loom began to smoke. We were testing the lightning illumination circuit which consisted of a high voltage source for fluorescent lighting. Apparently a plug was put in on a harness that managed to get a 308V circuit shorted to a ground circuit. The 308 came from a ballast and though the ballast was fused at its input it wasn't fused on its output. So the resulting short burned several wires in the cockpit and caused 9 months of delay pulling out all the wiring that was affected and replacing it with all new and all the testing had to be redone.