Been looking around for an answer but haven't found anything too close to my exact issue.

So the issue.

~Turn key in ignition to first position, everything lights up fine, battery turns on.
~Turn key to engage starter, I get one click from the engine (which sounds like the starter solenoid to me) and everything in the vehicle dies. All power, including door lights etc.

~I can jump the vehicle just fine. Starter sounds fine etc.

~I have measured the battery with a multi meter. With it turned off it sits at 12.5, when the vehicle is on (after jumping it) it climbs to about 14.5. i.e. the alternator appears to be charging the battery just fine.

Here's the odd part. I jumped it yesterday and drove around for about 15 miles. Came back, measured the battery (14.5) shut it off and it slowly dropped to its 12.5. Watching the mulit meter, attempted to start, got the one click and everything died, battery did not change voltage at all. I sat around for a bit thinking, then I popped out all the relays under the hood (starter, headlights etc) trying to see if another would work in place of the starter relay. Found that none were the same, popped them all back in. Jumped back into the driver seat and noticed the door lights came back on, decided to give it a try for the hell of it and the freaking thing started up just fine. Shut it off, started it again just fine. Took out the starter relay, obvisouly didn't even kick over to the starter but also nothing died, all the lights stayed on. Popped it back in and it started just fine again. Called it day, tried starting it this afternoon and just got the click and everything died.

For the record the battery is only about 4 months old and from what I can tell holds a charge just fine.

My first guess is it's gotta be something with the starter contacts. I just don't understand why everything in the vehicle dies and its only sometimes.

Had something similar to this happen on my first gen 4runner. Turned out to be a fusible link (battery to fusebox) and a badly internally corroded battery cable (battery to starter). Hope that helps.

1986 Toyota Supra
1987 Toyota 4Runner
1996 Toyota 4Runner

Had this happen two weeks ago....ended up being a broken battery terminal. It was split on the bottom, you couldn't really see it. I tried to tighten the cable and could tell something wasn't right with it.

To be clear, the battery cable termnal was broken,, not the battery post.

After I got back from work today I tested the battery, still good. Tried it, got the one click and all dead again. So then I tried testing the ground, put my multi meter on the red and black to the body. Was only reading about 4 volts. wiggled the negative post and it shot up to 12.5. Vehicle starts just fine.

On my way to the store to get new posts. Hopefully its just the posts and not one of the actual wires...either way it appears to "just" be a short.

Good to see you track it down, thanks for letting us know what it was.

Classic symptoms of a bad battery connection. Even if they look clean take them off and hit them with a wire brush. If it's not the terminals start looking at your major ground connections between the battery and the chassis. There are very few connections that will cause the whole truck to go dead and they're all close to the battery.

Edit: Posted that before I read that you figured it out.