Electrical issues in starter/ignition system

Hey all, first post here so forgive me. Lately I've been having a very hard to trace electrical problem in my 2000 Tundra. I've never been a natural at understanding circuitry so I could really use some more expert advice/opinion. Some background, I've had my 2000 for almost two years and haven't had any issues with the starter or ignition system, or with anything electrical. I drive it somewhat regularly as a daily driver and for long trips (I also have a 2nd gen tundra for work and split usage about 50/50). These issues popped up seemingly out of nowhere.

The Problem
About a month and a half ago, I went to go start my truck up after letting it sit for 6 or 7 days, and for the first time since owning it, it had a little hiccup but got itself going. I was in a rush and didn't pay too much mind to it. Drove about 15 minutes to my destination and shut it down. Went to drive home a couple hours later and there was no juice at all. Popped the hood to jump it, only to see this:


(+) C clamp was melted in half, (-) side was fine. I disconnected (-), replaced (+) clamp, then attempted to reconnect both. With (+) back on, I attempted the (-) terminal only to be met with sparks and the starter motor instantly engaging, with no key in the ignition at all. The battery was reading proper voltage.

Got it towed to a shop where they said the starter relay was stuck closed causing continual operation and a massive load therefore melting the lead C clamp. They replaced the relay and said that it was fixed, but it was now misfiring pretty bad and they also had to replace spark plugs and ignition coils. Once I got it back I replaced the battery for good measure and it ran fine, until yesterday...

After enjoying the pleasures of a functioning vehicle for about two weeks, it appears the same or similar issue has reoccured, with an unsolved root cause. This time I started it up but immediately recognized something was off so I pulled the key... only for the truck to keep running indefinitely without it. I had to disconnect the battery to shut it down. Back at square one.

Doing my own research and trouble shooting has led me to these findings:
As in the past, the truck will immediately start and run just by reconnecting the battery without any key. Both the starter circuit and ignition circuit must be active, and when doing this the starter motor does not seem to disengage itself.

My battery and starter relay are both new and seem to be functioning regularly. If I remove the starter relay and attempt to reconnect the battery, there is no action in the starter but there will still be sparks and I can hear some other system activating. So, while I'm sure my starter is probably quite haggard by this point, this should eliminate the possibility of a sticky or stuck starter solenoid bypassing the control circuit, and there must be something else going on.

I also tried disconnecting the wiring harness for the ignition switch to theoretically leave both the starting and ignition control circuits open. This had zero effect on the problem.

Another note: Normally during these tests the truck will start and run immediately once I connect the battery, and will shut off either immediately or shortly after disconnecting the battery. On one of my tests, however, the ignition system shut down upon disconnecting the battery but the starter kept running on its own. I had to pull the starter relay to shut it down in this case. Not sure if this is a hint to something or not.

Minimal accessories present as well. I have a standard aftermarket stereo, and the truck was wired for trailer brakes when I purchased it, but have never used them and haven't touched that wiring at all.

At this point I'm at a loss, and I'm assuming I will just have to send it back to the shop to have them trace wires for shorting/grounding issues. Any input would be incredibly appreciated, thank you!!!

 

FGT Tundras are mostly Denso. I would probably start with a close inspection of the under hood fuse box for burned or melted wires/terminals. If you haven’t replaced the starter, you might want to pull the intake manifold and look at that. Rodents love to chew wires under the intake. If you pull the starter, any parts store can test it. Post more pics in the FGT section. Update your profile with vehicle specifics ie 2000 AC 4WD 4.7. Read @shifty`s megathread.