Battery Short randomly losing ECM power w/ign on

[Gump12b]

(Hopefully) Final Update:

I wanted to make sure I ran at least 1k miles before this post.

Got a new ecm with factory cal last week, installed and the truck ran for a couple hours (got it pretty warm) and tried the tap test which the ecm passed with no issues, I couldn’t drive it in this state without the demandate program. Got it reprogrammed with help from unilevers (thanks again), I let the truck run for a long time (to get it hot) to make sure there were no issues, then I buttoned everything up in the cab (had it all torn apart chasing wires when I assumed that the first replacement was good) and I took the liberty of installing a fan override switch while I was there.

I ran the truck for about 60 miles without any issues and I decided to try it out working Monday.
Ran 400ish miles Monday with no issues, gotta admit I was a little skiddish at first because I hadn’t tried it loaded since it was fixed. I’ve had no issues as of today (Fri) in over 1600 miles with the “new” ecm.

I am positive the truck is indeed fixed for now, that is of course until the inevitable next thing pops up. This was a totally preventable ordeal which kinda sucks. I wasted a lot of time and money chasing solutions for issues I didn’t have (meaning the wiring when it was the ecm). I’m going to chalk it up as an (expensive) learning experience, I came out of this knowing a lot more about the wiring of my truck and I learned firsthand not to blindly trust any electronic part. I would likely have spent about 10k at a shop for them to do what I did after I replaced the ecm the first time and chased wires only to reveal no issue.

As a recap, I am a firm believer that the cause of the original ecm losing power was a battery short. The terminal fuse the feeds the power bus where the 2x 100a bus fuses and power to the ecm feed off was blown. The 100a fuses showed signs of heat but were not blown. The alternator fuse (10a) was blown. The real kicker here is that the power feed for the ecm had a 30a fuse in it :tire: which did not blow. This power feed should only have a 10a fuse in it because the ecm should never need more than 8a to function. Correct me if I’m wrong but I’ve come to the conclusion that somewhere between 8-30a went through this ecm effectively ruining the power feed within the ecm. I would imagine the short resulted in a pulse greater than 10a as the alternator fuse was blown but less than 30a made it to the ecm before the terminal block fuse let go. This leads me to believe that if I had a 10a fuse (like it’s supposed to) instead of that 30a fuse then my ecm would still be fine. I should’ve just been able to remove that 4th battery and replace those couple fuses on the side of the road and been able to make it back for proper repair.

Takeaways:
1) CHECK YOUR BATTERY FASTENERS, Regularly....this all could’ve been prevented had I caught this issue before the battery rubbed through.
2) CHECK ALL OF YOUR FUSES, after I found that 30a fuse where a 10a fuse is supposed to be I was furious and I double checked every other fuse is my truck to make sure it was the proper rating. You never know what someone else has done to a preowned truck.
3) NEVER ASSUME A REPLACEMENT PART IS “GOOD”, Just because the part is oem does not mean you’re not getting a defective part, I would have saved a couple weeks of after work late nights and pulling out hair chasing every wire on this truck when in fact, my replacement Oem Reconditioned Part had issues from the factory.

I would still be where I was a month ago without this forum and all of the information on here. I want to thank everyone who has taken the time to read and respond to this and other threads. Huge Thanks to Rawze and Unilevers!!!!!