Working on the truck today (e-fan install) reconnect battery ground, side post bolt touches battery, unsuccessful catching threads, re-try, arks from side post, headlights flash, hear fuel pump prime, engine starts & runs, remove ground truck still running, remove keys from pocket, insert in ignition switch, turn to run, turn to off, engine still running, engage starter barely scrape teeth, turn to off, engine stops.

This has happened three or four times in fifteen years. Always have to scrape starter to kill engine. With the V6 to V8 swap it has different ECM, starter and wiring harness. This time it slammed the speedo needle to 120MPH burying on the stop pin and staying. Thought it killed the stepper in the Escalade cluster I just installed. I was happy to see the needle start operating again after hitting a few bumps on the hwy.

Has anyone ever seen a GMT800 phantom start just by connecting the battery? I'm always somewhat skittish reconnecting but this time after almost losing the cluster I'd like to get to the bottom of this weirdness. Any ideas... maybe the BCM??

 

sound like you have a loose wire some place around the fuse block or ecu since you was working in that area for the efans maybe? bad ignition switch? if you have a aftermarket remote start that might be it since you said the lights flashed

 

 

Ign relay possible staying hung shut. Swap it with another relay.