I'm helping a friend with his 1991 GMC sierra 5.7 TBI which is now actually a 305ci Rebuild I installed for him. Then I installed a used 700 tranny soon after due to loss of 1st and R. Ok, runs strong he loves it, as do I, but we still haven't put not one highway mile on engine due to the fact that the ECM/IGN fuse blows and has blown at the same EXACT driveway 5 times consecutively! I can pinpoint when the fuse will blow. Soon after the truck shifts into high gear. Replace the fuse drive it back home, plus around the block cpl times plus 15-20 minutes idling and the fuse won't blow. Take off down the road and bam same spot every time fuse blows. I've been around the World, went to 2 World Fairs and watched Buzzards mate, but I ain't ever seen anything like this!!! What'* up? I would deeply appreciate any help at all! Thank you.

Could be something in the transmission harness shorted like the lockup circuit that it doesn't use till its warm.

I will surely check. I have thoroughly went over the wiring inside the engine compartment that prior owner has apparently done some of their handy-work on. It'* a natural born mess as my Granny would say!!! Thank you and I will keep ya'll posted!

If its been hacked, then that could be the issue too. I don't have the schematic, but I think there is a lot on that circuit. Could move just right to short at that point

I don't think anything'* been hacked. The truck ran fine before I had to put new motor in it due to marred cylinders from overheating. I'll find the schematic. This is the only issue left with his truck then he'll have a strong running vehicle for years. I hope so cause he'* spent way more than he wanted to! lol Thanks Brother.

The harness pinched anywhere since the engine was replaced?

YAY!!! Problem solved my Brother! Thank you for your advice! The problem was where you said to look. I started at the connector to the transmission and there it was. All 3 wires into the connector were somehow bare about an inch up. So, I cut them and put heat shrink down to the connector then butt-connected the wires back and took it down the road. The wires were ok when I installed the tranny. So, I don't know what happened to them but it don't much matter now I reckon!!! Once again thank you!

Glad you got it fixed.