What was the reason for unhooking the battery in the first place?

I'd start with checking for corrosion on the battery terminals. Then a multi-meter and check for about 12VDC on the battery. Then fuses to the ECM. If you can find a pinout reading, you might can check the supply voltage to the ECM just to be sure.

If all that is good, I'd probably search on the internet a little more, but I think the ECM might be fried.

Last edited by WoodrowFCall; 09/01/19.
Well, we don't rent pigs, and it's better to say it right up front because a man who does like to rent pigs is — well, he's hard to stop.