Is there a fuse protecting the ECM ???

theres ecm fuses in the battery box i believe at least on my century i had

 

did you check fuses behind glove box and under the hood.

 

should be #8 20 amp fuse

 

Positive reversal will go through all fuses. The ECM is likely gone along with anything else plugged in that has a brain. Once it powersup on the wrong polarity shorts out instantly.

I install computer power supplies when building machines or replace them and for the most part the polarity is idiot proof. If it does not flow correctly for any reason there is no more computer.

In a fuse it carries current. Either the right kind or the wrong kind. It does not know.

What fuses do know is that when there is too MUCH current they get hot and fail. That is all they do. Fail. Before there is a fire or nearly so.

 

On my 2006 kw t2000 there was a 50amp fuse on the wire running from the battery box into the cab,it on the engine bay driver side.
I shorted the cables once and the truck went completely dead,took me half a day to find it. Its called megafuse.

 

Jump the truck from the alternator and c what happens,this will allow curent to run backward and bypass the megafuse if there is any.
Dont attempt to start the truck.

 

did you check the battery box too

 

i would check the battery hook ups again. 9 times out of 10 its the driver not the car. check any grounds and check the ecm plugs