I bought a 1990 jeep Cherokee xj it ran great after a while we found out it had a cracked block which was a motor out of a 1996 Cherokee so I got a 1993 Cherokee motor after installing I now cannot get it to start it turns over great has plenty of fuel pressure in the rail I reach of what the problem could be and I got the crankshaft position sensor so I replaced that the I replaced the coil still no luck also my friend did hook up the battery backward which I believe is causing a problem and on the fuse panel as soon as I turn on the key it melts the acc wire and the radio fuse I've tried everything I'm completely stumped any body have any ideas? If you think you have an answer please pm me

 

You'll have to find and fix the accessory short first, then check the fusible links that fan out from the start relay. An accessory short may have opened one up.
What year electronics are on the motor? Did you swap in the '93 block only, or more?

 

Did you use the flexplate from the "original" engine that ran, or the one from the donor engine?

PM you? Why?

Wedonthavetimeforthatandweneedtokeepeverythinginth ethreadtokeeptrackofthingssowedontwasteeveryonesti meandenergy.

 

flexplateisthebigdiskwithteeththatthestarterdrives

 

hehe longest sentence ever!

 

Notanymoresincemyedit.

 

Finally making some sense haha

 

edditedabovewhohastimeforspaces

 

ohgoodlordmyeyeshurtnow

 

Just the block

 

The battery being hooked up backwards has burned wires.

We can't help you from here.

Tell your friend "thanks a pant load".