I have been working on a '92 Cherokee/xj with the 4.0HO that is blowing a 30amp fuse under the hood labeled "Electronic Control Mod". The starter will still turn but it will not fire. The fuse blows immediately when the key is switched to start. I disconnected the connector attached to the ECM (drivers side next to airbox) and tried to start and the fuse did not blow (of course the engine didn't start either). I also disconnected the fuel pump (to be sure it wasn't bad) and the fuse still blew.
After some very helpful advice from Willys55 (thanks again) and a break in the weather I've continued troubleshooting. When the a fuel pump relay is removed (and the ecm is still connected) the fuse does not blow. If I put the relay in but disconnect the wires to the fuel pump (going into the gas tank) the fuse blows. I've found that this circuit also includes two items: a "ballast resistor" and a "ballast resistor relay". I found the resistor and it seems okay. This might sound ignorant but how do you test a relay? I have a good multimeter but have long forgotten how to test relays. Where is the "ballast resistor relay"? Could this be my problem? Any other pointers would be helpful. Thanks.

 

On my friends 98 grand it did the same thing. Ended up the o2 sensor wire was melted away and grounding out on the exhaust. taped it up and fixed the problem. Worth a try.

 

I appreciate it but I failed to mention that I had already checked that. I also checked to make sure the coil was good.

 

Is the relay a factory relay from the fuse box, or a typical relay?

If it is a standard relay there should be numbers on the back 85, 86, 87, 87a and 30

If 12v is applied to 87 and 85, when ground is is applied to 86, Connection 30 should read 12v's