I bought another XJ this past weekend. This time for my daughter. She's super excited, but the story I got was that they replaced the alternator and it's still not charging. So I immediately suspected a bad ECU.

I checked for voltage and ground at the field wires and got no 12v, but I was getting ground. I pulled C2 and C3 off the ECU, checked continuity of the 2 wires going to the alternator and everything checked out less than 1 ohm. Pulled the ECU and opened it up. There is a HOLE on the board at the C3 connector, right at the pin (pin 25, if I remember).

I did some digging and it seems that quite a few people have had this same issue, and I have kind of wondered if it's somehow related to either shorting the terminals with dropped tools or maybe water since the alternator does sit low and the field terminals are exposed at the very bottom.

That being said, has anyone considered (or gone through with) putting a fuse on the field circuit? It seems silly to me that it wouldn't have a fuse there and risk burning the ECU traces or components.

For reference, it's a 1998 2.5. 2wd 5 speed base model XJ.

 

I had a 2000 XJ when Hurricane IKE did its thing in southeast Texas. I was parked in the garage and got water deep enough to cover the starter I guess as later the starter started acting up and had to replace it. Never have been in water up to the alternator but it can't be good. Never seen a ECU with a hole in it, but never opened one up! Be interesting to how you go about fixing or just replacing it. I run a 96 XJ, 4WD, with auto trans now and recently picked up a spare ecu for it.....just in case as they say!

 

Seems like a good idea. Should have had one from the factory.
Where would you put a fuse that it would be out of the way and not get corroded from salt, water, etc?
Could also cut out a chunk of wire and use a fusible link but thats as not easily replaceable if it pops for some reason.
Im surprised the wire didnt melt if there was enough current to kill the board like that.