From some internet searching, we think the ecu is bad in my buddys 90 XJ (4.0, AW4, 4x4). I was considering taking the ecu out of my MJ parts Jeep to see if it would work, just to see if it actually is the ecu in his before he spends the money on a new one. My only question is would the ecu out of my 4.0 AX-15 MJ make his 4.0 auto XJ run? Just to make sure that is the issue?

 

First what have you done to come up to this conclusion?
Second I do not know if it will work on an AX15 but somebody else here will.
What is it doing? No spark or fuel?

 

Has spark and fuel. Cranks up and immediately dies. The whole fuel system is new, as well as most of the electrical system. Fuel pressure is good. Spark on all 6 cylinders. The ecu and tps are the only things left that haven't been replaced. I found a couple threads through a google search where people had the same problem and it was the ecu. And I've had a couple friends tell us it was the ecu. Which is how I came to that conclusion.

 

CPS replaced?

 

Yep, swapped that out today. No difference.

 

Does the '90 have the fuel pump ballast resistor and if so, have you bypassed it to see if that is the problem? Just unplug the two connectors and clamp them together while starting.

 

Yep, the 90 XJ and 89 MJ both have the ballast resistor. Swapped the MJ ballast resistor in the XJ, then tried bypassing it. Made no difference.

Like I said, everything except the tps and ecu have been replaced. Nothing but the ecu and tps are over 2 months old, all the wiring under the hood is brand new (I rewired it when he bought it, the idiot owner before him caught it on fire because he didn't know what he was doing). A lot of parts have been swapped in the past week trying to get it running again, this is the 7th day its been stranded on the trail, and we really need to get it going.

 

One kinda odd thing though, that I forgot to mention. The fuel pressure is good, and the pump runs when the engine is turning over, but doesn't prime when the ignition switch is first turned on. The entire fuel system is brand new too, everything from the fuel pump relay to the tank itself. And we swapped in my spare fuel pump, and it does the same thing, works fine just doesn't prime.

 

I would think you could get away with swapping them and seeing if it would stay running real quick

 

I hope so, I'll probably try it tomorrow while he is at work unless someone tells me for sure it won't work.

 

Only Cruiser or CCKen would know for sure lol