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Discussion Starter · #1 ·
About four years ago I swapped a 1998 4.0 with an OBD 1 computer into my 1981 Wagoneer. It's been running fine and without issues. Recently I swapped the battery out and now the engine will not start. The starter will engage and crank the engine.

I have checked and can verify that the fuel pump is no longer priming when the key is on. It is also not running while cranking. I have jumped the relay with the power distribution center and the fuel pump will run - so all of the wiring from there is good. Thinking that I just the problem popped up when I changed batteries, I ran new battery cables between the Starter and PDC as well as a new ground, but the problem is still there.

I have not checked for spark yet (as not having fuel seems to be at least one problem). What would cause this to occur? Is it likely that my ECU died?
 

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confusing -
are running a 1998 PCM?
1998 is OBDII - so IDK what we have.
the relay is installed in a 1998 PDC (underhood fuse box) OR something else -

YOU jumped relay was thhere 12v to relay load side?
 

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Discussion Starter · #3 ·
The Jeep is becoming a frankenstein of parts from other cars / years... I really should put it all in a spreadsheet someday.

The engine is a 1998 block (I believe from an XJ). The ECU / Wiring harness is from a 1992 XJ (OBD 1). The fuel pump relay is installed in the under hood PDC that is part of the harness. To verify that the fuel pump is working, I supplied 12v to the load side of the fuel pump relay and the pump runs without issue.

All that to say, when I turn the ignition to On, the fuel pump no longer primes (or runs), but I know the issue is not (1) the pump itself or (2) the wiring after the PDC.
 

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then the control side of relay - and we have a PDC from the 1992 XJ -
I don't have quick access to that old a diagram BUT will try, in mean time

there should be 2 fuses for FP the load side you say is good the contro side has a fuse also - give me some time

NOTE - Are you aware the fuel pump will only get 12v for 3seconds if jeep is not running, makes troubleshooting tricky - 2nd person is helpful
 

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Discussion Starter · #7 ·
A little injury took me away from this for a while, but I wanted to share an update for future readers.

I traced all the wires in the diagram above and confirmed that everything was working as it should when power was applied to the correct wires. Eventually I traced the wires back to the ECU, and ensured that there were no breaks or shorts in any wires. This led me to replace the ECU which solved the problem.
 
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