91 cherokee electrical problem... Any ideas?

[crashandburn]

I'm working on a 4.0 91 cherokee and I had it running perfect. All I had left to fix was a short that was popping the fuse for the tail lights/gauge lights. Got that sorted out by running a new wire from the headlight switch to the tail lights and another to the cluster, and cut the old wires from the switch. I can't see that affecting anything else.

Now it won't start. No spark, fuel pump not kicking on. I can't find any blown fuses. I plugged my OBD scanner in, and the computer is not communicating with it. I grounded the fuel pump wire at the computer and the pump came on. I found a constant power wire that was hot and a key on power wire that also works. I changed the crank sensor because it didn't ohm like it was supposed to. No change.

Did I fry the computer somehow?

 

[Fordum]

The computers are tough. I'm sure you could have fried it, but I would look elsewhere first.

This is a classic problem I get called out to fix again and again. Somebody does something innocuous to a piece of equipment and it quits working.
And lots of times it has no relation to the problem.
But, that is the starting place. ALWAYS assume that what you did caused the problem. Retrace your steps and if possible, restore things the way they were. Its likely that one of the wires you cut loose might have been a feed for something else and just happened to also feed the headlight switch.

 

[crashandburn]

Just got in from the garage. Tied in the wires that I cut. No change. To make matters even worse, the schematics for the PCM that I found online do not match what I have. The connector going into the PCM has 3 rows of pins, everything I found online has 2. I'm starting to get a little frustrated.

 

[crashandburn]

Alright, I found the right pinout diagram. Looks like I have power in the right places. But no spark, no fuel.

 

[FAB454]

no pump. how about injector pulse?

 

[crashandburn]

Don't know, how would I check that without fuel pressure?

 

[ktmoutfront]

Hit me up if you want me to look at it. Wiring is not a big deal if you know what to trace and where to start.

 

[GWeakland620]

My brother had an identical issue. Had a early 90's Cherokee he was fixing for a friend that had a wiring problem. He ended up frying the ECU. Replaced it and the problem was solved. Not saying that is your issue, but sounds a lot alike.

 

[crashandburn]

Thanks Wade, I might take you up on that. You busy Friday or Saturday?

 

[ktmoutfront]

Friday would work.

 

[crashandburn]

My brother had an identical issue. Had a early 90's Cherokee he was fixing for a friend that had a wiring problem. He ended up frying the ECU. Replaced it and the problem was solved. Not saying that is your issue, but sounds a lot alike.

I forgot to mention my Check Engine light is not coming on. Was his?

 

[GWeakland620]

I don't remember hearing anything about a check eng light.

 

[FAB454]

No check eng light is a good indicator the ecu isn't working (failed or missing power/ground).

 

[FAB454]

Wade, if you need diagrams let me know. Theres only about 10 pages for the truck.

 

[blazinzuk]

As much as I hate to say it it really does sound like your computer.

If it's got power going in and nothing coming out.

 

[GWeakland620]

Call LKQ in Denver and have them look for a computer for you. I bet it's cheap.

 

[crashandburn]

I have a co worker with the same year cherokee. I yanked the ECM out of mine last night and I'm gonna stop by his house tonight. I'll swap in my ECM and se if his rig starts with it. If it doesn't start, I found my problem. If it does, back to the drawing board.

 

[crashandburn]

Looks like it's the ECM. I brought mine over and put it in my coworker's jeep, and it wouldn't run. Time to find an ECM.

 

[ktmoutfront]

At least now you know.

 

[crashandburn]

Wow that was fast. I just pm'd you.