98 Cherokee XJ troubleshooting

Hi all, first time posting. Wish I had found this forum years ago.

I'm no stranger to XJs and the 4.0 engine. Recently, I had a chance to rescue a 98 4dr 2wd 5 speed 4.0 XJ for $1k. It only has 140k original miles and is pristine, but was going to be crushed because of crank\no start. I grabbed it, but after 2 days, I'm still stumped. Found this video:



about the 96 XJ, which is very similar, and it's helped a lot, but...

I have a good battery, connected with the key on. I have tested all fuses and relays and they're all good. I had an aftermarket security system in-line with the starter wire in the ignition harness that was muddying the waters, but I have removed and reconnected wires to stock.

Put a spark tester in-line with a plug and thought that I was getting no spark. Turns out my tester does work, but still pretty sure there's no spark. Coil checked good. Checked 12v to the back of the coil, but nothing. Also noticed no fuel pump prime when I turned the key on, and no fuel in the rail when I pushed the Schrader valve. Figured it was the ASD relay.

Extensively tested the relays but that all tested fine. Figured out that it could be a sensor not allowing ASD to fire, so I made a jumper across the ASD to see if that would improve my reading at the coil. It did, I have 12v, and I assume spark.

Started probing things, and found that I have .7v on my CPS instead of 5v, which eventually lead me to the posted video. I plugged in an ODB dongle and have no comm. There's something wrong with my 5v reference circuit, and I'm trying to determine if it is internal or external to the computer.

I have tested the power and ground pins to the PCM, and they're good. Both of my 5v refs coming out of the PCM read .7v. I went through and disconnected the sensors from the primary 5v one by one, no change. When I unplugged the cable at the PCM that contains the secondary 5v (rather than cut the wire as he does in the video), still no change to the primary. I'm thinking it's internal to the computer.

Like I said, no blown fuses and no bare wires. I have visually inspected the harness and not found anything alarming. I don't want it to be the computer because they're difficult to find for the manual transmissions. But I feel like I have pretty much isolated everything attached to that 5v ref wire, and I still don't get 5v or comms to the computer. What am I missing?

Thanks all.

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Cut the wire. Disconnecting the PCM is going to throw a wrench in the trouble shooting process. Everything goes off line.
By cutting the 5v feed near the computer, you can prove if the rev circuits shorted to ground , or if the PCM just can't supply 5v any more. Also be aware that you may need to cycle the ignition to get the 5v ref back, or to get the 12v your looking for at the coil connector.

The above is good advice. If you don't want to cut a wire (you need to cut both because the 5v ref circuits are probably tied together inside the PCM), here are some other things you can do.

I recommend you unplug all connectors at the PCM. Check resistance of each 5v reference wire going out of the PCM to battery ground. Should be "OL" = infinite resistance / no continuity. Then check resistance of each 5v reference circuit to each sensor ground / "low reference" circuit at the PCM connector. These should be somewhere around 50-500 Ohms, I would think. If it's like 1 Ohm, then you have a partial short either in a sensor or from 5vref to sensor ground wires within that circuit.

Another thing you can do is mimic the computer -- buy a voltage converter (really cheap on eBay, like <$10. That can take a 12v input and you can turn the dial to set it at 5v output. Hook up its outputs to the connector disconnected from the PCM (positive to the same 5v-ref and negative to the sensor ground pins). If the 5v-ref does not get pulled down, that indicates a bad PCM.