I'll have to remember to do that on Wednesday. That might be difficult to navigate in the dark

 

Some developments:

Before this point, I could twiggle some stuff around or do the magic ritual to reset the PCM and I'd get some spark activity.
Now, that's gone. There is no spark activity at all.

1. After bypassing the pump ballast resistor and cleaning some grounds, the fuel pump is working as expected. Primes before start, pressure is good.
2. Gas is definitely getting to the engine. I can smell it, I'd guess the injectors are pulsing?
Ruling out fuel system issues.

3. Checked the coil contacts. Shiny clean. No issue there.
Did a "screw driver test" on the bare coil while cranking. No spark at the coil.
Removed the coil. It was quite rusted, so I decided to test it.
It failed both the resistance checks. Not only was the resistance wildly unstable, the receiving terminals (+/- posts) never reached the desired resistance of <1 ohm and the - post to output post resistance was too low (<5 ohms).
Since this was the first component I tested that failed the test thoroughly, I excitedly replaced it.
Tested new coil. Passes resistance checks.
Passes continuity test at distributor (terminals are making contact).

Still, no spark.

4. Re-checked CPS.
Resistance is still nominal at 220-230 Ohms.
However, performed the crank test of CPS. The cranking AC voltage was <0.25v, but present. Should I replace it? The service manual says so.

5. Replaced headlight switch. Totally unrelated, but it needed to be done. It works, doesn't short out.

 

Do the CPS mod on my website first.

 

We replaced the CPS.

The crank voltage improved to 0.4V - 0.5V AC, but still no spark.

At this point I guess it's just dead. When I started spark was intermittent, now its just gone. I have no idea what else could be wrong. If it's some random rubbed out wire behind the dash I'll never find it.
Should've trusted my late father's intuition and just left it last year...

 

Well, here's a development.

While replacing the CPS did not immediately get spark back, I did the ritual to reset the PCM and now its sparking consistently.
So it's a "have fuel, have spark, no start" - it gets extremely close to starting. Fires here and there, but can't keep itself going.
Today I replaced the cap and rotor. That didn't change anything, but the old cap was corroded so I guess it doesn't hurt.
This is the second time that resetting the PCM/ECM/ECU dramatically changed the operating condition, so I have to get under the dash and start looking there.
Some time next week I'll get under there and pull the ECU, see how it looks.

I've also got to check the vacuum and airflow, too.

Overall this is very confusing. I think the mechanics were right in that its a bizarre electrical issue. When I check the reference voltages at the sensors I can test up top (MAP, TPS, etc), they are all good. Their voltages also check out across their own wires and also back to the battery itself. That would seem to indicate that the ECU is fine. Tracing the CPS wires out of the harness would help. I should also check the voltages being supplied to the ECU at its connector as well.

The battery voltage gauge on this Jeep has been consistently leering and leering lower and lower. For the past year, even if its been starting and running, that gauge wouldn't exit the red.
I've read in other posts that the ECU is what ultimately sends the signal to that gauge. Of course, it's a 30 year old gauge in a rusty jeep. The other gauges back there don't seem to be troubled. I'm confident that the speed, temperature, RPM, oil, and fuel gauges are all fine because their "good running" positions haven't drifted much over the years (and the speed gauge matched the little speed-check radar stands that tell you how fast you're going in this town).
It's just strange the way it started to fail was utterly stopping at high RPM, and now we're at this.

 

I fixed it

and its honestly pretty dumb! So once earlier I had inspected the dipstick ground stud and cleaned it as best I could, but did not remove it. I couldn't get to it! My wrenches would just slip off of it, or I didn't have enough room.
I also was checking the voltages coming _from_ the ECU : voltages to the ICM, MAP, and TPS. They were receiving the correct voltage. I figured if the ECU wasn't properly grounded, it couldn't send these voltages, so I'd move on from that.

But after replacing the CPS, coil, and dist. cap (which to be fair did all fail testing), there really wasn't anything left that could be causing the problem.
Well, you know there's lots of things, but I really really had to check that ground stud. Cruiser54 and everyone else wouldn't be on about the same thing if it wasn't a serious problem.

Well I checked it. I don't know how this happened. There was no sign of corrosion here, just violence.


Badly damaged ground wires (that go to dipstick tube stud)

Badly damaged ground wires (that go to dipstick tube stud)

Fixed ground wires

Well, thank you guys for pointing me in the right direction, even if it took me a month or two to get there.
Some rock or something must have been tossed up there at high velocity.
Of strange note, my oil pressure sending unit seems to have perished. It's pegged all the way right. I'll have to keep an eye on that. Wasn't like that before...
Everything else seems to be working as normal, though. vehicle now starts and runs as expected!

 

That is a prime example of the best advice posted on this forum as to electrical problem fixes.......check the grounds, don't just look at them, take them apart and clean them. It was NOT me who posted that first on the board, but with XJ's and electrical problems it is a trouble spot for sure!

Glad you found your electrical "gremlin" !

 

Couldn't agree more. I coulda sworn that in my instructions the wrench size to hold the stud while taking the nut off was included. I'll go look.

 

Yup.

• Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. If the whole stud turns, you can use a 7/32″ six point socket or wrench to hold it so the nut can be removed. Worst case, cut the wires and remove the stud and nut. Install new terminal eyelets on the wires when going back together. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Surfaces must be clean, shiny and free of any oil, grease, or paint.

 

Nice to see you got it fixed. I threaten to give up too but so far I haven't yet.

 

You always hang in there.