Use push pins to spike into the sensor connectors:

 

Ah I see! I may pick one up in the future. Would be useful to have in situations like these.

 

Looks like whoever owned my jeep last picked up a spare pcm... Maybe I should try swapping that in to eliminate the PCM lol. That was a nice find.



Edit: Still didn't start. Guess I just gotta wait for my Cam sensor on Tuesday.

I read somewhere that if a cam sensor fails in the open position, the computer can think its not getting a crank signal. Once it warms up again I'll clean up all my grounds too to be sure.
Also recharged the battery over night, so of course there's no codes on the computer anymore.
If the cam sensor fails to solve it I'm gonna go through every electrical connection to figure this thing out.

 

I've experienced this exact symptom on at least 3 different cars in my time. Each time it was the ignition switch. It would have power to all the right things in the start position, but as soon as you release the key it would cut power to things like the computer, ignition coil, etc. Sometimes the car would even run for a few minutes and then die on me while in a drive. Fun stuff.

 

I think I would word that as "camshaft position sensor (CPS) failure can generate P1391 codes".

You know what would pick that right up?

An oscilloscope.

Meanwhile:

 

Changed the Cam sensor. Still having a crank no start issue. The first attempt started the jeep, it revved to around 1500rpms then died. Still showing a P0320 code
Ive now moved on to checking my wiring harness and grounds.
I also changed in a new cap and rotor, spark plugs and wires whilst I was in there for good measure.

Researching more into the grounds on jeeps, it seems that many of the sensors ground through the same couple of grounds. Which, when not providing enough voltage through it, can bring up sensor errors. Back from when it was still running I did have a transmission code pointing at one of the solenoids. Perhaps those were connected to my cps codes.

Gonna try cleaning my grounds up and will report back.

 

Cleaned all grounds. Only one that I'm suspect of is the one at the dipstick tube. Everything looked fused together, the bolt came out rather than the nut.

It definitely needed doing, most of them were pretty dirty.
But still crank and no start.

Now that grounding issues are out of the way, it's time for me to revisit the cps circuit. Perhaps theres a bad connection somewhere.

 

Hearing the fuel pump run and having fuel pressure isn't the same thing. Put a gauge on it. Then put a multimeter on the ECU and check for power and ground as the engine dies. You might eventually get lucky by replacing parts, but then again a room full of monkeys and typewriters will eventually reproduce Shakespeare. Test, diagnose, repair.

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Don't know if you read the original post, but I have code P0320. Which is in the crank position sensor circuit.
But I'll look at the fuel system whilst I'm at it cause why not.

 

After much research this past week I finally found some tips on testing the CPS circuit for electrical faults.

The general info I could find would tell you to measure the cps resistance.

However, this guide tells you how to check for voltage in each wire which will help immensely for others in my situation.

https://easyautodiagnostics.com/jeep/4.0L/testing-the-ckp-sensor-1

Will report back soon with results.

 

Testing of the CPS showed that it was in working order and that it was registering crank rotations.

So next up I decided to check my fuel rail for fun.
I stopped off at my local part source and loaned a fuel pressure tester.

I first tested my shrader valve to make sure it was working. Which it was. I then screwed in the pressure tester which read 0.
​​​​Turned key to the on position and still zero.
Jumped fuel pump relay to force pump on, still 0.

Looks like we've found a winner.
Odd that the pcm is interpreting no fuel as a cpm issue.
Ordered a fuel pump and will report back when it's installed.

thanks
@agreen

 

Not for nothin', but Post #2:

 

 

Man I'm just kidding. That's great news. I'm glad you found the issue finally.

Honestly I don't have an answer for why it's giving you that CPS code, but think of it this way. The engine was running, and suddenly it doesn't run any more. It doesn't have a fuel pressure sensor, all it knows is that it was running, and now it doesn't have the CPS signal any more for some reason.

Years ago a tech at the dealer I worked at read the CEL on a 97 Sebring when it came in for a rough running/hard start issue. The engine would barely run and you couldn't drive it. He pulled a code for low MAP voltage. At the time, a Mopar MAP sensor for that relatively new vehicle was like $200 or so, plus diagnosis, plus shop fees, etc. The bill was something like $4-500. The story is longer, but it ended up being that the engine was blown and could barely run on 3 of 6 cylinders, giving the MAP low voltage code. Sometimes codes are a symptom, not the diagnosis.

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