97 XJ sudden no spark conundrum.
 

Hey guys, this is going to be my first thread. Not because I haven't used the forum before, but because my XJ has been so reliable, I've only ever needed to do routine maintenance to it. That changed this winter. I was roaring up the road heading for lunch at our favorite deer hunting lunch-stop, when my 97 XJ with the 4.0 fell flat on its face going up the hill. I pulled the codes off with my scanner I keep in the glove box, and it relayed 2 codes dealing with the ignition coil. I'm an idiot and didn't record them before I erased them, but replaced the ignition coil that night, and it still wouldn't fire.

Since that night, my replacement parts are as follows : NEW OEM crankshaft position sensor, NEW OEM camshaft position sensor, NEW OEM ignition coil. I have good fuel pressure, however I have no spark coming from the new ignition coil. I have checked my 5v references to the CPS, CamPS, MAP, and TPS, and they all have 5.14v with the key on. I have checked the DG/OR wire in Pin 1 of the coil connector and found it indeed has over 12 volts when cranking. The ASD system checks out, I checked it even though the ignition coil gets power from it. My PDC fuses and kick-panel fuses have all been tested and found to be good.

While checking wire integrity, I did come across the connector to the camshaft position sensor had a MAJOR issue: ALL 3 wires were bare for about an inch out of the connector. My question is: with those 3 wires touching, is it likely my PCM is toasted in that circuit? I've uncovered all the wiring, tested with a multi meter, and the PCM acts fine. It just has no spark. Sorry for the length, just trying to show an attempt at doing all the diagnostics I can.
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Can this kill a PCM?

my mechanic mate obtained a '97 for free that had been sitting at a reputable auto electricians for 1 year.

He swapped computers from another '97, that he had in his yard, and that fixed it

Its my understanding the '97 has a factory immobiliser, so he must used the the key from that car presumably

You can order a computer from Rock Auto, check archives on this site for details, immobiliser vehicles need a VIN programmed

I am not saying that is your problem, but an issue can be in the harness, connectors, internal solder joints, or capacitors

Although it may or may not be the cause of the problem. The short answer to your question is YES. Any time a PCM monitored circuit has a direct short, it can damage the PCM.

You mentioned that the "ASD system" checks out. Can you provide a little more detail as to what you checked there?

Does the Jeep just crank over and over and not start? I had the same cps wiring condition and I had a new cps from my mechanics engine swap so I was a bit upset when I saw those wires corroded and all around eachother. I ended up pulling the battery and re splicing those wires to correct the situation. Still would not start. Right before I sent my jeep off to the mechanic i floored the pedal to the metal and she fired up! Worth a try. Mine is with my mechanic(a best friend) so if there is a revelation I will let you know...

I can hear the fuel pump prime in its initial surge which IIRC is a sign the circuit is functioning properly. I've even replaced the relay with a known good one, as well as a jumper wire to confirm as suggested a few times across the forum. I may be wrong on this, but since the DG/OR is lighting a good 12v to pin 1 on the coil connector, that means the ASD relay/system is working as it should.

My Jeep is cranking over fine. I replaced the NSS a few years ago, so I know what a pain that can be when it finally goes out. I've tried working the throttle as its cranking and that doesn't seem to yield a different result. I have engine RPMs on my scanner while it turns over, as well as a good pulse from the CamPS as the distributor shaft spins. I've taken apart every ground in the engine bay and cleaned the contacts. I've un-loomed the wiring harness from the coil to where it meets the CPS connector past the fuel injectors wiring.

Pretty much if CCKen has suggested a test for a no-spark situation, I've tried to do all of them I can find.

Have you had any luck on your end? My mechanic is getting spark and no spark conditions and is replacing the crankshaft position sensor, the camshaft position sensor(which I think was damaged) AND the distributor even though it is a month old...he said the conditions seem to be coming from those areas...kinda vague I know but my camshaft position sensor wires could have certainly shorted out all kinds of things. Odd that it occasionally starts though. So just checkin to see if you had any luck?

Thanks for checking back. Since I'm new, someone has to look at my replies I guess and I don't even see my last update.

With all the diagnostic testing I've done and talking with a mechanic who is a family friend, the Jeep has a date with a local shop next week that has the proper scanner that will pick up where I left off. I'm making a detailed list of all the things I've checked and they will take it from there.

So for the ASD, the first thing I did was listen. I can hear the fuel pump and the relays click when I turn the key on. The Jeep does turn over, so I have that going for me. I switched the relay with a known good one, no change. I tested pin 1 at the ignition coil connector DG/OR and get a good 12 volts there. From my understanding, that is indicative of a functioning ASD circuit, as that's where the 12v comes from. Just for giggles I also jumpered the relay, and that didn't change anything. Still no spark.

If the wires touched on the camshaft position sensor connector it would only really short out one thing. That would be the 5v supply coming out of cavity A17 on the PCM which provides power to the sensor and continues to ground on cavity A4 of the PCM. The signal produced on the 3rd wire of the camshaft position sensor (color Tan/Yellow) goes to cavity A18 of the PCM. With a scope, that signal could be checked and if it was proper, a damaged PCM would be ruled out (at least in this regard). For a spark/no-spark situation, it would be fairly customary to do a scope of the cam and crank position sensors (at the same time) to ensure that they are both producing reasonable signals that are in-synch with each other.

The fact that the PCM provided the ground necessary to close the circuit at the ASD relay (as noted by your presence of 12v positive at the coil), implies that the computer was happy with the information it was receiving from the cam/crank sensors. Typically if it can't make sense of motor position and rpm (via these sensors) it will not provide the ground to the ASD relay, thus disabling ignition (spark).

I would next want to see what the ignition coil driver (ground) from cavity A7 of the PCM is doing. I would check it right at the PCM to see if there is activity there. If there isn't, then something is more deeply wrong. But if ground pulses are happening on A7, but not at the coil itself, that could point to a simple wiring problem between the coil and PCM.

I've monitored A7 several different ways. I back-probed it with a paper clip to a test light, then with an analog multimeter, and finally a digital one. While cranking I observed no activity at all coming from A7.

I have the wires uncovered from the coil, all the way back behind the block, up to about the fuel injectors. The only damage to any wires were to a few ground wires, and to the CamPS sensor harness.

I assume A7 puts out a 5v pulse akin to the 5v pulse I confirmed coming from the CamPS, and the CPS when I pack-probed it. Wish I had a better scanner and the skills to know what I'm looking at. The information the multimeter is making me question the health of cavity A7.

A7 would be a ground. The ignition coil gets its positive voltage from the ASD Relay (followed by the ASD fuse). That positive voltage is constant and ready to go. Then the computer completes the equation by providing the ground via A7. But it is not on constantly, it is a driver, so it is going to "pulse". Its a bit complicated, but it provides ground which energizes the ignition coil (at around 12v). But the spark plugs need more like 20-30k volts (way, way higher). So when the computer turns off the ground (pulses) to the ignition coil, it causes the amperage to fall instantly, which produces a momentary spike in voltage (low amperage, but very high voltage, and it is this voltage that allows the electricity to jump the contacts in the distributor but more importantly the spark plug gap).

Perhaps you should go back and test A7 again now that you know to test for ground instead of positive.

Maybe this will end up being as simple as a bad coil (I know it is a newer OEM coil - per your post). But some of the results of the diagnostics you've done so far are leading in that direction. Testing A7 would get you more certain. Because if A7 is pulsing a ground, that would mean the computer is happy, and would leave only the coil and the wiring between the PCM and coil as culprits.

What's the best way to check A7 ground pulse with a multimeter?

Test lamps tend to show pulses better. Because a multimeter tends to just average the pulses together once a second (depending on the meter and how sophisticated it is). With a lamp, you'll actually see the light pulsing (usually - unless the pulses are so fast that it is just one continuous light).

Yeah, I haven't seen any activity on my test lamp coming from A7. I think someone else suggested an analog meter in another thread, and I didn't see any results either. Starting to look like a PCM issue?

If you can be certain that the ASD relay is getting a ground from the computer - and at the same time there appears to be no ground pulsing provided to the ignition coil via A7. Then yes, that does seem to be a PCM problem. I rarely say that, because PCM problems are jumped to too quickly without proper diagnosing normally. But based on my understanding, if the computer is allowing the ASD relay to close, that means it is happy with its sensor inputs from the crank and cam (and a variety of others). I can't see why it wouldn't be driving the ignition coil at that point. Unless there is more too it that I don't understand (very possible) like a security system getting in the way - or some other PCM behavior that could explain this without the PCM being "bad".

Analog meter for A7 wouldn't be any more helpful. Analog meters are typically suggested just for the TPS where one wants to observe that the voltage coming out of it is smooth all the way across its range with no "drop outs".

Did you test A7 by connecting a lamp red lead to a known good positive and the black lead to an A7 back probe?