Re:Re:Re:Re:Re:Re:Re:37 year old coil works, new OEM MOPAR won’t

Thank you for the details, I’m studying your drawings to get an understanding. As I’ve said before this really beyond my background and comfort zone. But we need solution so I’m trying learn. This really stretches me … thanks again.

I am curious what you were thinking why the 1986 coil worked for a time, my wife keeps a journal—it was 45 days, enabling us to make a number of trips to town.

I don't know what's wrong, but you have loaded the 12v feed and ground with your test light. As the last guy pointed out, it would be better to do it all at once. I would like to see your spark tester fire with nothing else connected (plugs, distributor etc) if you can pull a 3/4+/- spark then you have proven everything upstream. If you don't get good spark, I would want to rule out the new coils being internally wired different (pin 1 & pin 2 reversed)
This is my thought process.
I think you are close.
Cheers

Ps I can't see the diagram?

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My spark test, Lisle 20610, does not show any type of measurement like some I have seen. It just has a light. Also I am unable to order the MSD harness concept as we are unable to get to town now. Due to our location we do not get deliveries to the ranch.

I am a little confused on the wiring diagram for the MSD as it shows a wire directly from the ASD to the coil. I thought the two wires of the coil went directly to the PCM. The grey wire goes to PCM pin 7A and the GRN/Tan wire goes to PCM pin 25C. I understand the ASD is controlling the coil, I thought that was through the PCM though. This is where my lack of knowledge shows and confuses me. I will show this information to the mechanic’s father though.

The spark tester is fine. I have similar. I use it as a quick check but your test light is the best because you can adjust the gap very easy. You want to be able to make the spark work. The air gap represents resistance. If it can overcome 3/4" you know it will fire under pressure.

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Okay, thanks for the clarification. I just realized my other comments were supposed to go to the Cherokee Forum, my brain must have went on vacation. There is a thread there with a test recommendation those comments relate to.. We have family coming for 3 days so the Jeep will have to sit idle for a few days. Happy Fourth to each on here who is trying to help us with our situation.

July 5th update…

We had guests for the holiday and I took a break from the Jeep. In going back through the thread replies, I’m not sure or confused as to what I should try doing next. I am very hesitant at trying to reverse the polarity wires to the the coil as has been suggested by some. I am really concerned about damaging the PCM or a new coil, if I was convinced it wouldn’t hurt the PCM I would try testing either the 2020 MOPAR or 2022 NGK coil before I try the 2023 OEM MOPAR coil. I watched one Paul’s Premium videos on YouTube this morning on a V8 Jeep where he diagnosed the Cam sensor was the problem. That caused me to wonder could that somehow cause our issue?


I am still stuck on the question why the 1986 coil, hereafter called 86, worked for 45 days and the new 2023 OEM MOPAR coil wouldn’t. In going back through invoices and my wife’s journal we built a timeline of events for the Jeep. When the Jasper 2 engine was installed under warranty a new distributor was installed with a new Cam sensor. The mechanic said, the OEM distributor had a “wobble.” I figured replacing a then 25 year old distributor was okay. However, I now could not help but wonder if perhaps that was somehow related or contributing to the problem since the distributor is directly connected to the coil. In looking at the Cam wire coming out of the distributor I noticed it was twisted 2-3 times, I wondered if maybe the twisted wires were broken and that was causing problems. I had the older Cam sensor so I did swap the older Cam sensor that was previously working for the new Cam that came with the new distributor. Changing the Cam sensor did not solve the problem, I was hoping for a crossing the Red Sea kind of miracle. I didn’t really think it would make a difference but since the idea popped into my brain i figured I had to try making the swap. Plus, it was free and relatively easy to swap the Cam sensors for me.

I appreciate those of you with knowledge and technical skills being patient as I wrestle with understanding the various test and suggestions.

I have been trying to understand ferris48’s diagram and what I would do. I am sorry that I can’t follow what I would do exactly and what I would be testing in the procedure. I feel I am being a dense that I can’t follow the diagram but, I can’t—it isn’t the instructions, it is my lack of background. I have looked at it a number of times and I just am not getting it. I explain to my wife by pointing at my forehead and saying my understanding of some of this stuff is right here, pointing at my forehead but the replies like ferris48 are about 1” to 4” past the top of my head. I kind of get it but not completely. I greatly appreciate your patience in working and trying to help me understand what to do next.

I managed to look at the diagram online (not on the app). It's a good test of your control circuit. To do the test clip your test light to ground and touch your test probe to pin #2. This is your control/ground for the coil. If you had a bad ground the light would stay on. If you have a good ground/control it will cause the light to flash on and off. The only trick is that the light goes off when the coil is powering on. This is because the coil always has 12 volts and the control switches the ground. Well, your test light will see no potential difference when the control wire is grounded. But when the control write is "open" the circuit for the coil is completed through your test light. This is called a series circuit. Because the coil only has 1 ohm most of the voltage will "drop" across the test light.
Cheers

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Does this help? Setup your test light like this illustration. The coil driver in the PCM just switches to ground. Green is ground, Red is positive battery voltage. The wire going to the computer is the control wire - the one that's switching from ground to voltage. So long as you have switching occurring on the control wire and battery voltage at the coil, it should spark. One caveat though, depending on the resistance of the test light, it may prevent the coil from sparking. If the test light has too low of resistance it'll hold the control wire to ground and not let go I think. But on the other hand it'll load test the power side of the circuit for sure. But for now, just see if your test light flashes on and off on the control wire.

Now that I think about it, use a really bright light like from a headlight. I don't care if the coil is prevented from sparking (it's not sparking RIGHT NOW anyway lol), we're checking the wire integrity of the circuit.

mmorris923 wrote:

I am curious what you were thinking why the 1986 coil worked for a time, my wife keeps a journal—it was 45 days, enabling us to make a number of trips to town.

Probably a bad connection somewhere in the harness. No way all those coils don't work on this Jeep.

I’m still somewhat confused, let’s start at the coil connector—pin #1, the grey wire goes to A7 at the PCM. Pin #2, the Dark green/Orange wire goes to C25 on my Jeep at the PCM [I understand the Dark green/orange wire also is connected to the ASD via a splice]. When I test both of these wires from the coil connector to the back-probed PCM connectors with the connector pulled from the PCM I have continuity, at least I get a solid green light. Doesn’t that show the wire is good?

I response to a suggestion today on the Cherokee Forum I just did the following: I back probed pin #2 at the coil with a non-LED test light grounded and engine cranking no light, I also had the spark tester between the coil and distributor—no spark either. When I disconnected the coil connector and with the key with no start there was no light on either pin with the non-LED test light. When I began the testing process for the forums I did register 12 volts on pin #1 with key on.

I also was asked to check the injectors while cranking—I used a rubber tube as a stethoscope no sound of the injectors firing while cranking. When the engine was firing with the 86 I could hear the injectors. Side note, while cranking there is no RPM on the tach either.

My confusion lies in that, I thought when the 86 coil worked all the wires were verified as good along with my continuity tests and the voltage reading I took when the 86 coil worked. I have removed all the harness loom, untapped everything and checked the wires. The harness wires were amazingly in good shape, no brittle, cooked or hard wires, they were all very pliable and looked good. I did when the we had intermittent stalling to wiggle the wires to see if I could cause the stalling from perhaps having a bad wire.

The intermittent stalling began in July of 22 and increased in frequency until December of 22 when the engine would no longer start. The time between stalling also increased in frequency in that by late December the engine would only run sometimes for a couple of minutes before is would stall. When it stalled if I waited 15-30 minutes it would usually restart and run again, then return to stalling. It seemed to me that it was somehow related to something overheating that caused the stalling since it would restart after a time interval. When 86 coil the Jeep We did eventually find that wiggling the coil connector caused a stall, and that connector was replaced and soldered in with a connector from Summit. The wiring harness has been re-tapped and new loom applied—everything is closed back.

I do have communication at the OBDII to my basic Autel. There are no fault codes either on the dash or on the Autel. Apology for such a long response but I wanted review what I’ve done.

[Additional info: when the 1986 coil worked and nothing else would I wondered if the wires from the PCM might be the problem, so… I cut the grey wire at the PCM and Dark Green/Orange wire and ran new wire directly to the coil, cutting and soldering the wire at the coil connector. The 86 coil worked’ the engine fired and none of the other coils worked (I have an OEM MOPAR I bought in 2020 as well as the NGK from 2022 along with 6/2023 OEM MOPAR coil. All three coils have been resistance tested and were taken to town and swapped into another Cherokee and worked). When the new coils didn’t work I returned everything to the original wiring, re-soldering all the connections and testing for continuity. At that time the 86 coil was still working and after re-soldering everything the Jeep ran with the 86 coil.]

From pin 1 to 2 you have continuity. This means you probably have a good primary coil. You ohms tested this and found 1ohm this tell you more and says, good coil.

As for your test, do I understand you have 12v with your meter, but no light with your test light grounded? This would indicate high resistance on the power line.

Finally you mentioned problems with fuel injector and tach. This is where a real mechanic and not an electrician would be better, but I would assume your computer will not send spark or fuel to an engine that it can't determine crank position. The other thought I have, is if you have bad power to the coil, it could be bad power to the pcm. This might give random weird problems and not set a code. I would think a bad crank sensor would set a code. But again, a jeep mechanic would probably be better.
Cheers

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Thank you for the observations and suggestions. The. CPS has been replaced with an OEM MOPAR Crank Position Sensor when the Jeep was running and it ran really well. I understand in todays world that the quality of parts is poor but I’m doubtful OEM CPS would fail in less than 1,000 miles. I don’t know if I said on this Forum but every relay and sensor has been replaced. When we were having the intermittent stalling issue I justified that with the vehicles age. I bought the best quality from Rock Auto from the mechanic’s suggestion.

I mentioned the tach and injectors from a question on the Cherokee Forum. I assume the tach not registering means something to those who are knowledgeable—I don’t know the significance. Same goes for the injectors not firing. I would guess if the coil isn’t firing the PCM would not send a signal to the injectors, but that is my assumption based on my very limited knowledge. I do appreciate the time you’ve taken to comment and suggest procedures, I’ll keep plugging away until the solution is found.

The pcm has no way of knowing if the coil is firing. So it's the other way around, the pcm will not fire the coil if it does not know when to fire. That's why I mentioned cps. I hate replacing parts so I'm not suggesting a new CPS. If you had a scope you could inspect it, but that aside, are you getting 12v on your meter, but no light on your probe? (Pin 1 to ground)

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pin #1, the grey wire goes to A7 at the PCM. Pin #2, the Dark green/Orange wire goes to C25 on my Jeep at the PCM

Not to be too nitpicky, the diagram I'm looking at, C25 is the brake switch input.

When I began the testing process for the forums I did register 12 volts on pin #1 with key on.

If you no longer have ASD relay activation at key ON, then this is something other than a CKP sensor issue. Stick your test light on Fuse 20, and cycle the key on and off. The ASD relay output should power that fuse momentarily, among other things.

I have a chart that I made when testing continuity as well as one showing the pins on each of the PCM connectors. The grey wire goes to pin #7a (the black PCM connector) and the Dark Green/Orange wire goes to pin #25c it is labeled “Generator Driver” when I tested continuity this is the wire that lite up from coil Dark Green/Orange wire. I understand the Dark Green/Orange wire is spliced with the ASD.

I’m not sure what fuse 20 you are referring to; my ASD fuses in the Distribution Center under the hood is #10 & 11. In testing the fuses I cannot get the test light to come on with the key on or off and there is no voltage on those two fuses when checked with the Power Probe clone≥. In re-checking all the fuses with a test-light and the Power Probe clone yesterday afternoon I got a minus .07 reading on a number of the fuses inside the passenger compartment with the key off and on some of them they then registered 12+ volts with the key on. All the fuses in the Distribution Center register 12+ volts except for the two ASD (#10 &11) with no test light or volts showing.

Fuse #11 in the passenger compartment labeled PCM registered, minus .07 volts with key off and 12+ with the key. I wrote down all the values for each fuse with key on and off. What is with the minus .07 volt reading? That confuses me, I can understand .0 volts not a minus .07 is this significant?

Is it time to start over testing? If so, where do I begin? I am feeling confused and not sure I am following the right testing procedures or should I say order, am I jumping around and if so is that helping those of you who understand a whole lot more than me or is that adding to the confusion? Part of my problem, I think, is I’m still hung up on why the old 1986 coil worked. I need to disregard in my thinking and follow a good trouble-shooting order of investigation. I just don’t know what that order is. Appreciate suggestions on this.

There was no test non-LED light or voltage showing on the Power Probe clone. The CPS was replaced with a new OEM CPS in July 2022 when we started having the intermittent stalling issue.There is less than 500 miles on the CPS due to the vehicle sitting all winter not running in the garage.