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

1997 Jeep Cherokee new Jasper 4.0L engine, 150, 586 miles. The problem is no start with new OEM MOPAR ignition coil. OBDII is communicating, no fault codes at this time. The Napa AutoCare Mechanics are stumped, on a lark they tried an old used coil that is stamped “1986” under the id number, no brand name is visible; the 1986 coil is the only coil that runs the engine. Put the correct OEM MOPAR coil or an NGK coil and cranks but no start. The 1986 coil has had the primary and secondary resistance tested— primary is .8 ohms, secondary is 12.9k ohms.

Wrenching beyond basics is not my skill-set and with the shop mechanics stumped I’m hoping perhaps someone here can lend some insight on what to do. The mechanic has tried diligently to solve this problem, even making house calls to our remote ranch. We live on a remote ranch an hour outside of the small rural town of Lakeview, Oregon, the closest stop light is 2.5 to 4 hours away depending on which direction you drive. This has been posted on the Cherokee Forum under this same title. So far no ideas that have solved the issue. I am embarrassed because I have watched and downloaded a number of Paul’s videos trying to learn more about our Jeep’s issue but just found this web-site and forum today.

A little more information on our Cherokee. Every sensor and relay has been replaced, the CPS with an OEM. Fuel pump and distributor are new, fuel pressure is within spec. In ignorance I, have been guilty of throwing parts at the problem - I’ve justified that with the vehicles age. Spark plugs and wires are new. Mechanic has verified spark at the plugs. All the grounds have been checked and cleaned to bare metal. The battery ignition, ground cables have been replaced with cables from JeepCables.com. This is the fourth engine. Engine #1 was the OEM, engine #2 was a rebuilt engine at 95 k when the original owner didn’t check the oil (we bought the Cherokee at 100k in 2013), engine #3- was Jasper #1 which was replaced by the shop under warranty due to some failure, engine #4 is Jasper #2 which is currently in the Jeep. Engine #2, the rebuilt engine failed in 2021 with 3 cylinders losing compression. That’s a lot of in and out of engines on the wiring harness. In July of 2022 we were experiencing intermittent stalling and thought maybe the wiring was the issue.

I opened all the loom checking the wires. They were soft, not cooked at all and very pliable. I did check continuity on the wires back-probing the PCM to various sensors, relays and the coil. I was amazed that 26 year wires looked so good. While trying to find the stalling issue we discovered the coil to PCM connector would cause a stall when wiggled. That connector was replaced with a new Summit connector. After wiggling the main harness wires to try and find the stalling issue I re-tapped and re-loomed the main harness bundle.

We thought at one point that the PCM might be the issue. Bought a used PCM that came out of a rusted out running vehicle that was being parted-out, it has the correct numbers for this XJ, which runs the Jeep but doesn’t solve the coil issue. We later found out from the mechanic’s scan that replacement PCM was out of a 2 wheel drive vehicle, I put back the correct 4 wheel drive PCM. I’m not sure what difference the 2-wheel drive PCM vs. 4-wheel drive PCM would make. I was disappointed to learn that even though the PCM had the correct number it wasn’t for a 4-wheel drive, another lesson learned in what questions to ask when buying used parts.


A little more information:

I have carefully measured the connecting coil stud the distributor/coil wire attaches to; I also measured and noted the size of nipple grove on the end of the stud, thinking maybe there was a physical difference between the old and new coils. I have a fairly new set of old Bosch spark plug wires and compared the boot depth between the Bosch wires with the premium Napa wire set the shop installed with the new engine. The stud and nipple grove measurement difference were very minor. The boot depth difference between the Bosch and “Premium Lifetime Napa” wire set showed a measurable difference. That difference gave me a fleeting hope that perhaps I had found the smoking gun to the problem.

Another cautious hope dashed on the rocks; I can’t tell you how often that has happened in this on going patience-building process. When swapping the coils around only the 1986 starts and runs the Cherokee. I did have an observation from my measuring the coils, the 1986 coil does not have a brass connecting stud like the other coils, it’s aluminum. I don’t see why that should make a difference, but it is a difference.

I understand from one of Paul’s videos that it is a small percentage of mechanics that are well versed in diagnostic problem solving. If I knew of a good diagnostic tech within a couple of hundred miles I’d rent a u-haul and dolly and take the Cherokee to that diagnostic mechanic This really is a nice XJ, no rust, never wrecked with a ton of new parts and other mods for our needs on the ranch. I hope perhaps someone here might have idea that will lead to the solution. File this under, XJ Files— the answer is out there… any ideas?.

It's a 2 wire coil. Try swapping the two wires maybe the newer ones are weird backwards.

With the new coil(s) spark is present but the engine does not start, is this correct?

Inversed polarity as mentioned by ferris48 will cause a weaker spark.....a secondary scope comparison between old and new coil will allow to check polarity.

When I wired a coil "backwards" that way ignition tended to fail under higher demand/load, the engine would still start.

Secondary resistance of the old coil seems a bit high... Do you have conductivity in BOTH directions of the primary ? Just to be sure there's no weird diode thing going on, although VERY improbable on 2 wire coil.

I watched the mechanic test spark with the OEM MOPAR, the engine would crank but not start. The mechanic and his father, who worked for Chrysler when Chrysler owned Jeep, took the OEM and NGK coil to town to test and tried both coils in another Cherokee, which I’m told ran with both coils.

I’ll pass on the question about checking polarity in both directions, I don’t know how I would test that, I’m a retired pastor my automotive skills are limited to what I have learned from YouTube channels like Scanner Danner or Eric the car guy. I have tested continuity on both wires from the coil to the PCM and they show green on a simple continuity tester.

To measure resistance both ways you would use the ohm function on your multimeter, put the red probe on one terminal of the coil's primary and the black probe on the other terminal and read the resistance. Then you would just invert the probes on the two terminals and measure again.

You can do the same 2 measurements with the diode function if present on your meter.

Try to investigate wether there is actually a spark when YOUR JEEP does not start with the new coil. This information is crucial.

Question, did you confirm the resistance to be the same on the old coil vs the new coil? If different, by how much? Have you put a spark tester in line with the secondary to see if you have spark during crank no start? As others have mentioned, the polarity matters. I would go from primary #1 to secondary with a resistance test, then go from primary #2 to secondary and record resistance. One set of readings will be just the secondary ohms and the other will add the primary resistance. It's a small difference but with a good meter should be noticeable. Now confirm the new coils match. If none of this shows a difference I would try running jumpers to the primary to eliminate any problems with the connector. Good luck.

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Thanks, I understand you directions. We are going to church this morning, which because of our remote location pretty much takes the whole day after doing some shopping so I’ll follow your suggestions tomorrow. I will report the results, I’ll also measure and report the resistance on the new coils.

I’ll check and record resistances on the new coils along with the old and post the results here. I don’t have a spark tester. I know they are relatively inexpensive, but Napa in Lakeview is closed on Sunday.I think I remember seeing a YouTube on how to do that without a tester I’ll check into that.

As I mentioned to Juergen school, we’re going into town today for church and groceries so I’ll try to get your suggestions tomorrow. Because we don’t feel the Jeep is reliable with this coil anomaly we only use the Cherokee for what we consider important or essential trips into town .

You can use a test light. (Non led) just clamp to the distributor and probe the coil with a gap. Careful not to get bit.

I live a lot closer to parts stores but it might be 100 miles one way or 150 miles another way to find a good church. Lol

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juergen.scholl wrote:

Try to investigate wether there is actually a spark when YOUR JEEP does not start with the new coil. This information is crucial.

You can use a test light. (Non led) just clamp to the distributor and probe the coil with a gap. Careful not to get bit.

100% this. Use the test light and see what strength of spark you can get out of both coils. You may have already watched it, but this video demonstrates the idea:

Quick reply, Wow—thanks for the helpful instructions, I’ll print out all your comments and get to work tomorrow. We really need to try and come to answer to this situation. Thanks again …

Regarding resistance numbers and spark testing:

I was able to get a Lisle In-line Spark Tester, #20610. I placed the tester between the distributor and coil, the 1986 coil produced a very bright spark. None of the other coils produced a visible spark. Obviously this the issue the $64,000 question is why? I tested each coil identically, mounting the coil to the engine bracket with the ground wires attached, like I said each test was made exactly the same. This really seems to be a mystery to me.

Resistance numbers: I tested the resistance numbers following a chart from the Cherokee Forum on how to test resistance combined with the suggestion here to record the values for both coil pins.

Primary Resistance Secondary Pin 1. Pin 2
1986. Coil. 1.0 ohm. 12.23 k ohms. 12.23 k ohms
5/2020 OEM Coil. 1.1 ohm. 12.96 k ohms. 12.96 k ohms
8/2022 NGK Coil. 1.1 ohm. 12.36 k ohms. 12.36 k ohms
6/2023 OEM Coil. 1.0 ohm. 13.21 k ohms. 13.21 k ohms

The reason I have so many coils. In 2020 we experienced stalling with no start with a fault code I read with a basic Autel that said ignition coil, I didn’t keep the fault number. In ignorance, I bought a new OEM coil, fixed the problem— so it seemed. In 2022 after the Jasper engines were installed began having intermittent stalling again, also had an ignition coil fault code. I’d read part quality was sub-par and figured the 2020 coil was bad. At the time NGK was the only RockAuto choice available. When the 1986 coil started the engine, it was suggested to buy a new OEM as the life expectancy of the 1986 was unknown. I bought an expensive OEM from RockAuto, $200, thinking if this solved the problem it was worth it. That turned out to be wrong thinking. By the time it was discovered it didn’t work it was too late to return it to RockAuto.

The 1986 coil has lower resistance numbers and visibly has an aluminum stud instead of brass, that is the only known differences at this time. I hope this information is what was being asked for. I really appreciate everyone’s thoughts and ideas to this dilemma. It is a mystery to me why there is spark for the 1986 and no visible spark for the other three.

So your meter (most meters) will not give you the granularity to see which wire is common. My fault.

I would rig up some jumper leads to pin 1 and 2 then try it and then reverse it.

And one final test is to see if the mounting bolts are grounded. Just go from the frame to all the terminals and see if you have continuity.

Cheers

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I don't understand what you are suggesting, would it be too much trouble to explain it a little more for me? I don't understand your suggestion about using a jumper cable to the pins. Regarding grounds, I don't know if this is what you mean, but I have a Chinese knock-off Power-probe instrument that shows all the block ground points green, including the block coil bracket mounting bolts. All the ground points have been cleaned to bare metal.

Working on vehicles is not my skill-set, I'm trying to learn but I don't have the background to grasp your suggestion... sorry. If you have the time and can further explain I'd greatly appreciate it.

Don't worry I'm not exactly a mechanic myself. The jumpers are so you can reverse polarity on the primary coil.



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As for ground, I am curious if the housing for the coil is part of the circuit.
So testing the housing of the coil to the electrical terminal on the coil would tell you if it is "grounded".


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I’ve been told the coil housing is not part of the circuit. It has been suggested to make a jumper and reverse the wiring at the connector to the coil. With my lack of knowledge I’ve been cautious of doing that out of fear of damaging the coil. However, after seeing how there is no spark to the new coils I’m ready to try the jumper idea tomorrow. I’ll use the NGK coil first to see if reversing the wires will fire that coil. I’ll report back the results.

I asked the mechanic today if he thought if somehow the new distributor and cam sensor wiring could be related to this? Maybe a wrong part or wiring that was correct for the 1986 coil, I’m reaching for ideas to understand why the 1986 sparks and the spec’d coils doin’t. I appreciate each view and all of your various comments. Somebodies comment will have the solution or stimulate an idea that leads to the solution.

6-22-23 Update:
One of the mechanics had me identify which pin in the coil connector had 12 volts with a back-probe. That was pin #1 which is the pin closest to the engine block when the connector with the back-probe is plugged into the coil. With the 1986 coil installed the voltage read, 13.95-14.08 volts with the engine running.

I then installed the new OEM MOPAR coil, with the engine cranking—no start, it read 10.56-10.70 volts. The question is what is causing the voltage drop to the new OEM and not to the 1986 coil? I used my Chinese Power Probe clone to measure the voltage.it was the easiest way to reach into the back-probed t-pin.

I’m waiting for the mechanic to respond and hope maybe one of you also might have idea(s).

mmorris923 wrote:

what is causing the voltage drop to the new OEM and not to the 1986 coil?

It's because the starter is working. Once the vehicle fires up, the alternator will re-charge the battery and that's why you have 13+ volts on the running car.

With the 2-wire coil removed, one of those 2 wires will briefly have battery voltage on it when you cycle the key to ON. That's the power feed and the other wire's the control wire. It will have no voltage on it when you cycle the key. Remember, coil unplugged.

Backprobe this control wire and plug the coil back in.

Cycle the key again. The control wire should get battery voltage on it too if the connector from harness to coil is any good. Does it?

Have you tried reversing leads? Here is a good diagram of your ignition system. You can see if the center tap is reversed the primary and secondary coils will no longer be in series. They are now in parallel. The collapsing field will be fighting the wrong way.
Cheers

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