Fixed: 5.9 '98 ZJ died at stop light - Ignition coil issue
I used this forum a lot for solution ideas on my recent ZJ issue; I'm posting what I saw, how I approached the problem, and what resolved the problem, for anyone else who encounters a similar issue.
I was driving the ZJ to work and was idling at a red light. The engine suddenly died. Attempts to re-start failed; the battery voltmeter indicated a nice >+12V, cranking the engine showed the typical voltage dip and restore on the voltmeter. The fuel gauge showed >75% full. The starting motor could be heard clearly when cranking, the engine belt would spin with the crank, but the engine simply would not catch and fire up.
I pulled a spark plug and laid it atop the engine block. I then cranked the starter and watched the plug. I saw a small spark when cranking. The spark had a orangish-hue. I was able to smell gasoline when cranking (this was due to the valve's spark plug opening not being plugged, so the vapor of the gasoline injected into the valve was able to exit the engine block). This told me 5 things:
1. There is ignition voltage reaching the spark plug
2. The fuel injectors are at least partially functioning and providing fuel when cranking.
3. The distributor is at least partially delivering energy to that particular spark plug.
4. The ignition coil is at least partially energizing and creating high voltage from its 12V supply.
5. The PCM is at least periodically controlling the ignition coil (12V relay pulsing the coil to generate the high voltage).
The orange-ish hue coloration of the spark is not ideal. With a 0.04" gap, through air, this means that the voltage is probably around ~5kV-20kV. Orange coloration of the ionized air means a relatively low amount of energy is crossing the spark gap.
First thing I did was replace the spark plugs. The originals looked OK but since these are wear items, and I hadn't replaced them in a couple years, it could only improve matters. Re-testing with the new plugs still resulted in a orange hue to the spark.
I proceeded to remove the source cable from the distributor cap. It's the cable which attaches in the center of the cap; the distributor rotor spins to electrically connect each spark plug node cable in sequence, with this center node. The other end of this cable remained attached to the ignition coil.
I plugged a spark plug directly onto this source cable and laid the plug on the engine block. Cranking the engine resulted in the same orange hue. I was still able to smell gasoline vapor when cranking (due to the spark plug hole not being filled, the vapor was able to exit and demonstrate that fuel was still being injected into the engine during cranking, and that the previous gasoline smell was not simply due to residue in the engine).
This told me that the voltage reaching the end of that cable was too low. I disconnected the cable from the ignition coil and ohmed it out (using a ohmmeter). I recorded ~20kOhm (on other cables I've seen as high as 70kOhm). High impedance is typical for this type of cable, which is used for transferring high voltage and not necessarily high current (probably 0.1mA at most). This told me that the cable was very likely OK.
At this point I knew something was most flaky with the ignition coil itself. I removed the 2-wire signal connector from the coil assembly. I attached a voltmeter across the two terminals in the harness-side of the connection (the mating half of the connector which is not on the coil assembly). I cranked the engine and saw a ~12V pulse when cranking. This told me that ample energy was reaching the coil to energize it properly.
This told me enough to spring the $35 for a new coil. I replaced the coil and re-tested with a spark plug directly attached to the ignition coil, using the source cable (which would otherwise be used to attach to the distributor cap). I laid the spark plug on the engine block, cranked the engine, and finally got the blue hue spark I was looking for (with a 0.04" gap in open air, this indicates ~40kV-100kV+).
I re-installed the spark plug, reconnected everything (ignition coil signal connector, ignition coil to distributor source cable), and cranked the engine.
Finally got ignition. Problem resolved. Good luck to anyone else who encounters this type of problem.
DD '98 ZJ 5.9
fun '91 YJ 4.0 stripped for mudding