CesarBartz

Folks,

Bought my '96 ZJ (V8) and two weeks later the ignition coil died. Funny that the resistance test with the ohm meter would say it is OK, but not sparking at all.
Replaced coil and before installing, measured the winning resistance and all in line with the service manual (Chrysler manual).

About 1-2wk and failed again. This time engine started to stall every now and then and in a matter of 2 days it got worst. Would not start and seconds later, works well. Run 1min but hesitatingly at high rpm (>4000). I had a replacement emergency coil (from another car, very different in look) and once replaced, all working OK, which gives me the impression the ignition system is OK...

I am a mech engineer and messed around a lot with old car in the past. Find very strange this repeated failure....

Car is used, so maybe some Muppet changed the ignition circuit. Any resistor in this engine? Any chances of my system being "mean" to the new coil?

New coil is some cheap "imported", the only I could find quickly. I have a decent one on the way to my home (Magnetti Marelli) but am afraid of burning it too!

Thanks!

dave1123

Welcome to CF if someone hasn't already said it!

Thinking back to my classic car days, most of the 60's cars had resistors in the coil feed to reduce the input voltage to around 9 volts when running and 12 volts when cranking. I haven't played with jeeps enough to know if that's the case with yours or not. The ones I remember had 2 wires going to the coil primary, one from the resistor or resistance wire, and one directly from the starter solenoid, hot only when cranking.

You can try installing a resistor in the input wire to drop the voltage to 9 volts and see if the engine runs okay or not. I'm pretty sure GM and Mopar had resistance wire sections in them. Late 50's Fords had a resistance coil in an aluminum cage next to the coil. This stuff all started with the change-over to 12 volt systems.

Yes, it sounds like your coils are getting too hot from overvoltage. The Chrysler manual should tell you if you need less than 12 volts to it.

I just did a little checking myself and it seems the coil supply current comes from the ASD relay, so maybe the proper jeep wire has resistance built in. IDK.

CesarBartz

A bit more info... may help others.

1) Shame on me... as an Engineer I should have thought deeper and read more carefully the Manual - Chrysler tells "if you are replacing the coil, check the spark plug cables; if the cables are arcing, new coil will fail". Got myself a new set of cables and distributor cap (car is 20 yr; cabe was replaced some 8yr ago and cap no sign of ever being replaced).

2) I had both failed coils tested on the resistance of primary and secondary and both were ok, but one coil had just a 1-2mm spark and the other jumped some 10-12mm, but very weak spark. This was using a fully charged battery and manually switching the coil primarry with 12V.

If the problem was too much time on (say a bad PCM - aka "ECU"), I would expect the primary winding on both to be "cooked" and shorted (less resistance) or in an extreme condition to be open, as too much time on would likely to be a DC failure and then pure heat build up on the winding.

As the resistance value of both primary and secondary is ok, now I suspect the insulation has gone and then my vote is for the secondary. So, when a spark should be produced, it actually sparks "internally" on the secondary winding as the faulty cables let too much gap and then too high voltage built up on the coil.

Cut one of the coils with a hack-saw and the copper wires seemed to be "OK" on the primary. Secondary ones are too thin to be seen with naked eye.

While I wait next coil and cables to arrive, I am using an old-style aluminum "can" oil filled coil from a GMC 250CID 6Cyl engine from my ex-boat. This engine used electronic switching ignition with traditional distributor (like the 1st generation after breaker point contact and condenser) and worked well. with 6cyl, my guess is it would handle the V8. It actually works fine for 30min and then stops to misfire a lot and stalls. Too hot coil... wrapped some thick layer of toilet paper on the coil and poured water. the wet jacked cooled the coil and in 2min car is back to life... ok temp fix for my very short home-work-home commuting.

dave1123

Very good! Actually, shame on me! I knew about internal arcing from my experiences over the years and I must be getting senile to have forgotten about it. Good catch on your part!

The coils in the GM HEI distributors used to get hot and short if the wires were bad, basically because they had no heat sink. Also carbon-trailed distributor caps and rotors were common. I found one where the rotor had a hole blasted in it where it shorted to the rotor shaft.

I'm sure you have enough knowledge to know this, but find a distributor cap with BRASS contacts in it, not the cheap aluminum. They last a lot longer.

RJCannonZJ

Just a note for you guys, not really anything to help OP with his problem.

In old point/condenser distributor systems, the voltage drop was necessary to keep from frying the points, that's all. Modern systems can run the higher voltages, as long as the coil is sized so it won't overheat/ has a heat sink.

Old drag racing trick was to gap your plugs as big as possible, have a second motorcycle battery mounted in the car, at full throttle relay would put 24v to the coil. Much more powerful spark to burn all the extra fuel. For short 1/4 mile drags, coil would never overheat.

dave1123

I've seen finned aluminum cans that fit on the coil as well. I had a Mallory Rev-Pole distributor on my Chrysler Hemi that used dual points, dual condensors, and an epoxy-molded coil. The idea was to reverse the polarity of the coil every other spark, allowing a complete breakdown of residual magnetic fields. The idea was good, but the coil cracked from the heat. Twice! I went to a magneto.

CesarBartz

dave1123, RJCannonZJ thanks for the replies.

PROBLEM SOLVED!!!

The female connector on the socket (car harness side) was with the opening too wide. I was relocating my coil to the inside of the cowl (fresher location, good anyway) and in the experiments got to touch the low voltage harness while engine was running and it stalled. Then I started again and moved one wire at a time and found the green one was a bit loose, pulling to one side would kill the engine.

Opened the connector and gave a "hug" on the socket with pliers and tested each one separate on the coil pin for interference.

Learned something - old-style and individual flat connectors are not waterproof but you feel the interference every-time you connect/disconnect. Multi-way and gasketed connectors are hard to "diagnose" as the other ways and/or gasket create mechanical resistance you feel while moving them.

Will post the coil relocation as a "mod" suggestion.

dave1123

Just make sure your engine-to-body ground strap is good and it should be fine.