jeep 4.0 going through ignition coils weekly

[brokejeep]

I recently replaced my 4.2 L in my 81 CJ7 with a 4.0 out of a 92 grand Cherokee. The head gasket blew in the motor which in turn caused a knock and rather than fixing it I swapped in a 97 grand Cherokee 4.0.
I used a hesco wiring harness. The ingnition coil in my jeep will work for 30 minutes to 1.5 hours then my jeep will die, the jeep will not start unless the coil in replaced with a new one. I am wondering if I maybe hooked up the alternator wrong???? What could cause the coil to heat up and stop working so frequently?
I first thought ground but I have checked everything and still nothing I have gone through several coils this week trying to resolve the issue and Napa is not going to value my year warranty any longer. Any help is much appreciated
thanks

 

[DaveB]

I would call Hesco maybe they have seen the same problem before. Some wiring systems add resistance inline with the coil power feed to reduce the current going through the coil. Sometimes it is done with a resistor and sometimes the wire feeding the coil is resistive. On my old chevy truck they bypassed the resistor when you were starting the motor to give it a stronger spark but once it was running the resistance was switched back in to save the coil.
When I did my 4.0 swap into my cj I used an XJ wiring harness. I could look at the wiring diagrams to see if they use a current limiting ressitor.

 

[brokejeep]

I called Hesco and they had no Idea. They told me that it was something I had hooked up wrong or a bad ground. The plugs only go one way on the harness and I have checker\d my ground and replaced the ground strap. I was told on cjoffroad to check if I have a ballast resister, as far as I am aware the coil on the 4.0 has a internal ballast resister. Do you think it is possible the alternator is wired wrong and it is sending too much power through my harness and not tot he battery? Is the ignition system different between the 92 I was running and the 97 I am running now?

 

[UNSTUCK]

I had the same problem one time. After swapping to my 350. The longest I was able to make it run was about 10 minutes. I "upgraded" to an MSD high performance coil. The same thing happened. I went back to a Carquest coil, same thing. Then a Napa coil. Same thing. Just for kicks I went to the junk yard and pulled a GM coil off an old Astro Van. Works just fine now and I have 50 to 100 hours on it.
If you're sure you have good parts, I'd double check your tach signal wire. Make sure it is not grounded coming off your coil, if that's where the pick up is on this engine.

 

[1993yj]

In my experience with coils on the 4.0's, the aftermarket ones only last a few days. I went through a few in one week after the original finally went out. Went and got one at the dealer, and I am now running 6 years on it.

Unfortunately there are just some things that you have to get through the dealer, and the coil seems to be one of them.

 

[DaveB]

I called Hesco and they had no Idea. They told me that it was something I had hooked up wrong or a bad ground. The plugs only go one way on the harness and I have checker\d my ground and replaced the ground strap. I was told on cjoffroad to check if I have a ballast resister, as far as I am aware the coil on the 4.0 has a internal ballast resister. Do you think it is possible the alternator is wired wrong and it is sending too much power through my harness and not tot he battery? Is the ignition system different between the 92 I was running and the 97 I am running now?

The only way the coil would draw more current is if the coil resistance dropped or the voltage went up ( Current = Voltage/Resistance ). If the alternator was causing the voltage to go too high for some reason things would start to fail.
The ignition system is different between a 97 and a 92, how much of the 97 ignition system did you use ? I have some spare coils laying around that I could measure the resistance on if that would help.

 

[brokejeep]

That would be great if you could measure the resistance on the coils for me. The only part that stayed 97 was the distributor. I didnt know if it I would work so I left it in in tried. I still have a distributor for the 92 if you think it would be better. This problem has happened since i first swapped motors. My 92 was only on the road for 7 days before it went bad so It only happened once, I thought it was because of the cheap checker coil I used. Now this enginehas been in for 4 weeks and continues to burn up coils. I will attempt to get a stock coil and see if that helps. Thanks for all the input.

 

[timpanogos]

On my old chevy truck they bypassed the resistor when you were starting the motor to give it a stronger spark but once it was running the resistance was switched back in to save the coil.

On the older motors, this reduction of voltage/bypass at startup was used to prolong the life of the points, not the coil