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TEXAIR 06-14-2013 09:45 PM

Blowing through ignition coils- 96 4.0
I have started having problems with blowing coils on my Jeep.

I have a 1996 4.0 stroker installed in my YJ. It ran great for a couple of years. Then in December it blew the coil. I installed a new one and by February it did it again. Since then it has blown 2 additional coils.

I am wondering if there is a problem in the wiring leading to the coil. I know it goes to terminal A7 on the ECM.

My question is how do I go about checking the wiring properly?
What voltage should the wires have?

I am pulling my hair out here. Any help would be appreciated.

Que89YJ 06-17-2013 08:43 AM

You should try running a hot spark coil. There is a resistor wire on the stock coil setup but in your case get a MSD that is rated for 12 volts.

TEXAIR 06-17-2013 10:08 AM

So why would the factory coil keep blowing out? And would the MSD coil burn out also?

Also, where is the resistor wire located you are talking about?

Que89YJ 06-17-2013 11:10 AM

My guess is the resistor wire has been removed. When you go to start the start run circuit cuts in a full 12volts. When you let off the key and you are in run the voltage to the coil is supplied tthorugh the resistor wire and the coil should only be supplied with 8 volts. It was done to increase the life of the coil but still allow you good starting power. Without the wire the 12volts causes premature coil failures like you are experiencing. The MSD is designed for a constant 12 volts so will not fail. Check to see when you are at ignition on and then go to start at the coil. If your voltage stays high and doesnt drop down then you are better off with the MSD because it has a higher step up voltage for a hotter spark. You can gap out the plugs widers and get a better burn. With your stroker how did yor recalibrate for different air fuel on a stock ecu?

idaholtby 06-17-2013 01:44 PM

Que,...I'm confused. I am aware of a resistor wire being used on older CJ's and pre-OBDI rigs, but not on the later model years. I've checked the schematics for both a Grand and XJ '96 and neither use this setup. Both run and start feed the full 12vdc to the coil(one being battery, the other straight off the alt output), or am I missing something? To the OP, have you measured the primary and secondary resistance values on your failed coils to see what failed? Being what failed? The Primary or Secondary on the coil? Curious.

idaholtby 06-18-2013 03:18 PM

Any updates? Have you tested the failed coils primary and secondary with respect to their resistance value? How does this compare to a new coil? This info may help lead to the cause.

Que89YJ 06-19-2013 06:38 AM

You are right about the resistor wire. Still the only reason for the coil to blow is too much current. Maybe they are giving him the older coil. The higher energy coil will work better because it offers a higher resistance to the secondary with a larger cap so the current decreases. I am wondering if it is failing on the primary or secondary too.

JWELK 06-20-2013 11:31 AM


Originally Posted by Que89YJ (Post 15580504)
You are right about the resistor wire. Still the only reason for the coil to blow is too much current. Maybe they are giving him the older coil. The higher energy coil will work better because it offersa higher resistance to the secondary with a larger cap so the current decreases. I am wondering if it is failing on the primary or secondary too.

Aren't these coils "autotransformers"? If so, there is no real secondary, it is a single coil with 3 taps, ground, 12 volts and the output.


Jim W

Que89YJ 06-20-2013 12:31 PM

Jwelk I didnt think auto transformers were used until the DIS packs. The function and end result is still kind of the same. There is a resistance check for primary and secondary. If I get a chance I will check into it more. The auto's are actually grounding through the batter positive. You can get into some weird stuff with them. The failure is still the same though. High current, low resistance and the windings pop like a fuse.

JWELK 06-22-2013 08:30 AM

Que: I only brought it up as while he can still measure the resistance, depending on the location of the open, the reading might be difficult to interpret. There are 3 wires correct? You should have resistance between all 3, lower resistance between 2. The distributor output should read high to one of the inputs and higher to the other.
Resistance wire, what will they think of next? What was wrong with the "ballast" resistor from days of yore? I think your diagnosis of the wrong primary wire is correct.
And yes it is "grounded" to the + side. The "real" ground is through the spark plug.


Jim W

TEXAIR 06-22-2013 09:58 AM

Sorry for the delay in answering. Been traveling and not checking the site.

The secondary of the coils have been burning up. I finally found a shop foreman that was knowledgeable and discussed the problem with him.

First, I have been using after market coils. I still don't understand what the difference is, but I had a new factory coil ready to go so I will be putting it in. I have also checked all the wires and there are no shorts coming or going.

Second, he asked me if I ever seen a specific code from the ECM. (P0351) I said I had and he is telling me that is the beginning of the ECM failing. Funny thing is he tells me the computer may have been damaged by the first coil shorting out.

I have ordered a used ECM and it will be here next week. When I get it I will be installing it as well as the factory coil. Then I well take readings to see if it is good.

I will report more at that time.

Thanks guys.

Que89YJ 06-22-2013 11:30 AM

no p code for a yj coil. It is low side driver so there is no way to damage the ecm from a short. If it is secondary that is unusual. Short or low resistance in a plug or wire or a bad coil with weak windings. I don't think you want to waste the money on a ecm. Get a msd coil or a oem. Keep us posted.

Noltz 06-22-2013 05:06 PM

Disagree Que. I've seen plenty of Ford ECU's get taken out by bad coils. The internal primary windings short, causing low resistance and high current, damaging the drivers in the ECU. Also seen this on Caravans, but it's the injectors that burn out the drivers with low resistance. Had 4 ECU's in a row come in, all had one or another driver no good. Fifth one was fine. Not enough YJ's around here for me to have seen many in the shop, but the theory is sound.

Que89YJ 06-22-2013 06:42 PM

Tested the drivers in the lab and ran the short and open test and each pin on the ecu. You can dead short any pin on the controller. Just don't load dump it more then 3 times.

TEXAIR 06-22-2013 07:01 PM


I have already ordered the ECM. But based on something you stated, I guess I need to look closer at the plug wires or even the plugs.


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