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91/92 MJ Premature Ignition Coil Failure Fix / Relocation


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I know not why, but I've had three ignition coils go bad over the last two years. After installing a new coil, over time (six months or so) the secondary resistance increased 3K-4K over the FSM spec and the coils became intermittent, causing hard starting and occasional engine cutoffs. These were NOS Mopar coils too, not aftermarket junk. Replacing the coil always cured the problem - for awhile. In researching this premature coil failure problem, I found TSB 08-35-93 that addressed 93-94 Jeep ZJ 4.0 coil failures. The TSB fix was the installation of Mopar Ignition Coil Package P/N C3905840 which consisted a new Mopar 4762312 coil and a resistive feed harness that lowered and regulated the primary DC coil input voltage. This harness was incorporated into the 95 and up ZJ engine harnesses, so it must have been successful. If it worked on the ZJs it will work on similar year 4.0 XJs and MJs since they used a coil with the same primary and secondary coil resistance specs. I lucked out and found a C3905840 coil package on Ebay and snapped it up. Installing the harness is cake as it includes a new connector for the existing 2-wire coil connector (simply remove the wires and snap them in to the new connector), then plug in the new harness. An easy PnP mod, and even the wire colors matched my 91 MJ.

Mopar  Ignition Coil Package:

 

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Next, I always thought it was a stupid design on Chrysler's part to mount the ignition coil directly to the hot engine block on the HO 4.0 MJs and XJs. All that heat soak can't be beneficial to the ignition coil. So I decided to relocate the coil to a cooler spot in the engine bay. The right front shock tower looked like the best available spot. For mounting the new coil vertically I picked up a new ZJ ignition coil and mounting bracket kit on Ebay for $17.50 shipped. I didn't need the coil that came with it since the pins didn't match my new harness coil connector, so I later sold it on Ebay for $21. WIN! I also had to order a slightly longer ignition coil primary wire that I will install later when it comes in.

ZJ Coil & Bracket Kit:

 

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New Harness with Coil and Bracket Installed on the Shock Tower:

 

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I'm confident this mod will make ignition coil failure a thing of the past. An added bonus has been a smoother idle, as smooth as it can be with the Hesco cam, and much less engine turn-over time when starting. It starts almost instantly now.   :)

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I think even the major auto companies are sourcing a bunch of their older parts from China.  Unfortunately, I don't think the quality assurance standards are the same as the parts that go into new production 2016 and 2017 cars.  Replacement parts are just another profit center watched over by bean counters only.  That means C.R.A.P. with OEM Mopar name tags.

 

Good catch on Ebay! and a nice write up.  Moving the coil away from the block was a smart idea.  My 1969 Corvair had the coil positioned away from the engine block from the factory as the smog equipment increased engine operating temperatures from 1966 to 1968/9

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I think even the major auto companies are sourcing a bunch of their older parts from China.

 

True. You can see brand new Mopar ignition coils on Ebay stamped "Made in China" carrying a Mopar p/n. They were previously made mostly in Japan. I won't buy a coil that's made in China or most anything else if I'm aware of it. China's not there yet, just like Japan used to be.........

 

Don, mind taking another picture but zoomed out a little? I'd like to see how the wiring is routed now.

 

Hard to do because the hood won't go up any higher. For now it's just tie wrapped to the battery cable harness then branches off to pick up the original coil wire connector.

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I'm also curious why the engine would start quicker now too as I thought the primitive  ECU was the culprit for long cranking....

 

 

Well dang, that's something I've never even heard of. Interesting that it makes your engine start faster now.

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