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Thread: Auto-Electrical Question

  1. #1

    Default Auto-Electrical Question

    Anyone have any idea why I keep frying my ignition coil? I've gone through 3 ignition coils in 2 weeks.

    It's a 1993 Jeep Grand Cherokee Limited V8.

    I'm taking it back into the shop Monday, but I'd like to have an idea of what is going on here.

    TIA

  2. #2
    Plumber jimbo's Avatar
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    No clue. I do the plumbing, and pay good money to my ASE certified tech to fix the truck!

    But stay tuned, because no matter what the subject, there is probably an expert out there who will post in,

  3. #3
    DIY scratch-pad engineer leejosepho's Avatar
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    On some vehicles, there is a resistor for lowering voltage to the coil during normal operation, with that resistor being bypassed during starting. You possibly have too much voltage at the coil, and that could also be caused by a bad regulator in your charging system. Or, maybe some other ignition component is demanding too much current and overworking the coil.

    If the three coils in the past two weeks have been replaced by the same parts changer(s), it is time to find a troubleshooting mechanic who ultimately knows which part(s) to actually change.

  4. #4

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    Only one of them was replaced by my regular mechanic, the other two happened on my recent aborted vacation trip.

    The first one fried 2 seconds after the Jeep started. The other two fried on a bumpy hwy as I was doing 60-70mph.

    I'm sure my mechanic will do about $500 worth of diagnostics. I just wanted to have an idea what might be causing this.

  5. #5

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    Lee was on the right track...

    If you've got the 4.0L engine and 42RE transmission, there was a problem with these vehicles (which many indy shops don't know about) resulting in the premature coil failure you're describing. Most though had long ago been repaired under warranty. The fix is to install an upgraded coil and harness package, Chrysler part numbers 4762312 and 4762311, repsectively. Won't set you back too bad, both items should total around $115 list rom your local Chrysler dealer. Book time on the job is only 0.3 hours.

    Basically, you'll be installing an upgraded low resistance coil, and a "jumper harness" which contains and external resistor like Lee described.

    If you need detailed instructions on how to do this (they might be in the 4762311 harness pack, if I recall), I can step-by-step it for you. It's also good idea to disconnect the negative battery cable to "clear" the PCM.

    Problem should then be solved. Does your A/C still work, the evaporators on those are high failure too.

  6. #6

    Default Car Talk

    It's actually the 5.2 L V8 engine. Not sure about the trannie. The Jeep is at the shop right now. Does that info apply to the larger engine as well?

    I don't know if I'd classify this as "premature coil failure". I bought the vehicle 2 years a go and never had a problem 'til now. But now I can't go more than 20 miles without the ignition coil burning up.

    Other research and forums have suggested a bad voltage regulator, bad ignition switch, a short in the wires either coming in or out of the ignition coil, or a bad distributor cap.

    The A/C evaporator has been replaced, so you're right on with that one. Unfortunately, some seals are leaking, which I haven't addressed yet. Also, the speedometer stops working and only begins working if I bang on the dash. I was told the sensor is working fine and that I need a whole new instrument cluster.
    Last edited by Verdeboy; 10-07-2007 at 01:58 PM.

  7. #7
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by Verdeboy
    I don't know if I'd classify this as "premature coil failure". I bought the vehicle 2 years ago and never had a problem 'til now. But now I can't go more than 20 miles without the ignition coil burning up.
    A coil should last for the service-life of the engine, and your premature failures, caused by whatever, are now simply happening more frequently.

    I have an old rusty-trusty pickup that was given to me when it was about 15 years old, and two years and $1500.00 in parts later, I finally got it out on the road. About 5 years after that, the ignition module quit working right after the first frost of the year, and that happened for each of the next three years. I replaced the module each time, and each module lasted for a year until the next year's first frost ... and all of those premature failures seem to have somehow been caused by a bad distributor cap. Go figure, eh?!

  8. #8

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    Quote Originally Posted by leejosepho
    A coil should last for the service-life of the engine, and your premature failures, caused by whatever, are now simply happening more frequently.
    I just figured the first one went naturally after 14 years. Then I thought the second one might have been defective since it only lasted a week. The third one, however, only lasted 20 minutes.

    So, if we describe the one that lasted 14 years as a "premature failure" I just thought we needed a different expression for the one that only lasted 20 minutes. Expressions with lots of 4 letter words come to mind.

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    The voltage to the coil is normally about 6vdc when running, and is the full battery voltage when cranking. If the coil gets too much voltage during normal operations, it's like putting a 6v bulb in a 12v system...doesn't last long. Electronic ignitions are changing all the time, so it's hard to tell exactly what that should be unless you deal with it all of the time. If the resister is external, it could be bad, or the harness is messed up, or the ignition switch (as was already mentioned) is not switching everything back so the coil is getting the reduced voltage. Might be a relay that is stuck. You'd need the manuals to scope it out for yourself, or someone who works on that model often enough to know for sure.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #10

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    I suspect you have a MWBC and a loose neutral.

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