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What causes GM HEI coil failure?

Discussion in 'The Garage' started by madmike, Sep 25, 2006.

  1. madmike

    madmike 1/2 ton status

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    I acquired a freebie, 1983 K10, 305, SM465/NP208. When I got it, it wouldn't start, so I checked for spark at the plugs, there wasn't any. I put in a used coil I had laying around, and it fired right up. I thought, great! No money required! I put maybe 100 miles on it or so, and one day it stalled, and wouldn't start. Again, I checked for spark at the plug, and there wasn't any.

    Well, it was an unknown condition used coil I had thrown in there, so I thought that maybe it just failed because it was old. So, I bought a new one, threw it in, and the truck fired right up. I thought, great! She's good to go! That coil didn't last an afternoon, and there is no spark at the plug, so I know the coil went bad. Now I realize that something is causing the coils to fail. I checked the 'BAT' wire to the distributor cap, the wire is live when the key is on, and dead when the key is off, so I know the coil isn't energized when the truck is not in use.

    I tried looking at the wiring, from the fuse block down to the starter, and I see no obvious signs of damage. The fusible link wire to the distributor cap looked fine. There is a braided ground strap that runs from the firewall to the rear of the passenger side cylinder head, and another black ground wire from that point that seems to disappear into the wiring harness. That wire had cracked insulation, and there was quite a bit of green corrosion damage at that point. I went ahead and repaired that part of the wiring, but I'm not so sure that was the real problem anyway.

    Can a bad ground cause the coil to fail? I know the current has to complete a circuit. What about excessive voltage, like maybe the alternator is putting out too many volts (I did check it, it was 14.5-15 volts), but maybe it is spiking?

    I don't want to throw a third coil on there, not knowing how long it will last, and not being able to trust the truck. What causes a coil to fail?
     
  2. xpndbl3

    xpndbl3 1/2 ton status

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    could be a faulty ignition module. Luckily HEI distributors are dime a dozen and very cheap to replace if you'd like an entirely new one.
     
  3. diesel4me

    diesel4me 1 ton status Premium Member

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    yeah..

    Could also be the pick up coil--often they fail only intermittently,after the engine warms up and gets hot..then after it cools off it will start and run again,at least for awhile--sometimes days,before acting up again!..can be a real butt pain to diagnose,since many other causes have similar symptoms,like corroded wiring for example...:crazy:
     
  4. btolenti

    btolenti 1/2 ton status

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    Do you have the ground strap on the coil????

    DR31A.jpg
     
  5. madmike

    madmike 1/2 ton status

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    I'm not a member, so I can't see the photo you attached. Where would a ground strap to the coil itself go? There are three wires that exit the side of the distributor, and plug into the bottom of the cap. Maybe one of them is a ground?

    The odd thing is, the distributor that was in the truck had a separate 4 wire lead, that plugged into the firewall. I wasn't able to set the timing with that distributor in it, so I replaced it with a common '75-'82 one, and then I was able to set the timing just fine.

    What's even stranger is that two coils have failed, one in each distributor, so I don't think it's a electronic ignition module, or the pickup. I'm leaning towards the wiring.

    Anybody else have an idea?
     
  6. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    I don't know if it applies here , but an old timer told me not to leave the vehicle turned on without it running . He said it nots good for the points/module/etc . I know better , but they still put accesory on the switch for a reason :D
     
  7. chevy wrangler

    chevy wrangler 1/2 ton status

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    one time when my truck had points i decided to leave the radio on and the points fried, tried replacing them and it was easyer to go to HEI and it runs better and i can jam to the radio now!!!LOL
     
  8. bigbadchev84

    bigbadchev84 1/2 ton status

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    I would guess the ignition module, there about $20.oo bucks at the parts store, and they are easy to replace
     
  9. chevy wrangler

    chevy wrangler 1/2 ton status

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    how did you get a truck for free??? thats cool
     
  10. diesel4me

    diesel4me 1 ton status Premium Member

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    hmmm...


    You had an "EST" HEI distributor (Extended Spark Timing)--those had no vacuum advance unit,it was all done by the module,which has 5 or 7 pins,rather that the simpler 4 prong one in older HEI's....and you put an old style HEI back in?--that should be no problem,I did that to my 305 I had in the 79 C-10 I just sold recently,and it ran fine..

    All that style HEI requires is one "hot" wire to the "Bat +" terminal on the cap..the coil has a bent peice of flat steel, with a hook on one end, that goes under one bolt that holds the coil in, to ground it..usually it won't even start if that ground strap peice is left off..:crazy:
     

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