504 coil-related near catastrophe
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  1. #1
    Fellow Frogger!
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    Default 504 coil-related near catastrophe

    The blue Bosch 12V coil on my 74 504 GL has always been mounted on the alternator arm near the dizzy and the carb, an arrangement I've seen on several 504s. In preparing to install an electronic distributor from a 505, i first decided to test the coil from the 505 (disconnected from its electronic control module) and to temporarily mount this on the passenger firewall as this was the best fit. On a test drive the car stalled and to avoid having to walk a longer way home I coasted (with the ignition in the on position) for about a mile, maybe 60 seconds. As I came to a stop, however, thick, grey, burning electrical smoke started pouring out the hood . I'll cut to the chase and tell you that the coasting without the engine firing delivered current to the coil that could not be discharged through the plugs, apparently overloading the low voltage ignition circuit, fusing the points in the dizzy, overheating and melting the wires in the coil, and damn near burning up my 504. Had the coil been mounted to the alternator arm, inches from the fuel pump and carby as usual, the intense heat and smoke might have turned into a spectacular 504 explosion. Needless to say, when the blue Bosch went back in, it went onto the firewall and not the alternator arm. I'm going to think about packing a fire extinguisher as well.

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    As an aside, the metal case from the 505 coil, after removing the burned up coil and ECM, made a very nice firewall-mountable holder for the blue Bosch coil and the new ECM that now link to my electronic distributor -all's well that ends well

  2. #2
    Fellow Frogger!
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    Default

    Quote Originally Posted by rpieper View Post
    The blue Bosch 12V coil on my 74 504 GL has always been mounted on the alternator arm near the dizzy and the carb, an arrangement I've seen on several 504s. In preparing to install an electronic distributor from a 505, i first decided to test the coil from the 505 (disconnected from its electronic control module) and to temporarily mount this on the passenger firewall as this was the best fit. On a test drive the car stalled and to avoid having to walk a longer way home I coasted (with the ignition in the on position) for about a mile, maybe 60 seconds. As I came to a stop, however, thick, grey, burning electrical smoke started pouring out the hood . I'll cut to the chase and tell you that the coasting without the engine firing delivered current to the coil that could not be discharged through the plugs, apparently overloading the low voltage ignition circuit, fusing the points in the dizzy, overheating and melting the wires in the coil, and damn near burning up my 504. Had the coil been mounted to the alternator arm, inches from the fuel pump and carby as usual, the intense heat and smoke might have turned into a spectacular 504 explosion. Needless to say, when the blue Bosch went back in, it went onto the firewall and not the alternator arm. I'm going to think about packing a fire extinguisher as well.

    As an aside, the metal case from the 505 coil, after removing the burned up coil and ECM, made a very nice firewall-mountable holder for the blue Bosch coil and the new ECM that now link to my electronic distributor -all's well that ends well
    Frack me.Happily alls well that ends well.

  3. #3
    Fellow Frogger!
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    Coils from points vs electronic systems differ in their primary resistance.
    A very low primary resistance coil (from some electronic systems or points system with the series resistor) will overheat if the points are closed continuosly, such happened in your case.

    Cheers

  4. #4
    1000+ Posts Beano's Avatar
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    Far out !
    The alternator arm is not the perfect place for a coil anyway, as it gets a lot of engine vibration there.
    Good to know you're fire-aware. Always keep an eye on the piece of fuel hose between the pump and the carby, in your car and any friends' cars. It's slightly pressurized in there and bad ends (hose clamps done up too tight for a few years) or a small split can cause fire, being right near the dizzy. I've prevented fires in a few cars by pointing out bad hoses.

  5. #5
    1000+ Posts 504-504-504's Avatar
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    Went to set off for work one morning in 504 but it wouldn't start.
    Completely dead, not even a click from the starting solenoid.
    Lifted the bonnet to find that the wire running from alternator to battery was now a collection of copper ingots lying on the block.
    It had to happen, alternator was shorting out, lights flickering at idle.
    Like Russian Roulette, this time the alternator stopped in the shorting position.
    Was amazed that the car didn't catch fire.
    Pays to keep a clean engine bay.

    Paul

  6. #6
    1000+ Posts Beano's Avatar
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    You've got me worried about my 505 now ! Is it practical to put a fusable link there, in series ?

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