> > > >IF the spark plug fires then it will clamp the primary to a lower voltage > >based on plug voltage and transformer turns ratio (and other 2nd order > >factors). If the plug does NOT fire then primary bvoltage may rise VERY > >high. In an automotove circuit you MUST design for somethng to dissipate > >the energy when the plug mis-fires. > > Or when the mechanic pulls that plug wire to see which cylinder is > misfiring. Wouldn't THAT be a tragic design? :) > > Actually I've seen a failure that turned out to be very similar to that. It was a '86 Buick skyhawk (IIRC). No spark, test light on the coil primary blinked when cranking- so I replaced the coil. Ran fine, came back two days later with the same symptoms. I replaced the coil and it ran fine again. I assumed I had gotten a bad coil and I sent the car back out. Came back a week later. Long story short, after the fourth coil from a different supplier I traced it back to a wire on the pickup that was rubbed through. Net effect was that the coil was firing when the distributor was pointing between cylinders and the voltage went high enough to cause flashover inside the coil. Seems like I always get the weird failures.... -Denny -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist