Superperformance - Having engine trouble - need help

[patrickt]

Originally Posted by RockBit

Patrick - what does the capacitor do?

The capacitor is installed "across" the positive and negative feeds to the MSD box. If it was just a wire it would be "short-circuiting" the two wires. But, it's a capacitor, so once it reaches its fully charged state it stops the direct current going across it but continues to act as a "shock absorber" for direct current voltage spikes, thus protecting the MSD box from them. Plus, if your alternator is leaking AC current out, a capacitor across the lines acts as a short cut for the AC to pass right through and head back home without ever going in to the MSD box to help muck up the electronics.

[RockBit]

Originally Posted by patrickt

The capacitor is installed "across" the positive and negative feeds to the MSD box. If it was just a wire it would be "short-circuiting" the two wires. But, it's a capacitor, so once it reaches its fully charged state it stops the direct current going across it but continues to act as a "shock absorber" for direct current voltage spikes, thus protecting the MSD box from them. Plus, if your alternator is leaking AC current out, a capacitor across the lines acts as a short cut for the AC to pass right through and head back home without ever going in to the MSD box to help muck up the electronics.

Great explanation. Thanks. I’ll put one on mine.

[Blas]

If you don’t see a big red box under the dash, look for an more square looking aluminum colored box. Could be he still has the original duraspark unit installed.
Blas

[Gaz64]

Otherwise known as a noise filter.
Absorbs the "ripple" between the supply positive and negative rails.
So the supply the module sees a "clean" supply.

[patrickt]

The answer, because I know Gaz is sitting there with bated breath, is that on our old points-based distributor, when the points open to break the ground path, and the secondary field collapses to generate that 40,000 volts, the return path from the spark plug is via the cylinder head/engine block to the negative battery cable, though the battery itself, out the positive side of the battery, through the positive lead to the primary side of the coil and back home to the secondary winding. That question will win you a free beer at your next 19th hole. Now, back in the 70's my friends and I thought of ourselves as genius car mechanics so we would test your alternator's output by pulling the positive cable off the battery, while the engine was running, and if the engine kept running then we knew the alternator was working -- what geniuses we were and, as memory serves, we just assumed that whatever alternator/regulator/ignition part that got burned up in our "testing process" was just broken before we got our greasy mitts on it. Remember, that 40,000 volts has to find its way back to the coil somehow for the car to keep running. An MSD box is even more sensitive -- and everything is running though it, including the return path of the spark (because that's the only way to get back to the coil). And that's just one aspect of the ignition and grounding process, but this will at least make Gaz happy....