RebelTillDeath3

This isn't a problem, but rather a debate I always somehow get into.

I believe a coil cannot be "bad" if the car is still able to run, it's what I've been taught almost all my life.

The guys at work seem to believe that if an engine skips/misses/stumbles that the coil is bad. How is that possible? If the coil is not working properly, the car isn't even going to run.


I'd just like to know the truth behind both of these arguments.

five7kid

iTrader: (14)

A coil might have enough to fire the cylinders under low-load conditions but not high load. It might be able to build up the field and fire under low RPM conditions but not high.

Taking out a few windings by shorting to each other might not kill the entire coil, like an open would, but it will reduce the voltage it can put out.

RebelTillDeath3

That was their argument, why does it matter what load the engine is under? The same amount of spark is still going to ignite the fuel/air in the cylinder.

RB83L69

The difference is, when the engine is under load, there's more air/fuel in the cyl; the more mixture there is, the higher the resistance to electric current; the higher the resistance, the more voltage it takes to overcome it and produce a spark; and if the coil's insulation is weak, then at the higher voltage, the spark that the coil produces will arc to ground internally inside the coil, instead of jumping across the spark plug gap.

The coil acts like a constant current source. That is, regardless of whether the load across is a direct short or a very high resistance, the coil will tend to output the same current. Consequently when the mixture is thin and has low resistance, that given amount of current will flow, and it won't take very much voltage to cause it to do so; but when the resistance across the plug gap is high because of denser cylinder gas, the coil will develop more voltage to try to overcome it, up to a point.

Ignition systems have got to be one of the simplest parts of a car to understand; yet there's almost as much McDonalds parking lot voodoo drivel about them as there is about what oil to use. Get yourself a scientific education that inculdes a little electromagnetic theory, and the mystery will instantly be cleared up.

RebelTillDeath3

Electrical know how is not something I'm too great at, and I didn't even think of resistance, that clears it all up.

pvt num 11

I suppose that's why anftermarket ignition parts (MSD among others) like to claim higher voltage numbers for their coils and whatnot, as well as having low-resistance wires. More voltage at the source + less resistance in the wires = more voltage going to the plug, in a nutshell.