nicklanigan
Contributing Member
Just a note for anyone interested - make sure you ground out your plugs or use the kill circuit/ignition off etc when doing a compression test.
Eating a slice of humble pie here - and I'm an electrical engineer - for years I did compression tests by simply pulling out the plugs, leaving everything dangling and using the key switch to crank over the motor. I got away with it for years also.
Along came a new power pack, and I decided to replace my original coils (40 years old), with modern CDI ones. And yip, my old method killed a coil. It kills them in a specific way - ohms test is just fine, but a high voltage path has been created in the coil to ground and the coil is now useless.
It gave me pause for thought though - why did a modern coil fail when older ones had been fine for years of bad testing procedures?
The spark plug, or gap tester effectively provides a limit to the secondary coil voltage - once the voltage is large enough, a spark is formed, and the secondary voltage collapses. I'm guessing this would limit the secondary voltage to 5,000-10,000V. Take away the route to ground, and the secondary coil voltage will go far far higher.
In a theoretical world, a secondary coil with no route to ground should be able to handle an extremely large voltage - the limit though is the insulation between the high voltage and anything connected to ground. You are at the coil manufactures whim here about what the upper limit they have designed for. I'd imagine they have designed for 2-3x regular operating voltages.
The power pack would also be at risk - potentially the risk of a power pack failure is lower than a coil failure, but neither is desirable.
Turns out in my case, the old coils simply had better insulation, or the old power pack wasn't as capable of creating such a high voltage in the secondary coil as a new one.
Doesn't matter, as either way, it's just a risk of something being destroyed.
Eating a slice of humble pie here - and I'm an electrical engineer - for years I did compression tests by simply pulling out the plugs, leaving everything dangling and using the key switch to crank over the motor. I got away with it for years also.
Along came a new power pack, and I decided to replace my original coils (40 years old), with modern CDI ones. And yip, my old method killed a coil. It kills them in a specific way - ohms test is just fine, but a high voltage path has been created in the coil to ground and the coil is now useless.
It gave me pause for thought though - why did a modern coil fail when older ones had been fine for years of bad testing procedures?
The spark plug, or gap tester effectively provides a limit to the secondary coil voltage - once the voltage is large enough, a spark is formed, and the secondary voltage collapses. I'm guessing this would limit the secondary voltage to 5,000-10,000V. Take away the route to ground, and the secondary coil voltage will go far far higher.
In a theoretical world, a secondary coil with no route to ground should be able to handle an extremely large voltage - the limit though is the insulation between the high voltage and anything connected to ground. You are at the coil manufactures whim here about what the upper limit they have designed for. I'd imagine they have designed for 2-3x regular operating voltages.
The power pack would also be at risk - potentially the risk of a power pack failure is lower than a coil failure, but neither is desirable.
Turns out in my case, the old coils simply had better insulation, or the old power pack wasn't as capable of creating such a high voltage in the secondary coil as a new one.
Doesn't matter, as either way, it's just a risk of something being destroyed.
