Hello Gents.....I gotta get this off my chest.<br />- I'm no expert either!!....But I know that ignition systems are capable of producing very high voltages (30,000 50,000volts).<br />The spark plug presents a load to the ignition system and keeps the voltage down to around 4000 10,000 volts (depending on plug gap, cylinder pressure, mixture).<br />-When you pull off the plug wire to test for spark, you remove the load from the ignition system and the voltage rises to its maximum.(30KV 50KV). Heres the interesting part. The ignition system is not designed or insulated to handle the maximum voltage it can produce, because under normal operation, this high voltage never occurs. If you dont believe this, try holding a pulled off ignition wire on a CDI system. Itll arc right through the wire to your hand because the wire can't insulate that high a voltage.<br /> <br />-If you pull a plug wire off the plug, crank or run the engine, increase the gap by moving too far from the block, youll lose sight of the spark. Where did it go? Especially on CDI, the voltage is so high that its going to find a path to ground somewhere, even if its not through the plug wire. It may find a path through the jacket of the plug wire, then through your hand to ground. Not only did it hurt your hand, but your elbow as well when you jabbed it back on the frame!<br /> <br />-It also did a bad thing to the plug wire (it only takes once). As it passed through the insulative jacket, it punched a small hole (as lightening would do through your roof!). Now the wire is damaged, and over time, moisture can enter and supply an alternate path to ground under certain conditions (high engine load, high atmospheric humidity) thus robbing the plug of applied voltage. This of course would result in a misfire. <br /> <br />-Damaging the plug wire is elementary. Where did the spark go if it didnt pass through the wire jacket and your hand? Dont forget, 30KV 50KV can jump an air gap of up to 1 (pending on atmospheric conditions). Trust me....it went somewhere (CDI is not forgiving). The most common place for the arc to occur when plug wire pulled off is through the coil windings to ground, thus burning an alternate path to ground inside the coil. Since its inside the coil, its not visible and the damage goes undetected. Each time the arc occurs, it burns more and more insulative material inside the coil making it easier for the arc to occur. The worst thing is that the coil doesnt always fail 'right away'!! Thats why coil failure and pulling wires off while running arent practically associated. It is common to find failed coils after the wire failed open internally. I can hear some saying yeah
.yeah
..yeah, Ive seen wires left off the engine before and it never damaged the coil Thats very common, because the spark was occurring through wire to ground, not through the coil. If you want to prove this, just remove the wire totally from the plug and coil from YOUR boat and take it for a spin. Itll either burn a carbon trace on the coil tower to ground, or damage internal insulation. Either way...coil is junk. <br /><br />-The recommended method of checking for spark is to install a device with a safe air gap (not pulled off wire held next to block and hoping you dont pull away too far). There are a multitude of different tools out there for cheap. Some look like (and would be easy to make) a regular electrode plug with the ground tang broken off. It also has an alligator clip fastened on to prevent you from losing precious ground (I believe they take approx. 30KV to jump their gap) Others have an adjustable air gap, which again should not be maladjusted. If none are available, use a conventional plug held securely to the block.<br /> <br />-A good bad example:<br /> I had an engine hooked on an oscilloscope. As I was maneuvering around the engine, I accidentally tugged on the probe wire from the scope, which pulled off the coil wire from the distributor. The engine quit, so I simply reinstalled the wire and attempted to restart the engine. NO GO. When the wire was pulled away on the running engine, it fell near the engine harness, which housed the signal wires from the distributor. Since the ignition pack is designed to handle about 40V signal input, it couldnt tolerate 40,000V. After putting in a new module, I restarted the engine. Now the scope didnt work. When the wire was pulled off, not only did the arc occur to ground, but also to the scope interface cable as well. The input section of the scope samples a portion of the actual voltage, not all!!! Luckily I found the Interface chip at a local electronics supplier and was able to fix my screw up on my own time.<br /><br />**My point is this....If we recommend to a non-tech person to remove the plug wire to check for spark:<br />-risk wire, plug boot, and coil damage (everyone pulls it away just to see the pretty lights)<br />-risk shock through hand which results in dropping of wire<br />-if wire is dropped > risk damage to pack, rectifier/regulator, coil, wire, tach and any electronically sensitive devices.<br /><br />Lets make safe recommendations.<br /><br />correct me if I'm wrong.....I'm going deer hunting!!
