ATV Electronics series: Part 1 - ignition systems
Of course, the primary purpose of the ignition system is to create a spark. Years ago, a fellow named Kettering came up with a pretty good way to make a spark. Very basically, his invention consisted of two sets of finely wound copper wire - commonly referred to as the ignition "coil" -.that when powered with electricity on one winding would create a sudden jolt, or spark, of electricity on the other when the electrical power is interrupted. To keep up with a spinning engine, the electrical power is switched on and off via a contact switch that's controlled by a cam lobe driven by the engine. Pretty darn simple, and pretty darn easy to diagnose with basic tools. However, this conventional "breaker" or points-type ignition had reliability issues and was limited to low rpm applications. Enter the age of high tech electronics: integrated circuits, transistorized ignitions, and CDI...
The good news is that CDI, or capacitor discharge ignition, was all that points-type ignitions weren't - they had no moving or wearing parts, could produce a heck of a spark, and could run very high speed. Bad news is that the ignition system now became a "black box", both literally and metaphorically. No spark? Might as well start swapping parts until you the spark magically reappears. Not a bad proposition for a dealer that has parts sitting on the shelf he can try with no obligation. But to spend a hundred non-refundable dollars on simply a hunch is a tough pill to swallow.
|Schematic of a typical CDI ignition system|
Fortunately, we can break down the operation of the CDI to some basic circuits - the guzzinta's and the guzzouta's (inputs and outputs). Whereas the points-type ignition has only two circuits (power and ground), the CDI has 5 primary circuits. On the guzzinta side we have the obligatory duo of power and ground, but with the added complexity of a circuit to tell the CDI when to fire (trigger circuit), and another to tell if to fire (kill circuit). What's left is the only circuit on the guzzouta side: the power to the ignition coil (fire circuit).
So let's discuss those circuits and their possible failure modes. The CDI's ground (DC negative) is always connected and provides the connection back to the engine's own voltage ground point or reference, which is the same reference the spark plug uses when creating a spark. The key piece of info here is that wherever the CDI makes its ground connection has to be essentially the same ground reference as the spark plug - make sure there's clean metal all the way between the two connection points, and virtually no resistance (ohms). If the CDI makes its ground connection at the frame up near the gas tank, be assured that the circuit from there is good all the way back the cylinder head. Don't rely on a ground connection through engine mounts. They can be intermittent, dirty, corroded, rubber-bushed, or painted. Proper grounding would dictate a ground strap from the engine directly to the wire harness.
Next thing we need is a way to power the CDI. Some CDI's are powered by 12vdc from the machine's electrical system, but most others generate the requisite electrical power from an exciter coil underneath the flywheel. As the magnet on the flywheel passes the exciter coil, electrical current is generated and in the case of a Capacitor Discharge Ignition or CDI, that charge is stored temporarily in a capacitor for a split second until the CDI is told to fire. Since the CDI is already tied to ground, there's only a single wire connecting the exciter coil to the CDI. To check this voltage produced by the exciter is tricky because it requires the engine to be turning. An electric starter can produce the necessary crank rotation, but diagnosis by means of kick starting is difficult since the output voltage varies with engine speed. To check, use a digital voltmeter (DVM) and set to AC scale. Connect one DVM lead to the exciter output wire, and the other to case ground. Most shop manuals will list a minimum AC voltage for both running and starting. Be CAREFUL when testing, the output from the exciter coil can reach 200 volts!
The power stored in the CDI's internal capacitor needs to be put to work, but the key to proper engine performance is doing it at the right time. That's where the pulse generator comes into play. Again, underneath the flywheel lies yet another magnet and coil combo, but the sole purpose of these two is to precisely identify to the CDI the position of the crankshaft. That is, it tells the brain box where the piston is and therefore when to fire. Basically, the CDI waits around for the pulse generator to tell it that the piston has just hit some point before TDC, and then waits the appropriate amount of time (dependent on rpm and spark advance) before energizing the guzzouta circuit that sends power to the well-recognized ignition coil. This circuit is diagnosed similarly to the exciter coil above, but instead connect your DVM to the pulse generator output. The voltage is much less and should be listed in your manual.
The last CDI circuit is the "kill circuit" which is how the CDI knows to quench the spark and kill the engine. Typically this circuit is switched to ground, and some CDI's require this circuit to be closed, while others require it to be open for engine shutdown. Most safety tethers, kill switches, and ignition switches utilize this circuit to control engine operation. The easiest way to troubleshoot a no-spark malfunction is to pull the spark plug and check for spark by unplugging this circuit. If no spark, then connect a ground wire directly between engine ground and this circuit (on the CDI box side). If still no spark, the problem is likely with another circuit.
To complete the whole ignition system, we still need to produce enough voltage to jump a spark across a 1mm gap, inside a running engine. This part of the system hasn't changed much over the years - it's still a good ole ignition coil with a small wire going in, and fat one going out. It's a little different since we no longer have 12vdc going in, but rather nearly 10x that!. It also requires a good ground connection so insure that there's no corrosion, mud, or paint separating the mating connections. Although it's easy to blame coils for electrical problems since it's difficult to test (resistance checks are not always reliable), they are not commonly known to be failure prone. More likely is a bad spark plug cap or connection to the fat plug wire. With some coils passing as much as 60,000 volts make sure your plug wire and cap has no exposed breaks or possible leak paths that water can penetrate. When the coil fires it will seek the path of least resistance, and if there's a way for 60,000 volts to get to ground easier than through a compressed air-fuel mixture (spark plug gap), it'll take it. And water does that job quite nicely.
Stay tuned, next month we'll get into the charging system of an ATV's electrical system.