Outboard Ignition System: How It Works
All outboard motors require three basic things: fuel, air, and a source of ignition. If your outboard is getting fuel and air but has difficulty starting, misfires once it's running, or doesn’t start at all, the problem could be ignition system failure.
Before you can diagnose what the problem is, you should have a basic understanding of what your outboard ignition system does and how it works. An outboard’s ignition system is responsible for producing an electrical spark to ignite the mixture of fuel and air within the cylinder(s). In the case of a two-stroke motor, the fuel and air mixture also contains a small amount of oil to provide lubrication for the engine.
The spark must occur at the precise moment the piston has fully compressed the fuel- air mixture within the cylinder. And the spark must be strong enough to ensure complete combustion of all the fuel. Additionally, if the engine has more than one cylinder, the ignition system must deliver a spark to each of the cylinders at the precise moment of full compression.
The ignition system must also adjust the precise timing of each spark to correspond with the speed of the engine. For example, as the engine speeds up at higher RPM, the ignition system needs to generate sparks more frequently, and vice-versa as the engine slows down.
Outboard Ignition System Components
An outboard's ignition system has a stator that generates the power to supply the ignition system, but also supplies power to recharge the boat's batteries.
The stator is located beneath the flywheel, and uses the rotation of the engine to generate electrical power. As the engine rotates, it also rotates the flywheel, which has magnets attached to it. The stator's wire coils convert the rotating magnetic fields into low voltage AC current. But before the batteries can be used, it has to convert this current into low voltage DC current.
The job of the regulator-rectifier is to convert the AC current into a DC current. The low voltage AC current from the stator passes through a regulator-rectifier unit, which converts the power to a low voltage DC current. The regulator rectifier also regulates the amount of current flowing into the ignition system so there are no power spikes that might damage other components.
Although most outboards such as Yamaha, Johnson, Evinrude, Suzuki, Tohatsu and Mariner use stators, some more recent Mercury and Honda outboards use alternators, which is basically a stator and regulator-rectifier rolled into one. An alternator generates AC current, converts it to DC current, and regulates the current's output into the ignition system and back into the battery just like a regulator-rectifier.
Pulser Coil and CDI Module
A pulser coil is a switch that sends a trigger signal to the outboard’s capacitor discharge ignition (CDI) module. The module takes a steady flow of low voltage DC current from the regulator, briefly stores it, and allows the current to charge and the voltage to build up.
When the CDI module receives the trigger signal from the pulser coil, it rapidly discharges the now higher voltage current into the ignition coil.
The ignition coil charges the current passing through it to an extremely high voltage. As the current from the CDI passes through the coils within the ignition coil, it’s charged to as much as 40,000 volts, depending on the outboard. This high voltage is the level required for the energy to jump across the spark plug gap and create a robust spark.
Spark Plug Wires
The spark plug wire carries the high-voltage current from the ignition coil down to the spark plug itself. These wires are heavily insulated to prevent the extreme high voltage current from escaping by arcing across the powerhead or to any other grounded metal it can find. Any voltage loss along the spark plug wires means a loss of strength in the spark from the plug itself.
On more modern outboards, the ignition coil connects directly to the spark plug and does away with the need for the wire. This type of outboard has separate ignition coils for each spark plug.
The high voltage current reaches the gap at the end of the spark plug and jumps across, creating a spark. Conditions must be precise for the electrical current to make this jump to the spark plug. If the gap is too wide or narrow, the electrodes are corroded, or the firing head of the spark is contaminated, the spark won't be strong enough or may not happen at all. Likewise, if the current is too weak, the spark may not be enough to cause effective combustion of the compressed fuel and air mixture within the cylinder.
Some modern outboards use technologies different from the typical ignition system. For example, some newer outboards have done away with much of the ignition system components such as the CDI and pulser coil.
They’ve replaced them with an all-in-one control box capable of performing all the regular ignition system functions and more. These newer ECM modules and electronic control units incorporate sensors for engine RPM, water temperature, manifold pressure, oil pressure, and electronic fuel timing. Using this data, the ECUs and ECMs ensure incredibly precise and efficient ignition timing systems.
All outboard ignition systems obviously have a method of shutting down the motor. On smaller outboards, it’s usually a simple on/off button on the tiller handle, while larger units typically have an on/off key switch.
Some kill switches create a short circuit that starves the ignition coils of the electrical current long enough for the ignition system to stop making sparks. Other kill switches create an open electrical circuit in the on position, but break that circuit in the off position, killing electrical power. Either way, a problem with your outboard's kill switch can just as easily prevent the ignition system from functioning properly.