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Beyond the Basics: Outboard and PWC Ignition Systems
pinging as a cause of engine damage. The knockretard spark control is operated by one of these two additional devices.
The knock-control module receives an electrical signal from a sensor, called a knock sensor, which is screwed into the engine block. This sensor “hears” any pinging inside the combustion chambers and sends an electrical pulse to the knock-control module. The module then sends a signal to the ignition module, ordering it to retard the ignition timing in small (3-degree) increments until the pinging stops.
In addition to controlling ignition timing to eliminate pinging, the Thunderbolt V also controls engine over-revving, acceleration, spark advance, and idle speed. It also has a feature called mean best timing, although this feature is not available on all engines.
Mean Best Timing
MerCruiser’s Thunderbolt V system uses a sophisticated feature called mean best timing (MBT) that fine-tunes the ignition timing during light-load cruising. The ignition-control module searches for the perfect setting for ignition timing by automatically adding a few degrees of advance and waiting to see if the engine rpm increases. If so, the module will add a little more advance until engine speed stops increasing. If the rpm drops for any reason, such as a change in sea conditions, the module will automatically retard ignition timing as needed. This is true electronic wizardry at its finest.
Idle-Speed Spark Control
The idle-speed spark control automatically adjusts ignition timing so that a specific idle speed is maintained under different operating conditions. This is accomplished by making small spark-advance adjustments and is only used within a speed range that generally falls between 400 and 700 rpm. This variation in exact speed specifications is one of several reasons that if module replacement is ever needed, the exact module for your engine (as determined by the serial number of the engine) must be used.
Don’t be fooled into thinking that a module that looks just like yours is the correct one for your engine; the internal calibration of the look-alike could be very different from yours.
Acceleration Spark Advance
All ignition systems need some sort of acceleration spark advance mechanism. Older systems with breaker points used a mechanical advance with centrifugal weights attached to the plate inside the distributor to which the points were attached. As engine rpm increased, it generated centrifugal force and the weights moved the ignition points relative to the center shaft within the distributor. This changed the place where the points opened and closed, and adjusted the timing of the spark. On newer engines with electronic ignition, timing is controlled by the ignition-control module or, on the latest computerized engines, by the onboard microprocessor.
This change in timing allows more time for the fuel-air mixture to completely burn as engine speed increases. The faster the engine turns, the more time is required for combustion, and the more the timing must be advanced. When the engine is accelerating, the ignition-control module may add more spark advance to the “base timing” (the starting point for timing on all engines). The amount of spark advance added depends on how fast engine rpm increases. Rapid throttle changes induce rapid timing changes.
On outboard and personal watercraft (PWC) ignition systems, some of the parts are located under the engine flywheel. So if your diagnosis leads you here, you may need the services of a professional mechanic who has the tools needed to get at these parts. All manufacturers include test procedures in their workshop manuals that use special test equipment, such as Stevens or Merc-o-tronic ignition system testers. This equipment is too expensive to be a part of your tool kit. Unless you do this sort of work daily, it just isn’t practical to have this stuff. This section will show you how to narrow down the most common ignition problems using simple tools and your multimeter.
Capacitive-Discharge Ignition System
If your engine was built after 1975, it most likely has some variation of a capacitive-discharge ignition
(CDI) system, which works by charging a capacitor and releasing this charge to the appropriate igni-tion coil at just the right time. We have already taken a brief look at this in the overview of how out-board and PWC ignition systems work earlier in this chapter.
Different engine makers use different names to describe the parts they use in their CDI systems, but they all are similar.
Magnets carefully positioned on the engine’s flywheel induce an electric current as they rotate past specially designed coils located very close to the magnets. One of the coils under the flywheel is called a charge coil. As the flywheel magnets spin by, this coil sends a fairly high alternating current voltage to the ignition-control module, which is sometimes called the power pack, or CDI unit. This will be around 200 volts AC, depending upon which system you have.
As already mentioned, the other ignition coils found under the flywheel are called sensor (OMC), pulsar (Yamaha), or trigger coils (Mercury). I will call them trigger coils here to avoid confusion. The trigger coils send electrical signals to the CDI unit to tell it which cylinder to work with at the correct time.
Next you’ll find the CDI unit itself. This device is the brain of the system and serves several functions. First, it converts the alternating current from the charge coil into usable direct current. Next it stores this current in the built-in capacitor mentioned earlier. The CDI unit also adjusts timing by changing the interval at which the trigger coil sends a signal to the module. The timing changes with any change in engine rpm and is adjusted by a change in the relative position of the trigger coil to the flywheel magnets.
A timing plate to which both the charge and trigger coils are mounted controls this adjustment. Mechanical linkage connected directly to the engine throttle linkage constantly adjusts the timing relative to the position of the carburetor throttle. In addition, the CDI unit electronically controls the discharge of the built-in capacitor and sends this voltage to the appropriate primary side of the ignition coil for the correct cylinder.
The CDI unit may also have electronic circuits within it to limit engine speed to prevent overrevving, and some even have a circuit that reduces engine rpm if for any reason the engine begins to run too hot. Some of the larger engines may automatically advance ignition timing during initial start-up and when the engine is running at temperatures of less than approximately 100 degrees.
Manufacturers often use one CDI unit for each bank on V-type power heads. One module will control the odd-numbered cylinders and the other will service the even-numbered cylinders. Once the voltage leaves the CDI unit, it’s sent to the high-tension coil, which is similar in design to the inboard-system type already discussed. Here, the voltage is stepped up to anywhere between 15,000 and 40,000 volts, the voltage that’s required to jump the air gap in the spark plugs. The high-tension ignition coil has two sides, primary and secondary, just as it does on an inboard system. It’s really two coils combined into one neat, compact case. Figure 7-10shows the internal construction of a typical ignition coil with primary and secondary windings.
The coil works by using magnetic induction, just like one on an inboard engine. The voltage generated by the primary winding creates a magnetic field around the secondary winding, which has many more windings than the primary coil. The CDI unit controls the rapid turning on and off of electrical flow in the primary winding, thereby turning this magnetic field on and off. The effect of this is the same as described earlier. The rapid movement of this magnetic field past the secondary windings in-
PRIMARY 12 Turns 12V 20A SECONDARY 120 Turns
To circuit
Fig. 7-10. Typical internal construction of an ignition coil, showing the primary side windings (with fewer coils) and the secondary side (with more windings).
