POWERBOATER’S GUIDE TO ELECTRICAL 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. 110
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.
Beyond the Basics: Outboard and PWC Ignition Systems 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
