In The CBX Ignition, Part 1, we examined the CBX’s ignition system, focusing mostly on the pulsers. In this installment, Part 2, we consider this ignition system’s spark units

Kettering Revised
As was mentioned in Part 1, the CBX ignition system, called TPI by Honda (Transistorized Pointless Ignition) is nothing more than the old Kettering points based system revised and made pointless. Hence the name. The interesting part is that TPI needs two parts to do the job that just one did before. That is, in a TPI system, the points were replaced not only by pulse triggers, but also by transistors. In other words, the point ignition system’s points did two jobs. They timed, of course, and they also switched. TPI’s pulsers (variously called triggers or signal generators) therefore took over the timing chore, while its transistors (spark units, also called igniters) assumed the switching duty. Nothing else was changed. In fact, all of the parts in both a points system and a TPI system can be completely interchanged, as long as the pulsers and transistors are kept together, because they work together. Restorers of old points ignition motorcycles often use ignition coils from later TPI bikes with no ill effects. Moreover, the two systems actually work the same. That is, both battery/point and TPI systems work on a collapsing field principle. In the points system, the ignition coil is energized through closed contact points, resulting in a magnetic field inside. When the points open, the coil’s current is interrupted, and the field collapses. The collapse is so sudden that a second ("mutual") induction occurs at the coil, in its larger winding, which produces the spark voltage. TPI works exactly the same way. The ignition coil is energized through a conducting transistor, the pulser sends a timed pulse to the transistor which in turn disconnects the ignition coil, and spark voltage is generated by the resulting collapsing field. Notice the extra step? The timing and switching roles remember are split now between two parts, resulting in that added phase.

Standard Performance
TPI is nothing more than a points system made electronic. There is no extra spark energy. There is no more resistance to plug fouling, no added potential coil voltage, and no reduction in heat cycles (and therefore no reduction in heat related failures). In fact, assuming a carefully adjusted point system, there isn’t even an improvement in timing accuracy. TPI was emissions-mandated not due to increased accuracy, but due to the reduced maintenance that emissions warranties required. That is, contrary to what many believe, first generation electronic ignitions such as TPI were not developed to increase performance. They were simply early steps at reducing the kinds of maintenance requirements that were obstacles to long-term emissions compliance. However, this is not to say that TPI is all bad. It’s not. American Honda used the stock TPI on its first-year Superbike effort in the early 1980s. However, it doesn’t even approach the technology that is available today, and being a modified Kettering system, it actually presents a number of drawbacks. Two are serious enough to warrant mention here, and they’re both from the same cause. First, there is a susceptibility to plug fouling. Because the system works on collapse, the time from pulse to fire is kind of long, as ignition systems go. Where a modern system might get the whole job done in just 4 or 5 milliseconds, TPI needs 20 or more. Plenty of time for accumulating spark voltage to bleed off before delivering the high powered punch that keeps plugs clean. Second, again because of the way in which energy is used in this system, this ignition tends to run out of voltage at high rpm. But then, both are also true of battery/point ignition.

The Spark Unit
TPI’s spark units have easily recognizable failure modes. A common one is when an attempt is made to start the bike after it has sat awhile and the battery is low. During starting, battery voltage normally dips when the starter is used but with less to start with, the battery’s voltage drops too low for the spark unit to operate, and the engine will not start due to loss of spark. Meantime, the transistor is overheating, and will soon permanently fail. This is such a known failure mode that Honda later added time-out circuits to their TPI systems, as did also other manufacturers. Another possible route to failure is the unthinking substitution of ignition coils having too low a primary resistance. Such coils will burn out the spark units. You have no doubt heard of this. High performance ignition coils have higher potential voltage output because they have larger primary to secondary winding turn ratios. There are two ways to build a coil with a larger turn ratio - either leave the primary the same and increase the secondary turns, or leave the secondary the same and decrease the primary turns. The latter method was once very popular. However, it results in a coil having lower than stock primary resistance. While not big deal on a points bike, whose points will merely arc a bit more, the transistors in a TPI system will not endure the increased current flow through the coil for very long, and the spark units will fail. We’ll address this issue again in Part 3. One thing you do not have to worry about with spark units is failures due to loss of ground. Their metal boxes do not physically ground, but rather, the spark units get their grounds through the green wire feeding back into the wire harness. And here are a few tips. When the ignition system is acting up, but it is mostly when the engine is warm, don’t suspect the spark units. The pulsers are likely the problem. When the ignition performs badly whether hot or cold, then the spark units become more suspect.

Testing
Perhaps the easiest way to test a spark unit on a later model CBX is to simply substitute one of the other two on the bike. Of course, that only helps if the spark is visibly poor or non-existent, and doesn’t help if you are trying to judge the performance of the engine as a whole. While the professional tech will use a peak voltage tester, there aren’t any conclusive tests for the spark unit that the do-it-yourselfer can use, except to test all around it, and that is valid. It’s also accurate, if done carefully. Simply test the pulser and the ignition coil. All that is left is the spark unit. Part 1 described how to test the pulser, and we’ll look at how to test the ignition coil in Part 3.

Go to Part 3