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 set of points did before. TPI’s pulsers (variously called triggers or signal generators) took over the timing chore, while its transistors (spark units or 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. 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.

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 virtually eliminated maintenance that emissions warranties required. However, this is not to say that TPI is bad. It’s not. American Honda used the stock TPI on its first-year 1025cc 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 has all of Kettering's same 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 required for combustion. Second, again because of the way in which energy is used in this system, Kettering ignition is a voltage-hog. This means it equilibrium is easily upset by small amounts of corrosion in the wiring and connectors.

The spark unit
TPI’s spark units have certain 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. 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 by coaxing too much current through. The transistors in a TPI system may 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. 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. The best test for a spark unit is to test either side of it. 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