American V-Twin Service, Performance, and Repair
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                         Tech Tips

Welcome to tech tips.  We will cover a variety of issues including maintenance, performance, and customizing.  The topic on this page will be changed periodically, however you can download other tips located at the bottom of this page. You can also find our tech tips in Thunder Roads Magazine and the A.B.A.T.E. monthly news paper, The Bailingwire.


Coiled Up (Part II)

   Last month we talked about the ignition coil, how to identify different types, and the applications to different model years.  We can expand a little further with the model year discussion by identifying the coil’s role in the different types of ignitions systems, stock and aftermarket.

   Before we get on topic, a brief explanation of how ignition coils do what they do is in order.  From the ignition switch, via some form of circuit protection (circuit breaker, fuse), 12 VDC (volts direct current) connects to one side of the coil’s primary side.  The other side is connected to the ignition component (points, ignition module).  In the case of the breaker points system, current is passed through the contacts to ground.  The current flow through the coil generates a magnetic field on the primary side until the points are opened via engine rotation.  When the contacts open or “break” the ensuing collapse of the magnetic field induces a high voltage output through the coil’s secondary (spark plug side).  The amplifying effect is remarkable.  The 12 VDC is boosted to tens of thousands of volts in early systems, and well north of 40 thousand volts in modern electronic versions.  That is the energy required to jump a spark across the spark plug electrodes.  Post 1979, electronic ignition replaced points.  In lieu of mechanical contacts providing the switching function to ground, the ignition module provides the same via electronics.  Very cool!

   The first few years of HD electronic ignition systems were not good ones.  The V-Fire was problematic and the aftermarket found a niche.  While many folks were converting their bikes back to points, new products were coming to surface from the likes of Crane, Compufire, and others.

   One other fun fact is that HD changed their systems from dual fire (both cylinders fire spark simultaneously, one is wasted) to single fire in 1999.  Taking a nod from the aftermarket which introduced this technology many years prior, The Motor Company gave us a much improved ignition system.  Dual fire coils will have two terminal primaries while single fire will have three. The 99 and later went to an enclosed weather proof plug.

   Now we can discuss troubleshooting.  We know that earlier ignition systems used breaker points to time the spark. The coils that provided the energy for that spark were a 5 ohm coil.  When The Motor Company introduced electronic ignition (1980), the coil’s primary resistance was reduced to 4 ohms.  In 1985, higher energy systems utilized a 3 ohm coil.  The lower resistances of the coils allowed for increased current flow with a resulting increase in output.  Furthermore, 1999 Twin Cams and 2004 and later Sportys use coils with less than an ohm on the primary.

   How do you know when a coil is bad?  The two basic rules of troubleshooting are the presence of spark and fuel.  When you ground your plugs and roll the engine and there’s no spark, keep in mind that more times than not it’s a bad connection.  When we roll around on a paint shaker on two wheels we have to expect connection issues!  Check everything around the coil first.  The good book calls for resistance checks on the primary and secondary sides of the coil.  The results of this test can be deceiving. The primary side should read as discussed previously. Depending on the application, a few tenths of an ohm (late models) to five ohms (early systems) will do.  The secondary side will read in the tens of thousands of ohms (check your service manual for specific parameters).  The trick to resistance readings is that a bad reading (open circuit or resistance that is way out of spec) will be indicative of a faulty coil, but good readings are not necessarily proof positive that the coil is good.  If your resistance readings are good, conduct a dynamic test of the coil using a grounding tool.  HD service literature describes how to put one of these together and the procedure to use it.  It’s basically a ground wire with a capacitor connected in parallel. This allows you to observe the function of the coil as if the bike was running.  After all of this testing, and all proves positive, there is still the possibility that the coil malfunctions only under certain conditions.  For example, we had a bike come in the shop that would misfire at the 3500 RPM range.  Above or below that engine speed, it ran great.  If we were to test the coil by any means previously described, all would seem good.  What we discovered was that particular RPM developed a resonance that would cause the coil to shut down.  We could postulate that there was a break in the coil of wire that would remain intact under most conditions to pass the requisite amount of current to function as designed, but once the vibration reached a certain level, the connection would break and down she goes!

   Hopefully this gives you a better understanding of how coils work and a bit of troubleshooting knowledge that will save you some time and money.  The logical flow of diagnostics can keep you from buying expensive parts to replace something that may not be faulty.  Modern service literature from Harley-Davidson is well written.  Use the flow charts and be patient!

Ride safe!