® Resistor Plugs, Wires, and Related Issues

Most powersports vehicles (motorcycles, ATVs, personal watercraft, snowmobiles, scooters, and utility vehicles) come with resistor spark plugs, resistive spark plug wire, or resistor spark plug caps. Many have more than one of these things, and a few have all three. Just what are resistive ignition components for, and why do we need them?

The What and Why of Resistive Ignition Components
An ignition coil is a transformer. It has two electrical windings, that is, a primary winding and a secondary winding. The primary winding, depending on the system, is either turned on or off, and the resulting appearance or loss of magnetism induces voltage in the secondary winding. Because the windings vary considerably in size, this voltage is stepped up tremendously into the thousands of volts necessary to fire the spark plug. However, this high-voltage release is very messy (electronics techs call it electrically "noisy"), that is, it crackles and pops. The electrical noise presents two problems. First, it creates electrical emissions that coincide with radio transmission wavelengths. This radio frequency interference (RFI) intrudes on electronic communications to the point that it can cause Jay Leno to stand on his head on your neighborís TV! You can imagine the difficulty with emergency and law enforcement communications. The second problem, and one less known, is that an ignition systemís electrical noise also creates an electrical backlash in the ignition system that unchecked can interfere with the systemís integrity. RFI can also interfere with other electrical components on the vehicle, particularly the more sensitive solid-date electronics. Adding a carefully-chosen resistor to the ignition systemís secondary winding damps the sparkís electrical noise, with little effect on plug voltage, since there is a surplus of unused voltage at the coil anyway. This damping is much like the muting of a tone on a guitar string with your finger on the string at the fret. The note is the same, the vibration and tone are different.

The Effect of RFI on Communications
However, not all vehicles use resistive ignition secondary components for the same reason. There are four different applications of resistive technology on powersports vehicles. To begin with, on virtually all machines, the aforementioned RFI suppression for the sake of preserving electronic communications is the goal. In other words, the resistor plugs, wires, or caps are there to control electrical emissions that would otherwise interfere with radio and TV reception.

The Effect of Electrical Backlash on Ignition Components
Second, on some offroad vehicles, resistive secondary components of a slightly different specification are present to reduce the electrical backlash that spark creates in the coilís primary winding. This unwanted electrical feedback is harmful to the parts that drive the ignition system, particularly the ignition control modules (CDI boxes, igniters). Motocross bikes use resistive secondaries to protect the CDI boxes.

The Effect of RFI on Ignition Components
Third, also on offroad vehicles, resistive technology is used to reduce RFI-related malfunctions of ignition control modules that are close enough to the ignition coil to be subject to these strong, nearby radio waves. The CDI boxes or igniters on these machines are mounted close to the ignition coils, and therefore need RFI protection. Personal watercraft in particular, but also offroad motorcycles and ATVs, employ resistor technology to cope with this proximity RFI issue.

The Effect of Both Backlash and RFI on Onboard Microprocessors
Finally, any vehicle that has an onboard computer, no matter how basic, makes use of resistive ignition secondaries to ensure the protection of that computer and its many sensors from both RFI and electrical backfeed problems. Thus virtually all modern road bikes and every other powersports vehicle, whether or not it is fuel-injected, which has some form of computerized engine management system, therefore has resistors in its ignition system to protect these computers and related components.

Modern Vehicle Maintenance Issues
The outcome of all this is that one must be very careful when replacing spark plugs or servicing the secondary side of the ignition system on any of these vehicles. There are many late model personal watercraft and sport bikes for example that wonít even start if the plugs are not the right type, so sensitive are these systems to the issues already described. Other vehicles are not as sensitive, and will at least run, but will run poorly and/or do other odd things. These vehicles have resistor spark plug caps to protect their instrumentation as well. They will turn on their panel indicator lights, wiggle their gauges around, and do other strange things if connectivity to the resistors is faulty, or if non-resistor spark plugs are installed in them.

Older Vehicle Maintenance Issues
However, no road bikes made before 1979, and only a handful made before 1990, fit into more than the first of the four categories mentioned previously. That is, most vintage streetbikes have resistor plugs or caps only as a concession to the worldwide concern for the protection of radio communications. They do not have electronics onboard that are sensitive to either RFI or backfeed, and there are of course no computers to worry about. It is not unusual therefore for restorers and modifiers of these older machines to convert their ignition systems to complete non-resistor specification. The benefit of doing so is two-fold. First, there is obviously one less thing to go wrong. Ignition secondary resistors are usually carbon-based, and therefore deteriorate steadily. Eliminating the resistors in these systems simply eliminates them as a maintenance item. Second, removing the secondary resistance often increases potential ignition coil output, because it frees up more of the coilís voltage reserve to be applied to the spark plug. The real-world gain is the ability to use a larger spark plug gap, with the attendant increase in combustion efficiency that is often realized as quicker starts, smoother idle, and better throttle response Ė most of the same things high output coils offer, but without their cost.

Resistive Ignitions on Automotive vs. Powersports Systems
But, beware of thinking automotive when servicing or modifying one of these older bikes. Cars and motorcycles are thought by many people to be very similar in technology, but in many ways they are not. The two industries have traditionally taken very different approaches to many areas of design and manufacture, and spark suppression is just one example. We have already examined the fact that motorcycle manufacturers (Harley excepted, as with other things) put the resistors on the ends of the spark plug wires. That is, inside the plug caps. They have also always used plain stranded metal wire (stainless steel in Asian motorcycles). That is, good old-fashioned electrically conductive stuff. Car makers on the other hand add the resistor, not in the plug cap, but in the wire, by making the wire itself resistive, i.e. "suppressive." (Sounds like a dictatorial regime, doesn't it?) The wire is not really wire at all, but a mixture of carbon and fibreglas. It is therefore very resistive. Unfortunately, it is also, due to the materials from which it is made, very subject to deterioration and very difficult to make good electrical connections with. This is why the car world replaces spark plug wires frequently.

Aftermarket Spark Plug Wires
But you donít ever need to replace the spark plug wires on Asian bikes, unless they are broken or badly corroded. You should however unscrew the spark plug caps occasionally and do two things. First, resistance-test the caps. Pre-1990 systems should generally ohm under 5k while a few of the later ones will be 10-15k. Check the manual. Second, occasionally trim a little of the spark plug wire off the end so that the cap has fresh surface to bite on when reinstalled. If you do replace the wires due to their being too short or because of breakage, be careful. Automotive plug wire, no matter how nicely colored, is unsuitable because it is made of non-wire material. You also donít need to add any more resistance to the system. So keep that stuff away from your bike. Ask your local motorcycle shop to order it, not from automotive companies, but from Parts Unlimited, Dennis Kirk, and a few other suppliers. You can also go to a good old fashioned auto parts store and ask for Delco brand Packard 440 spark plug wire, which is 7mm, copper wire used on cars in the 1950s. It comes on a 100 ft. roll, part number 1851208. Either of these is very close to what the Asian manufacturers used stock. You then attach old-fashioned non-resistor automotive terminals and covers, and youíre done.

Kettering System Spark Plug Wires
The really old (pre-1979) points (Kettering) ignition systems had unique ignition coils. The coils on these bikes often have the wires molded in place. That is, the wire is not conventionally removable. It is possible, if necessary, to repair a bad wire on one of these old style coils by removing and replacing the wire, but it is a tricky operation. The wire must be extracted with a pick and the new one epoxied in because now the hole is enlarged. There are also spark plug wire splice kits available, but they make a really poor repair that will likely corrode, get loose, and leak voltage. Itís best to replace the coil.

Further Reading:
Powersports Ignition Evolution
High Performance Ignition Coils
U-Gap and Splitfire Spark Plugs
Optimizing the Honda SOHC Ignition
Honda DOHC, CBX Ignition System Troubleshooting
Reading Spark Plugs
GL1000 Ignition Tech

Email me
© 1996-2021 Mike Nixon