Resistor-type spark plugs perform a crucial function on ATVs. These spark plugs contain a 5-kilowatt ohm ceramic resistor core inside the spark plug shell. The resistor functions as a suppressor against electromagnetic interference, also know as radio frequency interference, when the ignition fires at high voltage. This electricity "noise" can disrupt vehicle radios, often in the form of a hissing, popping or crackling noise. There is a marked difference between resistor and non-resistor spark plugs regarding ATV ignition systems and performance.
ATV engines typically use a coil or magneto to produce a high-voltage spark that travels through the spark plug wire and to the spark plug. When the primary winding inside the coil is turned off or on, depending upon the type, it sends a signal to the secondary winding to activate. The secondary winding steps up the voltage from 12 volts to anywhere from 30,000 to 50,000 volts. This high voltage is required to create an arc between the spark plug ground and positive electrode. The high voltage creates a stray radio frequency noise.
The primary purpose of resistor spark plugs for an ATV is to allow them to soak up powerful radio wave frequency interference. Since the electromagnetic discharge of high voltage also has a radio frequency signal, it often interrupts and causes static in ATV radios, CB radios and GPS systems. The resistor plug acts as a voltage soak to suppress static, crackling and loss of communication signal. Without resistor plugs, particularly on an older model ATV, no suppression exists, and interference can be detected in communication devices at any engine rpm. The noise interference can take on an annoying whine, directly proportionate to the engine speed.
On many off-road vehicles, the secondary systems, including resistor spark plugs, are used to reduce the high electrical backlash caused by the coil's primary winding. The backlash, or extra electrical feedback, causes a disruption in the ignition system, such as the CDI boxes and igniters that are part of the ignition control modules. ATV vehicles that do not have such secondary resistor systems can show poor performance, such as periodic missing, rough idle and high-speed ignition cutout. ATV's and motocross bikes use resistor spark plugs to protect the ignition control modules.
Radio frequency interference can cause disruptive signals to an ATV's sophisticated on-board computer, which links to the control modules. Secondary electrical systems close to the coil will cause backlash and high RFI, since their voltage signal is significantly stronger. Resistor spark plugs suppress RFI and electronic backlash, keeping communication systems clear and on-board computers from harm. Non-resistor spark plugs can cause radio, CB and GPS static interference, as well as disrupt the entire ignition system. Newer ATV models with computer management systems will sometimes not start or run poorly when fitted with non-resistor spark plugs, due to the sensitivity of the computer and module settings.
ATV models manufactured before 1979, and some models prior to 1990, do not have computer engine management systems or microprocessors. Therefore, they do not have ignition problems related to radio frequency interference. These older off-road vehicles benefit from resistor spark plugs only in a communications application if they have been modified or fitted to accept a radio, CB or GPS system.
The advantages of a resistor spark plug are straightforward, in that it suppresses RFI, for communications and ignition systems. However, resistor material is often composed of carbon material, which deteriorates with age. The deterioration will cause a suppression failure, leading to radio frequency interference. This results in additional maintenance. Resistor spark plugs do absorb some of the coil's maximum reserve output, and this reduction applies more to older vehicles in which the owner has installed non-resistor plugs in a system that never had or required them. Older ATV models that do not have computers or communication systems do not suffer any RFI-related problems.
