High tension leads or high tension cables or spark plug wires or spark plug cables are the wires
that connect a distributor, ignition coil, or magneto to each of the spark plugs in some types of internal combustion
engine.
"High tension lead" or "cable" is also used for any
electrical cable carrying a high voltage in any context. Tension in this instance is a synonym for voltage. High tension leads, like many engine components, wear out
over time. Each lead contains only one wire, as the current does not return
through the same lead, but through the earthed/grounded engine which is
connected to the opposite battery terminal (negative terminal on modern
engines) high tension may also be referred to as HT.
DESCRIPTION
Spark plug
wires have an outer insulation several times thicker than the conductor, made
of a very flexible and heat-resistant material such as silicone or EPDM rubber. The thick insulation prevents arcing from the cable to an earthed engine component. A
rubber "boot" covers each terminal. Dielectric grease can be used to improve insulation; a small amount can
be applied in the inside of the rubber boot at each end of each wire to help
seal out moisture. Printing on spark plug wires may include a brand name,
insulation thickness (in millimeters), insulation material type, cylinder
number, and conductor type (suppressor or solid wire).
The wire from each spark plug is just long enough to reach
the distributor, without excess. Each end of a spark plug wire has a metal terminal
that clips onto the spark plug and distributor, coil, or magneto. There are
dedicated spark plug wire pliers, tools designed for removing the terminal from
a spark plug without damaging it.
To reduce radio frequency
interference (RFI)
produced by the spark being radiated by the wires, which may cause malfunction
of sensitive electronic systems in modern vehicles or interfere with the car
radio, various means in the spark plug and associated lead have been used over
time to reduce the nuisance:
1. Copper conductors (no suppression)
2. Resistor in spark plug with copper
conductor
3. Compressed carbon powder as conductor
in the lead to act as a resistor
4. Stainless steel wire wound as a coil
in the lead with a resistance of about 1300 ohms/meter since 1980s. This acts
as an inductor and a resistor
APPLICATION
Placing spark
plug wires back into their separators or holders during replacement helps to
keep them in place despite engine vibration, extending their life. A common
problem with spark plug wires is corrosion of the metal end terminals. Better-quality spark plug
wires usually have brass terminals, which are more resistant to corrosion than
other metals used.
Older engines
also have a wire connecting the ignition coil to the distributor, known as a coil
wire. A coil wire is of the same construction as a spark plug wire, but
generally shorter and with different terminals. Some distributors have an
ignition coil built inside them, eliminating the need for a separate coil wire,
e.g. GM High energy ignition system and some Toyotas and Hondas.
Many modern
car engines have multiple ignition coils (one for each pair of cylinders) built
into a coil pack, eliminating the need for a
distributor and coil wire. Some car engines use a small ignition coil mounted on top of each spark plug, eliminating the need for spark
plug wires entirely.