Spark Plug Wires
Design and Purpose
Secondary wires carry high voltage
produced by the ignition coil to the spark
Resistance wires are used in order to
lower the current flow through the wires.
Keeping the current low, by reduces the
magnetic field which needs to controlled
because it causes radio interference.
Plug wires must be covered with insulation
that will withstand heat, cold, moisture,
oil, grease and chafing.
Removing Plug Wires
Resistor type ignition cable has very
little tensile strength and could be
broken if pulled.
When the conductor is broken, a spark
will jump across the gap and the
conductor will disintegrate. Misfiring will
Pulling a spark plug boot straight off
the spark plug can damage the
conductor inside the wire. Rotate the
boot before pulling it off.
Resistance in Cables
Most plug wire are made of nylon,
rayon, fiberglass or aramid thread
impregnated with carbon. The carbon
helps reduce RFI but does not effect
Most plug wires have a resistance value
of 3,000Ω to 6,000Ω per foot. It is not
uncommon to have between 6,000Ω to
12,000Ω. The accepted value when
testing is 10,000Ω per foot as a general
Checking Plug Wires
Resistance hinders the flow of current. If
you suspect bad wires, measure the
resistance of the wire while gently twisting
and bending it.
Turn the rotary switch to ohms.
Touch the probes both ends of the wire.
Resistance value should be about 10,000
ohms per foot. Some wires may be less.
You should compare readings to other spark
plug wires on the engine to insure accuracy.
Plug Wire kΩ Ω inches inches/12 = ft Ω/ft
Check your understanding:
1. What is the purpose of ignition cables?
2. What is the purpose of insulation? (more
3. What may happen to wires that are
4. What effect does this have on the engine?
5. What is the material used as a conductor
resistance- type cables?
7 6. How are resistance values measured?