This short technical update is written to explain how to deal with the common problem of sparking ignition leads - that is leads that spark along their length or at any place other than across the spark gap.

With the advent of higher voltage ignition systems, electronic and computerised controls as well as sophisticated testing equipment, ignition leads are taking on a whole new realm of importance. Proper installation and maintenance is now of critical importance if the leads are to function effectively on the engine. In the past, because voltage potential was relatively low and the degree of sophistication involved in spark control was low, lead installation was simple and caused few problems. Over the past few years this situation has changed significantly.

Very briefly, experience has shown that most problems occur from incorrect installation of leads. The most common problem area is the spark plug terminal connection to the spark plug nut. When installing the lead ensure the spark plug terminal inside the boot is firmly ‘snapped’ over the spark plug nut. This is particularly critical on some leads as the spark plugs are not always easily accessible. If the terminal is not connected firmly to the spark plug nut, then the voltage in the coil will rise significantly. (See Fig.2)

On modern ignition systems the voltage will rise many thousands of volts above normal. This is often observable on electronic analysis machines, which will indicate a lead or leads having excessive voltage (High Snap KV). The usual problem is not the leads themselves but poor connections to electrical apparatus at either end of the lead.

Some leads spark
Check lead connection to spark plug

Engine idling roughly
Check both spark plug and distributor/coil connections

Engine losing power under load conditions
Check all terminal connections for good electrical contact

Analysis machine indicating High Voltage in some leads
Check lead connections to spark plug

Analysis machine indicating High Voltage in all leads
Check lead connections, both ends

Distributor or Coil boots ‘pop’ off distributor or coil towers
Boots need to be ‘bled’ of air locked inside the sockets. Slip a small screwdriver or pen inside the mouth of boot to release air.

Leads appear to ‘glow’ at night. Appears as a fluorescent glow, seen during periods of high ambient humidity
This effect surrounds all High Voltage conductors. It is technically known as Corona, and is not to be confused with the sparking associated with breakdown. Generally does not harm.

• Terminal depth from the end of the spark plug boot - should be about 20mm. A terminal located too deeply inside the boot will not contact the spark plug, thus causing a dramatic rise in voltage and consequently the leads to spark, particularly around the boot/cable junctions. A terminal located too close to the end of the boot will not be sufficiently insulated and will allow voltage to jump along spark plug porcelain and across to any metal parts.
• R/A distributor and coil boots may need to be bled of air during installation, otherwise they will ‘pop off’. Simply slip a small screwdriver or other object inside the mouth of the boot during installation. This will allow air to escape and cause the boot to be held in place by the vacuum inside the socket.
• Modern leads should be treated as high voltage conductors. Clean the leads with metho or turps during routine maintenance. Some grease, particularly graphite based grease, and dirt will conduct electricity and cause surface tracking and breakdowns.
• Keep leads well away from all exhaust manifolds and other hot areas of the engine. Silicone lead is able to withstand 250°C+ temperatures. Exhaust manifolds can be much hotter than this so keep leads well clear.
• As far as possible keep leads well clear of all metal parts.
• Top Gun standard 5mm, 7mm & 8mm spiral wire (suppressed) leads should be approx. 6,000 Ohms (+/- 10%) per metre length. Test with a multimeter from terminal to terminal, and multiply resistance by

• Change leads one at a time, starting from the longest, to ensure proper firing sequence is maintained.
• Push plug end on firmly and ensure a good connection is made. The terminal should 'click' over spark plug. Experience has shown that many failures occur from terminals incorrectly connected at either the spark plug end or the distributor/coil end. (See fig.1 and fig.2)
• Push distributor and coil end carefully, ensuring a positive connection is made. When installing right angle distributor and/or coil ends, trapped air may need to be bled from the distributor/coil socket to ensure the boot stays on. Slip a thin screw driver or similar inside the neck of the boot to release trapped air. Be careful not to damage the boot. (See fig.3)

• When replacing integral cap and lead sets it is important to also check and/or change the rotor button.
• If the tube assembly keeps 'popping off' the plug after installation, then check that the vent hole in the top cavity cover is unblocked. Pass a piece of thin wire through this hole to remove any blockages.
• Ensure no water or oil is in the plug chambers, as this will cause the tube assembly to break down or deteriorate.
• Always replace the spark plugs with correct type and size, as varying profiles will prevent leads fitting correctly.

• Never work on the electrical system when the ignition is on or the engine is running. Ignition voltages can be lethal.
• Always pull the leads by the boot and not the cable. Do not try to remove the boots off the lead as it may damage terminals and conductor inside the cable.
• Keep cable and boots well clear of the exhaust manifold and other "hot spots".
• Keep leads away from all metal parts to reduce electrical discharge and breakdowns.
• If leads appear to spark or glow, this may only be a corona phenomenon. Corona is caused by climatic conditions. Cleaning leads with methylated spirits can reduce corona, otherwise the effect is harmless.

Most overhead cam engines manufactured since the late 1980’s have recessed spark plugs which are connected to the ignition system by high tension cables and a long rigid tube arrangement. The rigid tubes allow the lead to positively connect onto the spark plug terminal and ceramic, while at the same time sealing the top of the chamber against the ingress of moisture and other contaminants.

Occasionally these rigid tubes melt while in service. This should not and will not happen when the vehicle is used under normal operating conditions.
The rigid tube material is made from Poly Butylene Terephthalate or PBT for short. PBT is an excellent material for these applications.
It has the following characteristics:
• PBT has a normal operating temperature range up to 210°C.
• PBT will not melt until it reaches 240°C.
• The hottest operating temperature of a spark plug ceramic is around 120°C, under normal conditions.

Under normal operating temperatures the PBT tube will function correctly for many years without incident. However, under severe engine overheating conditions the PBT tube will melt. This may come about from a cooling system failure, an overloaded engine, running the engine low on lubricating oil or in a limited number of situations from incorrect timing.

If the PBT tubes on an engine melt, we recommend that the cooling system be carefully checked. If the PBT tubes melt it is a consequence of engine and cooling system malfunction, lead failure is not the cause. Melted tubes will normally stop the engine from operating. This will limit the amount of consequential damage done to other engine parts. Most overhead cam engines are particularly sensitive to overheating, which can result in head gasket failure, camshaft damage and piston seizing.

If an intermittent miss occurs, especially when the engine is under load, this could be caused by the PBT tube breaking down resulting in voltage arc, see photo.