Why ignition coils fail – common causes

Back in OBM428, Bill Woolnough wrote to us about his ‘ignition coil blow up’. Here Tony Proctor presents a comprehensive explanation of what might have happened…

The most common causes of ignition coil failure is overheating due to excessive spark gap. Spark plugs that are gapped excessively large require additional voltage for the spark to jump from electrode to earth electrode; this causes additional load on the ignition coils and subsequently causes internal coils to heat up. During the excessive heating and cooling process, the coils expand and contract each time, diminishing their integrity and causing malfunction or total failure.

Additionally, oil-filled coils do not take kindly to their mounting clamp being overtightened to the point where the coil body starts to crush. This may cause a number of issues, from forcing the internal cooling oil to begin escaping around the coil top to an internal short which will cause misfiring, poor starting and intermittent loss of ignition HT until the coil can cool. It shouldn’t really make a difference if the coil is mounted upside down or laying down, but upright is best if no oil leaks are to be encouraged.

Vibration and excessive heat are poor partners to the coil. It makes you wonder why manufacturers placed them on the dynamo (e.g. on Morris Minors and Minis) where vibration was high but the cooling blast from incoming air through the grille etc helped cool the coil. It has been my experience that an oil-filled ignition coil with ignition circuit ‘on’ but engine not running may cause the internal heat buildup created by the coil windings to heat the oil to such an extent that it literally blows the coil top off, showering lightweight (thin) oil everywhere.

Coming down to earth

A causal effect of this may be due to a poor earth in the ignition circuit contact breakers (points), badly burned and not passing current to earth. Most likely is, in the case of an old coil, that an internal breakage has occurred of either the secondary coil winding (thin wire like human hair thickness) containing many windings or the primary coil winding a few hundred windings (string-like thicker wire). The break will touch when the coil is cool so a circuit made but, as the temperature increases, the break opens and the circuit is lost.

The other problem is that all of the coil windings are supposed to be insulated from each other with the exception of the designed circuit connections, so a breakdown in insulation will create issues.

Manufacturing procedures and material issues can also render problems almost straightaway or not long after fitting, in this case built-in circuit failures. Running with too big a spark plug gap will cause higher coil temperatures. Then there is resistive wire failures and other features. Incorrect voltage selection is another problem, eg 6volt coil used in place of 12volt. More modern coils are given heat sinks and the coil itself is resin or bitumen type, filled to reduce vibration, while the heat sink looks after cooling. A good tip to older bike owners with external ignition coils is to mount the coil on a thickish aluminium plate, e.g. 8in thick and 4in square (size to suit) and well earthed. If you shake an oil-filled coil, you will be able to hear the oil inside and there has to be an air gap to allow for oil expansion when heated.

Piston ring migration

The piston ring gap ‘migrating’ into lined up position is a well-trodden path. Two strokes need to have their rings pegged so they cannot rotate around the piston ring land (the grooves) and collide with the ports in the barrel. Four-stroke engines’ piston rings are free to rotate around their grooves as there are no ports to be avoided; as a consequence, ring and bore wear are much less and last many thousands of miles. It is good assembly and engineering practice to position the ring gaps ‘out of align’ to prevent initial and unwanted scratching caused by ring gaps if they were all lined up.

Another reason is that it prevents compression pressure leaking away, also allowing oil up which could cause excessive oil smoke on start up. It is the up and downward bore travel of the piston(s) together with vibration that causes the rings to rotate around the piston ring land grooves.

Naturally after many miles or hours of operation the ring gaps could line up, but only momentarily. Excess carbon buildup and questionable lubricating oil will cause the rings to gum up and stick in one position, which will hasten bore and ring wear.

The Honda lawnmower... ignition coil internal break in the coil winding being coaxed into contact with nail impact or poor earthing of coil to engine case being re-established by nail impact... remove flywheel, clean coil mount refit.

If the flywheel cover was in contact with the flywheel, it could be possible for the magnetism to be weakened. The only other ‘weak’ suggestion as to magnets reversing polarity is that someone placed a degaussing circuit next to the flywheel for a joke!

Hope this is useful. It’s really interesting reading other readers’ offerings – I must get out more!