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If you have lost spark, check resistance between tip of spark plug and ground. Most of the problems can be found there.

If you have a twin coil, please additionally  see our special information on this coil!

Check high tension (HT) circuit for possibly abnormally high resistance!

If the combined resistance of HT cable, resistor cap and spark plug is too high, than the system will not start or do so only with difficulty (especially with hot engine). If your system starts, runs, but refuses to restart when hot, you can bet that you have a resistance problem (or a carburation one). To test that, close the petrol cock (gas valve) a few meters before you stop and than check again starting with hot engine. Very often helps.

The higher resistance is, the more power is turned into heat, increasing resistance even more. A vicious circle. The problem is more likely to happen on twin outlet coils, as both outlets are in sequence and add.


Resistance can be checked quite easily with an resistance (ohm) meter.

The ignition coil(s) remain(s) installed. This means the wiring from the stator to the ignition coil, the HT cable(s) at the coil with the resistor cap(s). Place the resistor cap(s) onto the spark plug(s) which have been removed however from the cylinder head (so that you have access to the middle electrode)

On single spark systems check resistance between the tip (middle electrode) of the spark plug and ground (best the cooling fins of the cylinder, unless those are painted).

On twin spark systems  check resistance between the middle electrode of one plug and the middle electrode of the other plug.



connect to an Ohmmeter tuned to read between 5 and 20 kiloohms.
(click picture to enlarge)

Please note that checking resistance of the HT circuit is the only thing you can and are allowed to check on our ignition coils! Please refrain from experimenting with other checks! All our instructions also explicitely say so! more info see here
If you check resistance of secondary (HT) winding of the ignition coil with the coil not installed, please know that not all coils have ground on metallic frame! In coils marked 7091, 9523 and 9522  ground is the brown wire. In coils 7021 and 9521 it is the white wire.

If Ohm-values are higher than about 10kOhms than you should look for the reason and rectify the problem.

Reasons might be:

# faulty spark plug caps (even new ones might be faulty)
# faulty spark plugs (even new ones might be faulty)
# wrong spark plugs (such with internal resistor. NGK offers e.g. R (R for resistor) coded plugs.
# faulty high tension wires (never use non-copper ones)
# a faulty ignition coil (very rare, good coil has about 6.3kOhms resistance)

It would be best to check this value with an hot engine. (But take care not to burn your fingers, use protective gloves!)


In the picture on the right we have checked an ignition coil with HT wires completely without resistors and without spark plugs.

The instrument shows about 6.3kOhms - the resistance of the internal secondary winding of our ignition coils. This value is the same in all our coils.

Important: Please do not try to check values of the other wires at the coil. As the coil contains electronics you would not get any sensible values, but endanger the part.


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