It is difficult for the home mechanic to fully diagnose whether an ignition coil is good or not, as a full test would involve four tests:
- Visual inspection.
- Test primary windings – this is the low tension side of the coil
- Test secondary windings – this is the high tension side of the coil
- Test insulation – making sure the insulation material between the primary and secondary windings is not damaged, or has not broken down and that the bobbin that the coils of wire are wound onto are doing their job.
The third and fourth tests really need specialist test equipment to perform properly, as a multimeter which is powered by a small 9 volt battery will struggle to give you consistent, reliable and correct results.
The good news though is that the first two tests CAN be performed, and if you see issue here, you know that your coil requires replacement.
1 Visual Inspection
This is actually an important element – if the outer case is damaged, it can easily crush, break or short out the very fine copper wire (often thinner than a human hair) that reside directly behind the casing.
The copper wire is lacquered, so if any of that is scraped away, it can easily cause a short.
These fine wire windings form the high tension side (several thousand volts, but very, very low current, just milliamps) which are discharged to the spark plug when the points open, or when the electronic ignition is triggered.
Also bear in mind that the original coils were filled with oil – this would protect the windings inside, but more importantly would keep them coil. Later on, many manufactures switched to resin encapsulated, but having realised the benefits of oil-filled, many are now switching back. If the outing casing is damaged, it is possible that the oil could leak out, again rendering the coil useless.
In particular on a Norton Commando, these coils fail when the clamps are done up too tight.
The clamps will crush the outer casing a little, which is enough to damage the delicate coils inside the can, and either short out, or break totally.
If you like, you could check with your multimeter (using the same procedure as described below) that there is not a dead short between the centre pin of the coil (where the spark plug lead plugs in) and the outer casing.
2 Test the Primary Windings
The Primary Windings of an ignition coil are typically much more robust – they are usually in the centre of the unit, the gauge of the copper wire is much thicker, and there are just a few turns.
So, although this is not a part of the unit that often fails, it is essential to check the resistance of the primary windings because if it is too low or too high, it can easily damage an electronic ignition module.
An owner will often make an error when switching from points to electronic ignition, assuming that the coils will be perfectly compatible. This is often not the case.
With a multimeter, you can check the resistance value of the primary windings, and make double sure that your coils are
- functioning as they should
- compatible with your choice of ignition system.
So we are going to put our multimeter into “Ohms” mode as we are going to be measuring resistance.
This is usually indicated by the Ω symbol on most multimeters.
The display will read something like “OL” which means Open Line when nothing is connected.
If you cross the two probes together, it will show “0” zero being there is no resistance between the two probes.
Take you ignition coil, and put the two multimeter probes across the low tension side.
These are the two terminals that come from the points or electronic ignition.
It doesn’t matter which way round you connect the probes, as there is just a coil of wire inside – no other components that are polarity sensitive to worry about.
In the case of Lucas coils, here are the figures that you should be looking for:
Lucas 17M6 (LU47275)
- These are the 6 volt coils used on Norton Commandos from 1971 onwards
- Minimum 1.7 ohms
- Maximum 1.9 ohms
Lucas 17M12 (LU47276)
- These are the 12 volt coils used on the older pre-1971 bikes
- Minimum 3.3 ohms
- Maximum 3.8 ohms
When you move to electronic ignition, it is important to note that the manufacturers are very particular about the resistance of the primary windings of the ignition coils that you fit.
Here are the requirements specified by the main Electronic Ignition suppliers:
So note that the standard Lucas 17M6 coils are fine for all of these systems!
When you move from points ignition to electronic ignition, you change the wiring configuration of the coils from the original setup where they fire independently for left and right cylinders to a series setup, where the two coils both fire at the same time.
This is known as wasted spark ignition.
In series configuration, you are wiring the two coils together, so that you are looking at the total resistance of two Lucas 17M6 coils. In other words (1.7 to 1.9 ohms) + (1.7 to 1.9 ohms)
Put simply total resistance will be 3.4 to 3.8 ohms.
When you are shopping for a new ignition coil, it is tempting to go for the neat, compact units that you see on modern bikes.
This comes with a warning.
Many modern systems use micro ignition coils, which are working at smaller voltages and much smaller resistances.
For example, here are two coils found in modern ignition systems on modern bikes:
At a primary winding resistance of around 0.5 ohms – these are NOT suitable for the Electronic Ignition systems commonly found on a classic brit bike.
Trying to use one of these would simply fry the electronics in the Electronic Ignition system.
The other common upgrade is to move from dual coils to dual tower single coil.
On a Norton Commando, this change is contentious for some, as many believe the coils are part of the iconic Commando look:
However, others feel that a single coil is neater and tidier:
Whatever your feelings are, please make sure that whatever coil you go for checks in with the correct primary winding resistance.
The dual tower single coil that we have here, which is the Crane Cams 8-3006 unit used by many (and common on 80s and 90s Harley Davidsons), as supplied by Matt at Colorado Norton Works is 3.0 ohms.
This is right on the low side of what I am personally comfortable with.
I would much rather see it at around 3.6 ohms instead.