Testing Lucas coils '69 Commando

Sparks aren't my strongest attribute so can anyone please tell me how to test my coils, 17M12 Lucas, David

A low voltage (9v battery) ohms check can only show if the coil is Bad. The ohm meter test can NOT prove it is good.
Typically a home hobbiest, or even most professional repair shops, do not have high voltage test equipment to PROVE the coil is GOOD.
A primary test, secondary test, and an insulation quality test, all three passing will show if it is "good".
However a good coil is not necessarily appropriate for the intended application...

gtiller said:

Of course Dave is right - the guy is a genius and is never wrong, and he has a workshop of specialist test equipment.


However, for hobbyists, like me, the test I have outlined will show up the most common cause of failure (in my experience)

The primary windings inside the coil are the low tension side.
This is the part that most commonly fails - either by burning out, or by physical damage (like doing the clamp up too tight and crushing the windings)
The wrong resistance of this can also damage your electronic ignition too, if you have moved away from points.

Of course there are more thorough tests, but most of us don’t have the facilities to test.
I have tried to give you an easy to do test that will give you an idea as to whether your coils are ok or not in a way that doesn’t bombard you with terminology that is confusing.

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In addition to what you posted, there are a couple more simple resistance checks that can be done. Either primary terminal to the coil HT tower should not be open or shorted (I will edit this later today with a "typical" value). Also there should be an open circuit between all terminals and the coil's case. This is best checked with the coil mounted in place, as I have seen Lucas coils shorted when the clamp is too tight.

As Dave noted, this doesn't guarantee that the insulation won't break down at 25,000 volts, but for a shade tree mechanic it's a start.

When all you have is a hammer...

A story to show what dynodave said...

Bike that was running fine started running terrible and would not rev over 3000 RPM. After eliminating everything else, replaced the Tri-Spark ignition. While trying to set static timing. when the test light came on, it would come on dim and then stay on no matter how you turned it until you turned off the key. Thought it was a bad Tri-spark so tried another - same issue - reinstalled the original - same issue. Removed the Black/White wire and the Tri-Spark acted normal so it was something with the coils.

They were two Lucas 17M6 coils dated 1972. Each measured 1.8 ohms plus to minus. Measured 9,000 from the plus to the high voltage output. Measured infinity from any of the three connections to the coil's case. Generally, that means that the coils are good.

Installed a different pair of Lucas 17M6 - problem gone! I still can't figure out what's wrong with one or both of those coils as every ohmmeter test is good and the new set has the same readings. Of course, resistance is only part of the story with coils, there's also impedance and capacitance but they should not come into play when setting static timing.

I have removed my post, as it’s seemingly not correct

gtiller said:

I have removed my post, as it’s seemingly not correct

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Nonsense... that was the standard primary winding test with an ohmmeter, and is pertinent to the thread.
It would identify a bad coil, but not necessarily guarantee a good one.

My Emgo dual output coil, working with Wassell/Vape ignition, failed over the last 70 miles or so, going from an occasional miss under acceleration, to frequent missing at low rpm with any throttle, to crazy missing and spluttering as I struggled the last mile through town. Only just made it home, I think!

Something about the symptoms said HT to me. Sure enough, the HT was open circuit.

Coil hadn’t done a lot of miles, but it had been kicking around my various sheds for thirty-something years.

I’ve replaced the coil with one of the cheap Chinese ones that are all over EBay. I bought it for no particular reason a couple of months ago.

Old coil was the chrome type pictured.

marshg246 said:

Measured 9,000 from the plus to the high voltage output.

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dynodave said:

and an insulation quality test

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Not knowing for sure but, within the HV coil, a 9v ohmmeter test mignt have a few milivolts between adjacent wires passing just fine. Yet in use, where from ground to the output typically at partial throttle, you might generate only 7kv (to jump the plug gap) and it might still run. Open the throtle and due to cylinder pressure it can 'TRY" to go way up (16-17KV) and poor inter wire insulation breaks down. The reduced voltage from the coil then fails to jump the spark plug gap. A super over heated coil could damage the varnish insulation.

Triton Thrasher said:

My Emgo dual output coil, working with Wassell/Vape ignition, failed over the last 70 miles or so, going from an occasional miss under acceleration, to frequent missing at low rpm with any throttle, to crazy missing and spluttering as I struggled the last mile through town. Only just made it home, I think!

Something about the symptoms said HT to me. Sure enough, the HT was open circuit.

Click to expand...

And here’s the open circuit.

My favorite bad coil story is one in a car that would cut out during aggressive left turns. No problem with aggressive right turns or normal left turns. Troubleshooting revealed that depending on the orientation of the coil it could change what 'maneuver' caused it to cut out. We got it to cut out on hard right turns or on straight-ahead acceleration, all depending on coil orientation. Obviously some internal issue that we never tried to figure out, we just replaced the coil!

I'm totally looking forward to electric cars/motorcycles once they get sufficient range capability to be workable for my needs. I'd be happy to leave the world of all this antiquated crap behind! Sadly, at the moment, the electrics (cars or bikes) don't have anywhere close to enough range. I would need 800+ miles for a car; 300+ for a motorcycle - both at maximum speed limit (+) speeds.