Ignition coil resistance readings

[JohnT]

With points ignition, what readings would I expect to see on a good usable coil?
The Haynes book only refers to readings for coils fitted to electronic ignition engines.
Some years ago I ran a points ignition R80 with a K100 coil with no running problems so are the coils for both actually electrically similar?

[Rob Frankham]

BMW used the same coils (Bosche E6V) for all models from 1969 to 1984 whether fitted with points or electronic ignition... coils designed for electronic ignition were(are) available but they were(are) aftermarket. The original coils work perfectly well and the gain from using 'electronic' coils is minimal in most cases. The Primary resistance of a Bosche E6V is quoted as 1.2 Ohms so Haynes quotes at 1.15 to 1.32 Ohms are perfectly acceptable. Some users quote 1.5 Ohms. Bear in mind that most multimeters suffer from increasing innacuracy at low resistance ratings so any resistance reading below 2 Ohms and above 1 Ohm is probably perfectly functional.

Many aftermarket coils will have different resistance values and will work perfectly well.

A later (twin output) coil is designed for electronic ignition and will have a much lower primary resistance. This will work but it's not a good idea to use these with points because the high current switching will cause excess arcing and reduced points life.

Rob

[Kurt in S.A.]

What is the resistance on the secondary circuit. Seems that's important also.

Kurt

[Rob Frankham]

I've never found a reliable figure for the secondary resistance of a Bosch E6V coil. Quoted values vary between 7.5 k ohms and 11.5 k ohms. I suspect it may have varied over the production life of the coils. It isn't really a good test anyway... the voltage and current of the spark is dependant as much on the construction of the coil the DC resistance. In real terms, DC resistance isn't really an important parameter except as a broad indication whether the coil is faulty. The reluctance of the coil, which is much more important, is definitely affected by the construction of the coil and differs dependant on the frequency of the signal (i.e. the speed of the engine). In general, if a coil has a secondary DC resistance in the vicinity of 5-10 kilohms, I would say it's probably good. Clearly if there is no reading it's busted and if it's lower than - say - 5 kilohms, it's probably suspect.

If it's an aftermarket coil, you really need to get the specs for that coil as they may be very different to a Bosch E6V and the coil still may perform completely satisfactorily.

At the end of the day, if the bike runs and starts well, that;s probably the best sign you'll get that the coils are good.

Rob

[Kurt in S.A.]

I thought that the secondary resistance would be an important value. Seems like the amount of resistance in all those coils of wire is what gives you spark at the plug when the field collapses.

Kurt

[JohnT]

Thanks for your input.
Low tension results from both coils are 1.0 and 1.0 Ohms, and secondary 4.75 and 7.0 Kilohms. Both coils are Bosch and look original.
I should explain that this bike is in poor condition and been stored in damp conditions for some years hence having to check everything electrical. It now starts, idles and runs well now.
As you stated Rob repeatability of the lower readings is problematic.

[JohnT]

I thought this was all conclude but looking through my old Clymer manual it shows the link wire between the two coils going from a +/15 to a -/1 terminal. I reassembled it as found with the link wire going from -/1 to -/1. Does it matter?

[Rob Frankham]

I thought this was all conclude but looking through my old Clymer manual it shows the link wire between the two coils going from a +/15 to a -/1 terminal. I reassembled it as found with the link wire going from -/1 to -/1. Does it matter?

Yes, it does matter. The link should go from 1 to 15. 15 is the positive side of the primary coil, 1 is the negative (on each coil). Since the two primaries are wired in series, Positive on one coil will always be negative on the other. The engine may run with one coil wired backwards but for balance and best coil performance, they should always be wired the same way.

[JohnT]

Thanks Rob. Tank off tomorrow again.

[Rob Frankham]

Still, could be worse... it could be one of those new-fangtled machines were you have to half dismantle the whole thing to get to anything...

Rob