Why are the coils 6v and not 12v

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My Commando is running stock points ignition the same as it came out of the factory, it uses 6v coils. My T100C Triumph also uses 6v coils on a stock points system.

can someone answer why we use 6v coils and not 12v as I assume the 2 sets of points means only one coil fires at a time.

just curious on a day made for staying indoors.
 
I believe it's so they still fire if battery voltage is low.... though I may be wrong
 
It was in preparation for the failed Electric Start scheme, the plan was to push the ballast resistor out of circuit when the solenoid operated so the lowered battery voltage (because of the high current draw of the motor) went straight to the coils. Even if its was 11V then the coils would be ok as as soon as the starter button was released the ballast resistor would go back into circuit and the coils would see 6V again. I had the same setup on a VW Golf, worked very well on cold mornings until the dwell went out of spec.
 
It was in anticipation of implementing electric start at some point. With 6V coils and a ballast resistor you can bypass the ballast when the starter is engaged, which increases the voltage to the coils during starting. Most automotive applications with points ignition did that.

The wiring for the ballast bypass is included in the 72-up harness. Undocumented.
 
If the coils are wired in SERIES, then a 6 volt drop results across each coil.

Slick
 
texasSlick said:
If the coils are wired in SERIES, then a 6 volt drop results across each coil.

Slick
Click to expand...

Which would be the wiring of choice for a wasted spark IE, but not for points, it is as described with the ballast resistor!

On some cars of the period people swear there is no ballast resistor fitted on their vehicle, because the ballast resistance is included in the wiring loom with a special cable.
 
Last edited:
To quote Norton's own Workshop Manual:
'Six volt ignition coils are used in conjunction with a ballast resistor to allow their use in an otherwise 12 volt electrical system. This system has been adopted to ensure adequate spark with the battery in a heavily discharged state'
For the ballast resistors:
'When the battery current has dropped due to the heavy current taken such as by the use of an electric starter, the coils are fed direct , by passing the resistor and thus enabling the coils to work at their approximate potential'
:)
 
SteveA said:
Which would be the wiring of choice for a wasted spark IE, but not for points, it is as described with the ballast resistor!

On some cars of the period people swear there is no ballast resistor fitted on their vehicle, because the ballast resistance is included in the wiring loom with a special cable.
Click to expand...

I use wasted spark with a Boyer. My coil is double ended 12 volt off a Honda four. If you wire two 6 volt coils in series that is two more points where you have terminals which can come loose. And if you have an internal problem with one coil, how do you know which one ? If a double-ended coil fails, it just drops dead - it does not lead you on a goose-chase.
 
maylar said:
It was in anticipation of implementing electric start at some point. With 6V coils and a ballast resistor you can bypass the ballast when the starter is engaged, which increases the voltage to the coils during starting. Most automotive applications with points ignition did that.

The wiring for the ballast bypass is included in the 72-up harness. Undocumented.
Click to expand...
Undocumented wiring is in the 71 also since it would have gotten the starter first. 71 first with 6V coils/ballast and the new style switch gear.
 
When i went to school, i was told that cars had ballast resisters and 6v coils to make staring easier when cranking the engine. Basically, when cold, the ballast allowed nearly 12v to the coil, but when it warmed up, it reduced the voltage down to the coils requirements.
Dereck
 
For cars bypassing the ballast may help starting a bit, not so much on a bike with kick start only. Some bikes, Harley Sportster, with electric start and points have resistor built into the coil and it can't be by-passed. I suspect the lower voltage going through the points reduces arcing and pitting helping the points to last much longer.
 
Its so your bike will never run properly , so its less likely to blow up . ( pommy logic ) a 6 V coil may be suitable for a 4.000 rpm volkswagon. just .

Read this , or play on the road, etc.


Why are the coils 6v and not 12v
 
kerinorton said:
When i went to school, i was told that cars had ballast resisters and 6v coils to make staring easier when cranking the engine. Basically, when cold, the ballast allowed nearly 12v to the coil, but when it warmed up, it reduced the voltage down to the coils requirements.
Dereck
Click to expand...

No, there is no “hot/cold” switch in circuit!
 
kerinorton said:
When i went to school, i was told that cars had ballast resisters and 6v coils to make staring easier when cranking the engine.
Click to expand...

Yes, because the ballast resistor is bypassed by a direct 'feed' from either a relay or solenoid during starter operation so the 6V coils get the full battery voltage during cranking.


Matt Spencer said:
Its so your bike will never run properly , so its less likely to blow up . ( pommy logic ) a 6 V coil may be suitable for a 4.000 rpm volkswagon. just .
Click to expand...

As I understand it, a 6V coil can produce similar output to a 12V coil as it's not just about input voltage.

Matt Spencer said:
Read this
Click to expand...

It's about replacing 12V motorcycle coils with higher output car coils.
 
Triton Thrasher said:
No, there is no “hot/cold” switch in circuit!
Click to expand...
The nichrome wire slowly gets hot and as a PTC (positive temperature cooeficient) device slowly goes higher resistance, The mass of the ceramic keep the nichrome warming temp slowly therefore makes the time stretched out for a few minutes. The current drive to the coils, in time, goes down. I measure this decades ago and is very old information even though few know what I'm referring to . The effect is however rather small.
So even without a starter, the coil is driven a little harder when the key is first turned on....
Ballast bypass during electric start is the much more important aspect of the ballast.
 
" It's about replacing 12V motorcycle coils with higher output car coils. "

Concurrent test was about finding coils for the 750 Ducati , to run, and win, superbikes , at Daytona ,.

I think you may find it mentions what to look for , regarding coils. Both to avoid pitfalls and to have satisfactory heavy duty ingnition . Which was NOT the case in std. production British stuff
after they dropped the Magneto .

Cost Cutting might have come into it somewhere .

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If its not in that artical , there was ( The Ducati SS ) one . Where the Std British Lucas Ignition Coils were about as much use as damp ciggarettes . run em at your peril .

If you havnt got ONE HALF INCH ( plus ) of Hot Spark , youll be at the mercy of the Hondas .
which'd be a bit like getting shot down by a Zero . When youd been provided with a Spitfire.
To use an analogy .
The Better top line Aircraft had the better mechanics / serviceman . It stood to reason . And they wernt looking to get holes in there aeroplane .The inverse in fact .
 
Right after that article in Cycle came out I ran out to our local help yourself junk yard and got 2 6V Ford coils for $20. I used them with an old Boyer Mk 1a. Still have them on the shelf. It did improve running and starting over the stock ones at the time.
 
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Matt Spencer said:
If its not in that artical , there was ( The Ducati SS ) one . Where the Std British Lucas Ignition Coils were about as much use as damp ciggarettes . run em at your peril .
Click to expand...

I think you are exaggerating [Edit] and in "peril" of what, exactly? I and many other riders of British bikes use 6V Lucas or Lucas type coils which as far as I'm concerned work fine under normal conditions.

Your continual insistence that 6V coils are somehow inferior to 12V coils just because they are 6V I don't think is entirely correct. Perhaps dynodave could comment?


Matt Spencer said:
If you havnt got ONE HALF INCH ( plus ) of Hot Spark , youll be at the mercy of the Hondas .
Click to expand...

I think there's a bit more to it than that. In any case, I don't go 'racing' or attempt to keep up with modern bikes on the road when riding my British bikes so it's of no great importance if that's what you meant?
 
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