1968 Lightning Zenor Diode.

[tdc]

The mist is slowley clearing, with the 12v wireing, that had been mostly ripped out when aquired but it would seem that the finned  heat sink has developed a dead short ? Have I perhaps missed something when re instalinf the thing ? .

[iansoady]

Is that a short bweteen the Zener lucar connector and the heatsink? Remember that the Zener is still a diode and will pass current one way (usually positive connector to earth) at low voltage eg with a multimeter.

[tdc]

Thanks Ian .The thing is when the heat sink is earthed the fuse blows ?

[iansoady]

Presumably when the Zener is connected? If so I'd suggest a failed Zener - or incorrect polarity.

[tdc]

Yes I was thinking thing this also but as the m/c has never yet run I cannot say if it was ok before removal for clean up but as I have only ever been usd to 6v I am unable to say either way but as the hot wire comes directly from the Ign. switch would this reverse the polarity if you follow me or would the sink hot pick up fail to earth . If you follow me ?

[V500]

It should be marked as +ve or -ve, but Zeners usually fail O/C rather than dead short. I would guess, as already said, you've used the wrong polarity.

[tdc]

Thanks Ian . As found the battery was wired + earth As I am not yet secure in understsnding the finer points of the 12v system I will need to do more research . Thanks .

[ChrisG]

If, as you said the battery is wired positive earth, that is how the boys at BSA intended so correct! As such your zener diode, if correct will be marked with Lucas part no. 49345A. If different it's likely to be for a negative earth bike such as the later T160's or Norton Commandos, which had two zeners and I believe may have been negative earth.
I would recommend getting a new one from a well known supplier. C&D, Kidderminster are my favourites.

[tdc]

Yes I wil do that. Thank's Ian .

[rhyatt]

Sometimes these diodes are quite pricey , which could lead to considering the opportunity to convert all the "old" components for a modern reg/rec

[cornishbob]

whoever it was that converted mine to 12v did a reg/rec. nice and tidy it is too