BSA Service Sheet number 808 shows the wiring diagram for the A,B & M group models. My copy has the date October 1948 and it is for a negative (-)earth system. These machines changed to a positive (+)earth system some time after 1948. I am sure that somebody can tell us the date of the changeover.
As your bike is a 1940 model, the negative earth system is correct.
Re: + to earth?#15088 09/07/0612:22 pm09/07/0612:22 pm
Interesting... my recently purchased 1965 A50 also had the (-) side of the battery to ground. All of the literature I have suggests that it should be (+) to earth (BTW, it is a 6V battery). Question: if one of the previous owners got it backwards, what problems may I be facing?
2004 Triumph Daytona 1965 BSA Pre-unit Triton
Re: + to earth?#15089 09/07/067:13 pm09/07/067:13 pm
The BSA Twin Restoration book states that BSA started using positive earth systems in 1951. I take this to mean for all models and not just on the twin cylinder machines. It doesn't state in it that BSA reverted back to the negative earth system. However, it does mention that the the 12volt system was standardised on the twin cylinder models in 1966. On saying this, some machines had 12volt systems prior to 1966, but not all of them. Your A50 is probably correct with a 6volt system but it appears that it should have a positive earth. Do we have any electrically orientated guys here who can advise and maybe comment on my findings? Beezageezauk.
Re: + to earth?#15090 09/07/067:39 pm09/07/067:39 pm
One thing to remember BSA has a pos grd for some kind of safey thing a million years ago..anyway if your battery is completely dead it is very possible to charge it up backwards so what you end up with from the battery < is a negitive which is the postive..and a pos. which is neg.I just know this is got to helpful..Beezerbill
Re: + to earth?#15091 09/07/068:40 pm09/07/068:40 pm