Having read various topics on these electronic tachymeter, I did all the tests I could do first. Good 12v supply between the -ve and +ve terminals in the plug. And using the AC setting on my multi-meter, I had a good 8v on the S terminal when the engine is revved which dies down to 0 when the engine is stopped.
So decided to take a look at the sealed tachymeter unit...except someone has been there before me and I had quite a job breaking through the glue that they had used to re-seal the unit. But having got it apart, it all looked very clean and I could see what the problem was: the needle was on the wrong side of the needle stop post at 0.
I couldn't understand how this could happed but carefully put the needle back on the correct side of the post and tested it on the bike and it worked. So far, so good. But while moving the needle around the dial, I could feel the flux from the magnets in the "motor" and if I rotated from 0 to past about 8, the opposite flux kicked in and took the needle all the way round to the back of the post.
This cannot be normal but if it does this again, the only way I have to reset it is to open up the case again and move the needle in an anti-clockwise direction pass the 8 mark and then the opposite flux takes it back to 0 on its own. As I can't keep doing this, for the time being, I've put a needle stop at about 8 (see photo).
Anyone else seen this and what's causing it? Is there a cure? Apart from this, the unit is working fine.