Currently I have my tacho wired on the negative side and it works fine, once I get my ute completed I will try it as in the diagram. If it works then well and good. I do understand that it works of induction, I just never realised that the slight current fluctuation on the + side of the coil would be enough to trigger it.
Edit: my apologies, The tacho can go either side if the coil, I had the picture of the test light running from + side and of course it will stay on because it is creating its own earth. Thanks Steve
