I have re-drawn the wiring diagram

[christopher storey]

The problem you are encountering is that the S2 (which I think formed the basis of the wiring diagram you used ??) used an ignition triggered inductive electronic impulse tacho , which has a white lead from the ignition switch to the coil looped through a sensor on the back of the instrument itself, with 12 volts being provided to power the instrument by a green wire from, usually fuse 4 or fuse 7 . The photograph you show is the earlier AC tacho used from about 1959 to 1969 on various Jaguars, in which the instrument is completely independent of the car's electrical system, and is a closed loop of the type you describe from the generator on the back of the inlet camshaft, to the tacho. A series 2 car will have no provision for this on the loom and you will have to wire it separately , and a s1.5 may well be the same. If you have a multimeter, you can easily check the tach generator : it should produce about 10 volts AC per 1000 engine rpm, and contrary to what many people say, I have found these setups to be incredibly reliable

[PeterCrespin]

Agree totally with Christopher.

Be aware, however, that whilst reliability is superb for such simple circuitry and instrumentation, accuracy is a different matter. They will have been fine ex-works, but accuracy does fall off over the years as the generator eventually loses magnetism from heat and vibration.

As with any techometer, if you are thinking of revving close to the red line fairly often, it pays to calibrate it (with for example an advance timing light) and make sure it isn't reading low as these eventually tend to do.

Pete