Tach question
 

Does anyone know how an aftermarket tach works is there something inside spinning at a faster rate than the needle. Mine makes a ticking sound at higher rpm but sounds like it is spinning very fast?

Thanks

Aftermarket electric. But how the needle moves I thought it would use a spining gear inside to get the sweeping movement?

The 'motor' which moves the needle uses two coils wound 90 degrees apart (electrically). The electronics inside apply varying voltage (which causes current to flow in the coils) which cause a magnetic field to be generated. The needle is mounted to a magnetic core which is free to spin within this field, and will therefore allign the movement to those fields (two coils remember). The electronic circuitry in every tach I have examined uses a local oscillator and compares this frequency to the incoming frequency (from the ignition system). These two are compared and an intermediate frequency is produced which is used by the circuitry to determine the proportion and strength of the two fields. It's possible that some noise could be produced by the electronics (capacitors charging/discharging) due to the three frequencies in use, but I would expect it to be a very soft "hum" not a clicking. I have never seen a 'mechanical' movement used in an electrical tach, which uses a spinning movement, but it could be possible to use one and have a motor act as an input to the mechanism like a machanical tach.

As said be sure to eliminate the speedo or other guages (metal oil pressure lines bringing in engine noises etc) something vibrating with the engine etc... If it is truely the tach I'd be interested in any findings you make to as to the cause.

Thanks for all your help guys. I might just be the speedo