Originally Posted by 97 SL320

Also, make sure your input can handle the sensors max voltage + some head room otherwise it might start clipping and you will end up with eratic readings. Don't try to measure the voltage and convert it to RPM, you will end up having non repeatable readings. Frequency ( analog ) is all you care about, duty cycle won't get it done either.

The sensor is a coil of wire wrapped around a rod magnet, the end of the rod looks at the flywheel teeth.

This type of system is very comon on industrial diesels, look at " Murphy " brand for off the shelf tachs, speed switches.