It refers to a ring or wheel fitted to the crankshaft or the flywheel which has 'bumps' on the edge which cause variations in the current in a fixed sensor; the sensoroutputs a signal whose frequency depends on the RPM of the engine. This signal is often at an audible frequency, hence the term 'tone wheel' in English.

FWIW, the very first electronic organs ('organ' as in 'musical instruments') used the same method to generate audio signals having a variety of waveforms depending on the shape of the 'bumps' on the wheel..

Resultados de la búsqueda System and method for sensing position of a motorcycle crankshaft ... - [ Traducir esta página ]
A flywheel is arranged perpendicularly and concentrically with respect to the cranksha. ... Passage of the tone wheel past a sensor allows electromagnetic determination of the engine's angular position and rotational speed. ...
www.freepatentsonline.com/y2007/0163336.html - Similares
de T Pirone - 2007 - Las 4 versiones

--------------------------------------------------
Note added at 3 hrs (2009-09-11 00:18:00 GMT)
--------------------------------------------------

@asker: And thanks to you, Daniel, for providing an excellent excuse to go back in time (in a field that far closer to my own interests than motor-bikes...): http://www.umatic.nl/tonewheels_historical.html