Does anyone know what the electrical characteristics of a W terminal are on an alternator ?

It can be used by rev-counters to get an RPM reading, typically on diesel engines.

I would specifically like to know if its voltage increases with RPM, or is it a pulsed output, etc...

Thanks
Greg

The "W" marking on bosch/lucas alternators runs around 7-8 volts, it is taken from a single phase of the alternator although complete it is of course three phase, unless a very small one(some are single-but rare), usually used with STAR configuartion, rarely with DELTA.
The "W" gives a pulsed output.
tabetha

Thanks tabetha, I presume the pulse frequency increases with the speed the alternator runs at ?

Yes the pulse is pre rectification, so will be according to speed.
tabetha