read engine rpm from battery/alternator???

[Dave Williams]

-> On a GM altenator, there are 4 pins, one big and 3 small. The 3 small
-> are L for lamp, F for field, and an unlabeled one that we used in a
-> GM race program where rules would not let us use a wheel speed
-> sensor. That pins outputs a frequency proportional to rpm which we
-> used with some engineering math to get vehicle speed.

 Is that the same pin that GM Diesels used for 'tachometer' output?