read engine rpm from battery/alternator???

[Frank F Parker]

> Recently I´ve seen that Bosch´s latest rolling road reads the engine rpm from 
> the battery voltage. When connecting my multimeter to the battery I can read 
> different frequencies at different rpm´s caused by the alternator. This seems to 
> be easy but when connecting my 20 MHz-oscilloscope to the battery I can only see 
> a flat line around 14.0 volts. 
> Does anybody know how I can read the rpm from the battery using a 
> microcontroller like PIC or Basic Stamp II?
> Kind regards 
> Alex Wenning
> 
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.  You could take the AC signal and
do what ever you needed.

regards,

frank parker