read engine rpm from battery/alternator???

[KD6JDJ at aol.com]

>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. 

  Alex
  I dont know how much work you are willing to do with your alternator.
  If you are willing to disassemble it, it is simple to attach two leads to 2 
of the stator leads within the alternator. That will give you a very reliable 
ac signal -- always. 
  If you do try to read the AC off the 12 vdc lead, you will get many 
different kinds of ac component signals, depending mainly on battery charge 
level. 

    Jerry