Hi has anyone done this before?
Obtaining motor RPM from car’s altinator,
(PS: don’t have access to CAN ODBII)
I don’t need exact RPM, but a scaled 0-100% reading.
I’m considering using the LM2917, which is a frequency to analogue voltage
suggestions welcome
Touch a scope to the alternator and you will know if you get some useful reading or not 
Some suggestions from the web:
[url]How to measure the rpm of a car motor from the ripple battery? | Forum for Electronics
[url]RPM from alternator ripple? | Electronics Forum (Circuits, Projects and Microcontrollers)
But it doesn’t seem to be a simple thing to do.
Any particular reason you want to do it on the alternator? Is it a petrol or diesel engine?
How old is the car? Have you thought about using the cam/crankshaft position sensor to get the RPM? You might even be able to cheat and do like a lot of aftermarket tachometers and borrow the same signal that the car uses for engine management.
intention is for forklifts as a rough indicator only, the least invasive method is preferred.
I have seen some “gimmick gadgets” that plug into the cigarette lighter that are able to detect rpm,
and then broadcast a hotrod audio sound over FM radio to make the car sound larger and noisier than it really is.