need math to convert ac voltage to rpms

I'm performing diagnostics on a car and it has no tachometer, but I need to monitor engine rpms.  There is a pin on the ECU interface that you can connect a tach to, and it has AC voltage that I can read with my DMM.  Is it possible to sample the AC voltage and then convert it to engine rpm with some fancy math?  It is a 4 cylinder engine.

Thanks.

If you can measure the frequency of the signal (not the voltage) you should be able to convert it.

However, without knowing what kind of signal it is it is hard to say for sure how to calculate it.  I would start with something like this:

Frequency x 60

That will give you the pulses per minute as opposed to the pulses per second.  You can then look at the number you have and use multiples of it until you find a number that may fit.  For example, if this were a fuel injector signal you would need to double it, because they fire every other revolution.  If it is a 1 ppr signal then you can use it as is.  If it is a 2 ppr signal you will need to divide it in half. 

It may be easier to measure the frequency of an actual fuel injector because then you could acurately calculate the RPM.  Then, if needed, you can use the known RPM to calculate the RPM of the tach pin.

But, like I said, you can't do any of this measuring AC voltage.  This is because tach signals are not represented as voltage potential, they are represented as pulse widths.

Thanks, interesting to know.  My DMM wont read frequency, so that wont work for me.

My dad actually found an old tachometer he used to set the idle on carburated cars from the 60s, so he's mailing that out to me.  Hopefully that will work, if not, I'll just pick one up from an auto parts store.

You can pick up a Craftsman dmm that does do frequency for around $25 at Sears.  They are the cheapest I have found that do it - I'm sure there are others out there as well in (or even under) that price range.