tach signal

Can someone tell me the best way to test a wire for tach signal.  What values should I expect ad what to test in?  AC voltage?

AC voltage, you are looking for 1-5 volt acShawn

It should also increase with enging rpm.

I am trying to get it at the ecm but am unsure what one it is.  I found a three white wires but when i test the ac voltage i get nill.  i was told it was at pin 10 in the red connector. I tried all the white wires i found going to the ecm and no luck.  any place else i can get it EASILY?  Also if you unplug the wires from the ECM and then reconnect them does it cause any damage or lose any setting or anything?

Do you have your dmm set to read AC voltage?Sound Pressure

You know you have the right amount of pressure when your eyes start to water! Now you've got Juice!

I didn't catch what type of car you're working on, but you should be able to find tach wire(s) at the ignition coil where the spark plug wires connect. There may be a plug in harness that comes from the ECM. Example: Dodge Neon-3 wire harness at the coil has 2 tach wires.

I have the AC voltage option set on the DMM and I have a chevy silverado 2500HD with 6.0L.  I wanted to grab the wire at teh ecm because it is easier to tuck away and keep out of sight then.  I was told pin 10 of the red harness but how do i count the pins...do i start from the top or the bottom?