How does the tach signal work?

[jmgtp]

I did a lot of rewiring of my wire harness and I have 2 white wires - one of them is the tach wire (for the dash), the other terminates somewhere buried in the harness and is no longer used. My issue is to determine which is for the tach.

I know the easy way would be to hook one up and start the motor to see if the tach works and I could determine which one is for the tach by process of elimination - but currently the car has no exhaust and is indoors so starting it is not a reasonable option. Is there any way to trick it? Can I apply voltage or ground to the tach wire and (with the key turned to the run position) expect to see movement of the tach in the dash?

I'm not sure how the tach signal works - so I don't know how to make a fake signal to apply to each wire to test. Is this feasible? If I can't determine which is which I could hook both up since one wire is not connected to anything anyway. I soldered and shrinktubed everything to avoid poor connections and false grounds but I still don't like having it hooked up to a dead wire, could cause a short sometime in the future.

[pocket-rocket]

I believe tachs work on a square wave signal since for aftermarket ones you have to set a dip switch to tell it how many cylinders you have.

[robertisaar]

square wave is correct, though i can't remember if the ICM grounds the circuit or applies voltage to it to indicate an event....

either way, it happens once per complete engine revolution.

i imagine it works the same as the N* ICM though(wonderful explanation from GM on how it works), and the ICM supplies voltage, the ground is in the cluster.

[jmgtp]

haha well, this all of a sudden became too complicated! I was willing to try to ground the wire or power it to see if I could get the tach to move and thereby identify my wire. I took the easy way out and connected a wire, fired the motor up, and nothing registered on the tach, tried the other wire... and there it is, working tach. So I found my wire. It just made a lot of noise, prolly disturbed the neighbors and stunk up my garage a bit! But it only took me 2 minutes and was easy lazy mans way out!

Thanks for the replies guys.