Crazy tach reading, need help diagnosing.

[buckshot]

The tach in my truck seems to have lost its mind. Sometimes it reads right, sometimes it reads a little low, and sometimes it claims I'm idling at 3000+ RPM. The truck runs a little rough, and doesn't quite get the gas mileage it should. I'm trying to figure out a way of determining whether the tach is just buggered, and therefore not an issue, or if its accurately displaying the signal it is getting (and determining if the computer is being fed the same misinformation), implicating the ignition mod i'm assuming. I do not have access to an aftermarket tach right now, so just hooking up another isn't presently an option. I have a volt meter that can measure fequency, So i'm thinking i should be able to hook it up to the tach signal wire and work out the rpm from that, 3 impulses per revolution (its a v6) so divide frequency in hz by 3 and multiply by 60 to give me RPM, does this sound right? The problem with this is I can't find a wiring diagram for and 89 so I can't figure out which wire is tach signal to the dash and which is tach signal to the ECU, if someone has access to this information i would greatly appreciate it.

Thanks in advance,
-Chris