Tachometer Failure

On our 1993 Trans Sport, the tachometer went from intermitttantly failing, to rarely ever moving. Once in a very long while it will remind us that it once worked and then nothing again. To begin with, what are the common things I can look for that don't require a bunch of equipment? What are the most common reasons for failure?

Which engine is it? Tach signal is taken differently on 3.1 and 3.8.

Sorry about that. It is the 3.8 engine.

3.8 OK..so follow the Vin L path on this diagram
www.avigex.ca/xport/instrumentpanel.jpg
THere is nothing but a single wire between the ignition module and the tach...so most probable the problem is in the instrument cluster, connectors or the tach movement itself. FIve or six screws will remove dash upper cover; then light and wiper switch, and you have easy access to pull out the instrument cluster to check connectors .
Here is the procedure form GM shop manual:
Tach inoperative:
Disconnect instrument cluster connector
ENgine running
Use a digital volt meter: measure voltage at insteument cluster connentor terminal D7 to ground.
Voltage should read approx. 1.5 V t idle
Does voltage increase as engine speed increases?
Yes: replace Tach
No: Check for open contact i connector 121; if OK replace Electronic Ignition Module
Comment: If car runs normally, I would not change the Ignition Module just for the tach.....

Is that single wire traceable? I agree. Why mess with the module if the thing is running good? Maybe I could pull another tach from the wreckers next time I'm there. I could pull the dash and try it in there while it is apart; I could do the voltmeter check too. Thansk a bunch:)

This is a very late follow up this thread. The problem was indeed the ignition module. I replaced it with a wreckers unit and voila. The tach signal is controlled by the ignition module. Didn't have to pull he dash apart this time.
Thanks