tacho wire check

[nimhbk]

The wire into the back of the cluster that powers the tacho if working as it should, what values should it put out on a multi meter

 

[watoga]

There are 3 wires going to the tacho:

The yellow wire should be 12V as it comes directly from the ignition live.
The black wire goes directly to the chassis earth - so check it for resistance rather than voltage.
The blue/black wire which goes to the ECU gives approximately 1V at idle, and nearly 3V at approximately 2500 RPM.

Check here for more details:
http://www.gtirworkshopmanual.com/default.aspx?v=2295

Hope this helps,
Dave