Tachometer not reading correctly

I recently purchased a one owner 1990 Lumina APV 3.1 with 166k miles, it runs great, I'm just fixing some small things that have been neglected, one being the Tach, it shows idle at around 2200 and I can bury it pretty easy, I know it is off, anyone with a solution, or a place to start, Oh and the Temp Gauge pegs at the high side about 15 seconds after start up, I'll try replacing the sending unit this week end, all other gauges seem OK.

Thanks :screwy:

Grounding problems resulting in instrument panel readings going haywire have been reported. Try to pick some ground wire and splice new ground wire to solid engine and body connection....or take voltage measurement between a IP ground wire and engine ground...should read 0.
www.avigex.ca/xport/instrumentpanel.jpg
www.avigex.ca/xport/instrumentpanel1.jpg
www.avigex.ca/xport/tacherratic.jpg

Thanks for the diagrams LMP, they will be very useful, I will dig into it again this weekend, The sending unit for the temp gauge did fix that problem so all but the tach are working.
Thanks Again !