Hi,
Not sure how you are searching.! The post I wrote very recently was exclusively about the tachometer and how to fix it. To fix yours you need
1) Clarks garage, page
Gauges - Removing Late 944 (1985.5 and newer) Gauge Cluster
It will take between 10 minutes to half an hour to get the gauges out. They come out as a complete assembly, and it's not too difficult.
2) You could check pins 2,3 and 4 on the second loom connector against the circuit diagram for your 87 car. Thing to look out for on the Porsche diagrams is the gauge marked e-tach on the circuit diagram (in the instrument section) is really the speedometer, and the gauge marked speed is really the tachometer (engine speed). Germans eh?! Can't remember where I found the circuit diagram pdf's but they do exist. Might even be on Pelican. You might skip this step and go straight to trying to find out if your gauge works, because it's quick to try.
3) Try the post I've recently written. Plug the three connections from your tachometer straight into the loom, bypassing several potential points of failure and see if with a bit of judicious prodding of components it springs into life. If it doesn't you need to go back to step two and buzz these three connections out.
http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/684099-fixed-intermittent-tach.html
cheers
Rissole