engine rev low on instrument panel

My car: Corolla GLi 1993
Problem: The engine rev always shows very low. On idle it is lower than 500 rev/min. Engine is working ok, gear changes normally. when travelling at 100km/h, rev shows only 1000+. Anyone here knows what's the problem. Worst of all, it seems to be getting lower and lower!
I figured it shouldn't be the engine speed sensor or else the gears won't change correctly, so most probably something to do between the meter and the ECU.
Anyone know how to fix this easily? I've tried asking some workshops and they say either ECU or "I don't know how to repair that" DAMN..

Hmm, that's an interesting problem. If it were me, I'd hook my scope up to the IG- terminal under the hood (it's in the diagnostic connector on fender; has a gray cap). You should see healthy pulses. Here's a scope capture I took on my car ('90 Corolla):

http://picmcu.netfirms.com/Car-Distributor.GIF

Maybe a shop has a scope that they can put on it.

-Tim