What I found out about the output of the tacho is that you'll need to use a very high impendence load so you don't drain the signal.
It doesn't take much of a load to make the signal stay below 3v, and 3v won't run the tacho... Also, a quick short of the tacho output will result in popping a resistor in teh ECU (I found this out the hard way !).
So you problem could be one of two things. Someone in the past tried to hook up a after market tacho or shift light and stuffed it up resulting in a blown ECU, or the tacho your running now isn't compatible (is it the factory one ?) and the impedance is too low.
To test the signal you need an oscilloscope, a standard multimeter won't do it Just earth the ground of the oscilloscope to the ground of the ECU and probe the ECU tacho output, you'll see the signal being pulled to ground - just a normal square wave.
I can't help you with where to tap your SAFC into to get another signal