So if it was only looped and there was no physical connection made to the tacho, you couldnt have burnt the tacho out. Having said that, you still don't know if the tacho is actually any good from the start.
The only thing I can think of, and its highly unlikely, If you connected the loop wires back to front ( the loop has to run in a certain direction and power has to flow through it correctly) If it is wired backward the tacho will try to work backward ( ie, it will try to go from 0rpm to -0rpm, if it is this, what you could be seeing as reading correctly, is the needle bouncing off the 0 rpm stop. ( like I said its highly unlikely)
But you can still check it out anyway.
When wired up correctly there isnt anything realy besides a fauly gauge that could cause this.