Tachometer question

Just to make sure everyone does understand because sometimes I dont explain things well enough, Before any wiring is connected and tach was in box, needle was at 2k. Once I put 12v to tach without engine running tach jumps to 10K. remove 12v wire and tach went to 0, once started it appeared to function normally. I did not get to go out and ceck it yet tonight. Have to wait until The Voice is over! lol, I will be sure to go out tonight and see what happens again. I did this last night at 10:30 and only ran it for about 10 seconds.
Thanks for everyones help and thoughts on this.
Rod