surfrata

I have a 2003 EXT Trailblazer and the RPM needle is acting crazy. It does not want to go back to zero and stays up on like five or six RPMs. If I disconnect the battery, completly, it will zero out, but then it starts climbing again. I have checked everything I know how, even took the gauges display console completely out, to see if it was a loose cable. But it has one electrical plug that runs everything. What I am thinking is either; the gauges display console is going bad or there is an electrical plug, on the engine somewhere that controls that specific gauge. I have bought the HAYNES manual with negative results.

Can some one help me PLS.
Thanks in advance