In my 84 Z28 with my iroc guage cluster, my tach is not working. When i turn the key to on the tach jumps down to the needle where it is suppost to, but when i start the car it doesnt move. It was working fine a few days ago...cold it be a ground?

 

it sure sounds like it!
pull your tach back out and check to see if all of the prongs from the guage are locking into the metal brackets. i believe its the one closest to the top right corner is the ground. if its missing, it has fell behind the dash. if you dont want to dig through the dash. just wire a ground from the tach's ground post to the bolt hole under the steering column to the left or to another body screw!