Should be several wires on the hood tach, if I remember correctly. One for power for the lights, one from the HEI and maybe a ground? Make sure you have them all connected.
We disconnected ours and just keep it for looks now. It never worked right.
The hoodtach has 3 wires, a +, a ground & a tach. Connecting the + & the ground is illumination for the tach. Connecting the tach wire on the tach cpnnection on the HEI is not working