I had this problem. Im assuming with the wires your naming that your tach is a small one, like one i had. The signal wire (green) connects to to the blue wire like you have, but you say next to the firewall? Well it should be the same blue wire coming out of the coil on the dizzy. You want to connect that green wire as close to the blue wire on the coil as you can to get an accurate reading. That was my problem when i installed mine.