If your tachometer doesn't work after installing an Accel 275+ or 300+ CD Ignition Control, you may need a tach adapter. The green lead in the wire harness of these controls produces a 12-volt square wave signal, which can be used with most tachs. If your tachometer is triggered from the coil negative terminal (voltage trigger) using the white lead through points, electronic ignition amplifier output, or OE module for an Accel 275+ or 300+ CD Ignition Control, then this is the adapter for you.