This diagnostic tool is a necessity for any serious turbo enthusiast, HUNDREDS in use. It allows you to actively simulate an ignition under load and observe the actual spark as it occurs. You can easily check the spark intensity of each coil at any RPM from idle all the way up to around 8000 RPM. Unit attaches to the top of the 1986-1987 GEN 1 coil pack and plugs into the module, then to the car's battery. By turning the control knob, you can observe the spark through three spark gap windows at the top of the unit. Weak or non-existent spark activity at any RPM can easily be seen.