In some instances when you install an MSD ignition, the factory ECU may not be able to distinguish when the coil fires. This is the signal which is also responsible for firing the injectors (or the tachometer), which may cause a no-run situation. The Dual Channel Ignition Adapter simulates the original coil trigger signal thus allowing the ECU to properly trigger both the tach and fuel injection with the MSD ignition installed. In most cases, the adapter plugs directly plugs into the MSD's Harness. For vehicles using an MSD DIS-2 Ignition, only one 121-8912 is required. For DIS-4 applications, two 121-8912's are required.