Download AMC introduced Solid State Ignition (SSI) as a running change on some 1977 Canadian models. It is standard equipment on all 1978 and later American Motors built engines. The system consists of a sensor and toothed trigger wheel inside the distributor, and a permanently sealed electronic control unit which determines dwell, in addition to the coil, ignition wires, and spark plugs.The trigger wheel rotates on the distributor shaft. As one of its teeth nears the sensor magnet the magnetic field shifts toward the tooth. When the tooth and sensor are aligned, the magnetic field is shifted to its maximum, signaling the electronic control unit to switch off the coil primary current. This starts an electronic timer inside the control unit, which allows the primary current to remain off only long enough for the spark plug to fire. The timer adjusts the amount of time primary current is off according to conditions, thus automatically adjusting dwell. There is also a special circuit within the control unit to detect and ignore spurious signals. Spark timing is adjusted by both mechanical (centrifugal) and vacuum advance.