The Ford EDIS system consists of a controller module, coil packs, a crank trigger wheel and matching variable reluctance sensor. Running alone, the EDIS module will fire the spark plugs at 10 degrees BTDC, operating in it's "limp home mode".
Normal operation involves interaction between the engine management system and the EDIS module, using the PIP and SAW as communication lines. After each ignition event, the EDIS module requests the current ignition advance value from the engine management unit using the PIP line. The engine management system calculates the current ignition advance and signals it back on the SAW input of the EDIS module. To communicate the advance value, the SAW reply signal is a pulse with a duration that corresponds to the amount of degrees advance.
The formula for calulating the SAW pulsewidth is as follows:
SAW Pulse width (uSec) = 1536 - (25.6 x (desired spark advance in degrees))
Range: 64 - 1792us (57.5° BTDC to 10° ATDC)
Resolution: 4us (5/32nd of a degree)
The MegaJolt Lite Jr emulates the Ford engine management system, communicating the desired ignition advance value to the EDIS module calculated from it's internal configuration.