A computer program is used to calculate the best ignition timing for the current engine operating condition. Initial timing is never less than the initial advance. Basic advance is the advance value obtained from the timing map in line with the engine load and speed conditions. Correction advance is the advance that may be added for current engine operating conditions, such as warm-up, overheating, idle and knock control. The results of these ignition calculations are a switching signal that controls the timing of the ignition coil primary current.
Every system has a power feed and ground circuit for the ignition primary.
This can serve as a common diagnosis point on all ignition systems, including both the DI and EI control circuit and is usually easy to access. Generally there are three locations that may be used to check the primary ignition current. Directly at ignition coil. The method of choice is the inductive amp probe. Hook up for the probe depends upon the individual vehicle, scope and operator, but in all cases the engine must be running and the probe must be around the most direct power supply or ground in order to get proper readings.