The EEPROM 3981 came on all non California 1993 automatic transmission vehicles before VIN PZ229154.
Base Desired AFR Table. There are many more checks / modifications to the desired AFR as its determined, but this is the table that the PCM starts with.
The PCM figures volumetric effeciency by taking the first value from the main VE table, and adding it with the RPM VE table.
The ECM starts out by retrieving the base spark advance from the RPM vs MAP table. You'll notice that the highest the table goes to is 3600 RPM. After that the ECM uses the 3600 RPM / [Insert MAP reading here] table entry and adds advance based on this formula:
This formula is only used if the RPM is over 3600, and the RPM value is maxed out at 6000.
(((2 * ((RPM - 3600)/25)) * 11) / 256)(Drop the remainder) * (90/256) = Timing advance to add to base timing advance (HINT: the most spark advance this will add is 2.8 degrees at 6000 RPM and above.)
This table is added to the advance with a value updated with a lag coefficient of 11.7% (EXAMPLE: The current amount added from this table is 0, then vacuum has changed and 9.84 degrees is returned from the table. The actual amount added would only be 1.08 degrees. If the table value returned is still 9.84 degrees on the next cycle, the amount added this time is 2.04 degrees. If 9.84 is still returned, the next amount would be 2.90 degrees and so on.
Added straight to timing advance.
Added straight to timing advance only when the throttle is closed