Electronic Spark Control (ESC)

Fig. 121 Ignition System Wiring And Vacuum Hose Diagram:

Ignition timing control, utilizing both vacuum and centrifugal advance mechanisms, affects the time at which each spark plug ignites the air/fuel mixture in the combustion chambers. These systems determine ignition advance based on engine speed and coolant temperature. This system adjusts ignition timing differently, both during and after engine warm-up to maximize fuel economy and engine performance.
This distributor has two seperate vacuum advance diaphragms which operate on manifold vacuum; Diaphragm A is connected directly to manifold vacuum, and Diaphragm B also has a thermo-valve in the line. Both diaphragms are used for ignition advance.
This system is designed to improve cold engine driveability by advancing the ignition timing more than when warm. When the engine coolant temperature is below the set value of the thermo-valve, manifold vacuum is applied through the check-valve to the diaphragm of the vacuum advance unit. The thermo-valve blocks the air entering the vacuum line, keeping the diaphragm from returning to its rest position. When the coolant temperature is above the set value, vacuum in the thermo-valve is released, allowing the diaphragm to return.