Spark Control

Spark control allows the computer to determine the exact instant that ignition is required; then signals ignition coil to produce electrical impulses which fire the spark plugs. The computer eliminates the need for either vacuum advance units or centrifugal advance weights. Spark control operates in 1 of the following modes:

• Start Mode  - During cranking, an electrical signal from the distributor is fed into the computer, which causes the computer to fire the spark plugs at a fixed amount of advance.
• Run Mode  - Once the engine starts and is operating normally, the timing will be controlled by the computer, based upon information received by the data sensors.

Spark timing and dwell cannot be adjusted in the run mode. If the computer fails, the system will go into the start mode. This enables the vehicle to be driven in for repair, but performance and fuel economy will be poor. If the start mode fails, the engine will not start or run.

The amount of spark advance is determined by engine speed and engine vacuum. However, where it happens depends upon the following conditions:

• Advance From Vacuum  - Advance based upon engine vacuum is allowed by the computer when the carburetor switch is open. The amount of advance is programmed into the computer and is proportionate to the amount of vacuum and engine RPM.
• Advance From Speed  - Advance based upon engine speed (RPM) is allowed by the computer when the carburetor switch is open and advance from vacuum will not happen quickly. This advance from speed is programmed into the computer and is controlled by engine RPM. Advance from speed will build at a slow rate. If carburetor switch closes, advance from speed will be cancelled.