Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Electronic Spark Control System: Operation
The Spark Control Computer has two functional modes, "Start" and "Run". The "Start" mode operates while cranking and starting only. The "Run" mode operates after engine has started and during normal engine operation. The two modes never operate at the same time. When cranking and starting the pick-up coil sends a signal to the computer which is in the "Start" mode, the "Run" mode is by-passed. During this time a fixed advance is used. Advance is determined by distributor position (basic timing). After engine starts, the pick-up coil continues to send a signal to the computer, but the computer is now in the "Run" mode and "Start" mode is by-passed. The amount of timing advance is now controlled by the computer, based upon information received from the engine sensors.
The amount of spark advance is determined by two factors, engine speed and engine vacuum. At what point it occurs depends upon computer programming. Advance from vacuum will be provided when carburetor switch is open. The amount is programmed into the computer and is proportional to the amount of vacuum and engine RPM. Advance from speed will be given by the computer when the carburetor switch is open and is programmed to engine RPM.
If for some reason, there is a failure of the "Run" mode of the computer, the "Start" mode will come back into service and allow the vehicle to be driven. Performance and economy will be greatly reduced because of the fixed timing.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.