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.
If there is a "Run" mode failure, the start mode will take over. The engine will keep running, but performance and fuel economy will be poor. "Start" mode timing will be fixed. The pick-up coil signal is a reference signal. When it is received by the computer, maximum amount of timing advance is made available. Based on data from all sensors, the computer determines how much of this maximum advance is needed at that instant. If for some reason the computer fails, the system will go into what is called the limp-in mode. The driver can continue to drive the vehicle, but performance will be poor. If there is a failure of the pick-up coil (or both pick-up coils on dual pick-up distributors) or "Start" mode of computer, the engine will not start or run.
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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.
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