ignition

Spark Advance Control

The Spark Advance Control system maximizes engine efficiency by continuously adjusting spark advance timing to deliver peak combustion pressures when the piston reaches about 10' after TDC. Incorrect spark timing can have a significant effect on emission output and vehicle driveability. If ignition timing is excessively advanced during certain conditions, detonation will occur resulting in increased HC and NOx levels. Since NOx production is most predominant under loaded engine operating conditions, the spark advance system must ensure accurate ignition timing during these conditions. If ignition timing is incorrectly retarded, only partial combustion will take place resulting poor engine performance and increased emission levels.

Causes of Incorrect

On systems that use the ECM to compute ignition spark advance, there are only two conditions which are likely to cause spark timing to be incorrect; initial timing or a false input signal to the ECM.

The first step in troubleshooting emissions and driveability concerns should always include a quick check of initial ignition timing. Any error in initial timing will be reflected throughout the entire spark advance curve.

If engine load is miscalculated because of incorrect input signals, spark advance angle will not be appropriate for engine operating conditions. This will result in driveability and emission problems. Refer to course 850 for additional information on spark advance strategy.

The Effects of Fuel Octane

Toyota engines equipped with a knock detection system are very sensitive to fuel octane levels. Motor fuels with low octane ratings will cause the engine to detonate, which will in turn, cause the detonation retard system to retard timing. On some vehicles with advanced ECM operating strategies, an adaptive memory factor is used to track signals from the knock sensor. When detonation occurs frequently, the ECM relearns the basic spark advance curve, retarding spark throughout the entire engine operating range. This retarded spark curve will negatively effect engine performance and fuel economy under all driving conditions, even after a tank of higher octane fuel is purchased. The retarded spark curve will remain stored in the ECM keep alive memory until the engine is operated for a substantial amount of time on the higher octane fuel, or until the "keep alive memory" is cleared by removing power from the BATT terminal.

Purpose of Spark Advance Control Systems

The amount of spark advance needed by the engine varies depending on a number of different operating conditions. Generally, spark advance follows the following strategy:

•      spark advance increases with higher engine speeds for performance and fuel economy.

•      spark advance needs to decrease under heavy load conditions to avoid detonation.

They are many variables the system must consider when determining the proper spark lead time. Coolant temperature, fuel quality, and engine load are just a few of the many factors that can significantly impact ideal ignition time. The ECM determines proper spark timing by applying various input signals against a preprogrammed spark advance strategy or "map".

Fuel injected Toyota vehicles use either a mechanical or electronic spark advance control system. They are referred to as either conventional EFI ignition system (mechanical), Variable Advance Spark Timing (VAST) or Electronic Spark Advance (ESA).

From : Toyota Motor Sales, U.S.A., Inc. All Rights Reserved.