Ignition Timing: Description and Operation

IGNITION TIMING CONTROL
Ignition timing is controlled by the engine control module (ECM) according to sensor inputs. As with fuel injection, the sensed control inputs form two groups: primary and secondary. Primary control inputs are intake mass air flow (engine load) and engine speed; secondary control inputs consist of engine intake air temperature, throttle movement and position and transmission upshift (automatic transmission).

Ignition timing strategies are held in memory (EPROM) in the ECM and form an engine load versus engine speed matrix. The load and speed range of the engine is divided into 16 loads and 16 speeds (256 memory sites). Digital numbers representing ignition timing values fill each site. The resulting 256 ignition timing values cover the entire engine load and speed range. Ignition timing is then calculated from the ignition timing strategy according to secondary input correction factors.

The ECM drives the ignition module to switch the ignition coil low tension circuit and monitors the output signal for on-board diagnostics.

Ignition Control Chart:

ECM Output Signal:

IGNITION TIMING PRIMARY CONTROL
Ignition timing is controlled primarily as a function of engine load and speed. Engine load is sensed by a mass air flow sensor located in the engine air intake before the throttle housing. Engine speed is sensed by a crankshaft sensor located behind the engine damper. In addition to engine speed, the sensor supplies crankshaft position inputs to the ECM for ignition timing. The ECM processes the inputs from the mass air flow meter and the crankshaft sensor and accesses ignition timing from the ignition timing strategy. Usually, the load and speed at which the engine is running will be between load and speed sites. Two dimensional interpolation is used to calculate the correct ignition timing for the between sites engine condition.

IGNITION TIMING SECONDARY CONTROL
Secondary ignition timing control inputs consist of engine intake air temperature, throttle movement and position, battery voltage and transmission upshift (automatic transmission).

Intake Air Temperature
Ignition timing is corrected by the ECM for engine intake air temperature measured by the air temperature sensor mounted in the air inlet elbow. A portion of the ignition strategy (load sites 6 through 15; speed sites 4 through 15) is programmed with the temperature at which ignition retard commences for each load / speed site. In general, light loads/low speeds have retard thresholds set to a high temperature, approximately 100°C (212°F) while high loads/high speeds have retard thresholds set about 30°C (86°F). Above the threshold temperature, ignition timing is retarded at the rate of 2.25° per 10°C.

Dwell Angle
The dwell angle and peak coil current are controlled by the ignition module, located in the engine compartment near the ignition coil. A feature called "stall turn-off" is used to prevent coil overheating and battery discharge. If the ignition switch is left on without the engine running, the ignition module switches the coil current off.

Closed Throttle/Idle
There are separate closed throttle ignition strategies for gear positions Neutral and Drive, each with 8 speed break points for engine speed versus timing. With the engine at normal operating temperature, the Ignition timing at idle will be 10° BTDC in N (700 rpm) and 16° BTDC in D (580 rpm).

Ignition Secondary Circuit
The ignition secondary circuit consists of a distributor and a standard ignition coil. The distributor contains only a rotor arm and a cap. The distributor must be correctly positioned to index the rotor to the cap; however, it has no affect on ignition timing.

Closed Throttle Ignition Timing Control Chart: