Distributor Advance Unit: Description and Operation

The basic ignition timing map is calculated on two factors: Engine speed, and Engine load
The ignition map, Fig. 56, shows that ignition timing advances as engine speed increases. As intake manifold pressure increases (increased load), the amount of ignition advance decreases to prevent detonation.
Corrections to the basic ignition map are made based on the following conditions:

Engine coolant temperature below 5°C (40°F), and manifold pressure below 1.0 bar (0 in/Hg).
Manifold pressure above 1.0 bar (0 in/Hg).
Ignition knock occuring
Engine deceleration

Fig. 26 Coolant Temperature Correction For Ignition:

Whenever the engine coolant temperature is below 5°C (40°F) and manifold pressure is below 1.0 bar (14.5 psi), the ignition timing will be advanced from the basic curve, Fig. 26. This gives the air/fuel mixture more time burn at low engine temperatures.
For example, with an engine speed of 3000 rpm, and a manifold pressure of 0.8 bar (7.0 in/Hg), the basic ignition map calls for a timing angle of 28° BTDC. If the coolant temperature is -15°C (5°F), the ignition timing would be advanced to 35° BTDC.

Fig. 27 Intake Air Temperature Correction For Ignition:

When intake pressure increases over 1.0 bar (boost pressure), the intake air temperature is then used as the correction factor, Fig. 27. The basic ignition timing will either be advanced or retarded, depending on the intake air temperature. As the air temperature increases timing boost, the ignition timing will be retarded to prevent detonation.
For example, with the engine at 3000 rpm, and the manifold pressure at 1.3 bar (4.5 psi), the basic ignition map calls for a timing angle of 16° BTDC. If the intake air temperature is 60°C (140°F) the ignition timing would be retarded to 14° BTDC.

The knock sensor is used to prevent spark knocking ("pinging"). If a signal from the knock sensor is detected for longer than about 1 second, the ignition timing is retarded. To avoid errors, the signal from the knock sensor is only processed by the control unit when:

Engine coolant temperature is above 50C (122F)
Intake manifold pressure is above 0.7 bar (10 in/Hg)

Fig. 43a Ignition Advance Strategy:

When knocking is detected by the knock sensor, the ignition timing is retarded in 2.7° increments until the knocking stops (or a maximum of 8°). If no further knocking occurs, the timing is advanced towards its original programmed value in 1.3° steps, Fig. 43a.
If knocking continues after the timing has been retarded to the maximum 8°, the wastegate solenoid is activated to open the wastegate, and the engine control warning light on the instrument panel will come on. This tells the driver to ease up on the accelerator to decrease the engine load. The wastegate solenoid will close once the knocking has stopped and the ignition timing has been returned to its correct value.