Distributor Advance Unit: Description and Operation

The ignition distributor has no centrifugal or vacuum advance. It contains only the Hall sender and trigger wheel. The ignition timing advance is controlled entirely by the ignition control unit, based on signals from the following inputs:

Engine speed and crankshaft position
Engine load
Knock sensor
Throttle position

Fig. 34 Normal Ignition Map:

Fig. 35 Ignition Advance Strategy:

The control unit determines the optimum ignition timing and operates the Hall ignition control unit which triggers the ignition coil. pressure sensor located inside the control unit is used to measure intake manifold pressure.
Basic ignition timing is determined by engine speed and load. During normal operation, the ignition timing map looks like Fig. 34. As the engine speed increases the timing is advanced. As the engine load increases the amount of advance is reduced to avoid detonation or "pinging."
If a cylinder starts to "ping", the knock sensor transmits a signal to the ignition control unit. The control unit will then retard the timing for that cylinder 3°, Fig. 35. If the knocking is not stopped, the ignition can be retarded in 3° steps up to five times for a total of 15° if necessary. Once the ignition knock has stopped, the timing is advanced back to the programmed value in 0.35° increments.

Fig. 32 Ignition Map With Faulty Knock Sensor:

Fig. 33 Ignition Map With Faulty Pressure Sensor or Throttle Switch:

The control unit checks the knock sensor and its connections by measuring the background noises from the sensor which are present during normal engine operation. If a fault develops in the sensor, the ignition timing will be constantly retarded to its 15° limit, Fig. 32. When this occurs, the dashboard indicator light will come on.
The ignition control unit checks the operation of the pressure sensor and the throttle switches. If a problem develops in either of these components, the ignition timing is retarded to a predetermined ignition curve Fig. 33, and the indicator light will come on.
If the ignition control unit fails, an electronic bypass in the control unit is switched on. The ignition timing then remains at the starting value of 6° BTDC at all engine speeds, and the indicator light will come on.