900 Turbo Convertible L4-1985cc 2.0L DOHC Turbo EFI (1994)
Knock Regulation
Saab Trionic does not have a conventional knock sensor. Instead the ignition discharge module analyzes the ionization currents for all the
cylinders and sends signals to pin 44 on the Trionic ECM. This function is adaptive with respect to interfering fuel additives.
The ECM knows through the combustion signals which cylinder has fired and if the signal on the knock lead at the same time is above a particular
value, the ECM registers knocking on this cylinder. Ignition is then retarded 1.5° on this cylinder.
If knocking is repeated, ignition is further retarded, but to a maximum of 12°. It the ignition reduction is above a particular value on all the
cylinders, the amount injected is slightly increased.
If knocking occurs when the pressure in the intake manifold is above approx. 140 kPa, knock control takes place in a different way; first both the
fuel injection and ignition matrices are changed, and it this does not help the boost pressure is reduced.
The aim is to achieve good performance even in the event of knocking.
If there is an open circuit on the lead to ECM pin 44, basic charging pressure is obtained and ignition is retarded by 12° when the engine load is so
high that there is a risk of knocking.
