900 S Coupe L4-1985cc 2.0L DOHC Turbo EFI (1998)
Combustion Signals
The Trionic system does not have a camshaft position sensor. A camshaft position sensor is normally required for sequential knock control and fuel
injection.
The Saab Trionic must decide whether cyl. 1 or cyl. 4 is firing when the crankshaft position sensor indicates that cyl. 1 and cyl. 4 are at top dead centre.
This is done as follows:
One of the poles of the secondary windings of the 4 ignition coils is connected in the usual manner to the spark plugs. The other pole is not connected
directly to ground but to 80 V As a result, 80 V is always present across the spark plug electrodes except at the very moment a spark is produced.
When combustion takes place, the temperature in the combustion chamber is very high. The gases are ionized and start to conduct current. This means
that a current passes across the spark plug gap (without creating a spark).