TRIONIC Misfire Detection

THEORY OF DIAGNOSTIC OPERATION

Monitoring the Combustion Chamber
All Saab turbos use the TRIONIC Engine Management system which is ideally suited for detecting misfire. The ionization feature of this system allows us to monitor combustion chamber temperature. if a cylinder is cold when, according to the firing order, it should be hot, it is easy to conclude that misfire has just occurred. One very big advantage of this method of detecting misfire is that it gives us a true indication of misfire without the' danger of false readings caused by rough roads or unusual driving habits.

The following paragraphs from the TRIONIC shop manual do a good job of explaining just how the ionization circuits work,

"One pole of the secondary winding of each of the four ignition coils is connected in the usual way to the appropriate spark plug. The other pole is not grounded directly but is connected to an 80 volt source. As a result, there is a potential of 80 volts across the spark plug gap at all times except at the moment when the spark is produced.

When combustion takes place, the temperature in the combustion chamber is extremely high. The gases are ionized and begin - to conduct current. This causes a current to pass across the spark plug gap (without producing a spark)."

The TRIONIC system has always monitored the ionization current to determine the firing order and detect knock. Adding misfire detection became a matter of modifying the software in the ECM, the hardware was already in place.