MOTRONIC Misfire Detection

THEORY OF DIAGNOSTIC OPERATION

Crankshaft Speed Monitoring
1996 Saab vehicles equipped with a MOTRONIC engine management system (V6 and non-turbo 4 cylinder engines) use very precise monitoring of crankshaft speed to detect engine misfire. The principle involved is as follows:

The crankshaft does not travel at the same speed through all 360 degrees of rotation. It speeds up slightly each time a cylinder fires and then begins to slow down or "coast" until the next cylinder in the tiring order fires. Plotted on a graph, the change in crankshaft speed of a smooth running engine would be very predictable and form a sine wave.

If one or more cylinders are "dead" the sine wave becomes irregular as the engine slows down even further instead of speeding up each time a cylinder misses rather than firing.

By combining the information provided by the crankshaft position sensor with input from the camshaft position sensor the Motronic ECM is able to identify the offending cylinder and a DTC will be set identifying the bad cylinder. Some types of misfire (such as that caused by a fuel starvation problem) occur in a random pattern, making it impossible to pinpoint an individual cylinder. This type of problem would set the DTC P0300 "Random Misfire Detected".





Rough Road Detection
One complicating factor when using crankshaft speed as a means of calculating misfire is the fact that driving on rough roads may cause false misfire readings. A rough road will cause rapid changes in wheel speed which, when fed back through the drivetrain, can affect engine speed enough to create a false misfire diagnosis. The solution to this problem is to prevent the misfire diagnostic routine from running when driving on rough roads.

4 Cylinder Motronic
1996 4 cylinder Saabs with a MOTRONIC engine management system have a Vehicle Vertical Acceleration (VVA) Sensor mounted in the engine bay behind the top of the driver's side strut. This sensor is calibrated to respond to the rapid up and down body movements which are characteristic of driving on a rough road. The ECM is constantly monitoring this sensor and will not allow a misfire DTC to set if the "misfire" occurs on a rough road.

V6 Motronic
Vehicles with 6 cylinder engines do not have a VVA sensor. Rough road detection is accomplished by looking at wheel speed input from the ABS system. A surface which is rough enough to create a false misfire signal will also create rapid changes in wheel speed and, the misfire diagnostic will be disabled.