Circuit Description

PCM detects engine misfire by detecting variations in crankshaft speed. For accurate detection of engine misfire, PCM must be able to distinguish between changes in crankshaft speed caused by actual misfire or by rough road conditions. ABS system can detect if vehicle is on a rough road based on wheel acceleration/deceleration data supplied by the wheel speed sensors. If ABS system detects rough road greater than a predetermined threshold, this information is sent to the PCM via Class 2 serial data circuit. PCM can then take the rough road into account when calculating misfire.

Even if the ABS is malfunctioning and cannot detect rough roads, the misfire diagnostic will continue to run. However, if a misfire DTC is set, this additional DTC P1381 will also set, indicating that rough road data was not available during the misfire calculation due to ABS system malfunction.

Conditions required for setting this DTC are:

• DTCs P0300-P0304 has been set.
• Vehicle speed is greater than one MPH.
• MAP is less than 99.7 kPa.
• Engine speed is less than 3406 RPM.
• PCM received no ABS information for 2.5 seconds.