Rough Road Signal

Rough Road Signal

When the vehicle is travelling over a rough road surface the engine crankshaft is subjected to torsional vibrations caused by mechanical feedback from the road surface through the transmission. To prevent misinterpretation of these torsional vibrations as a misfire the ECM calculates a rough road level by monitoring individual wheel speeds from the ABS ECU on the CAN bus. the ECM determines the quality of the road surface by monitoring a CAN signal from the ABS ECU, which modulates the duty cycle of the signal in response to variations between ABS sensor inputs. Misfire monitoring is restored when the quality of the road surface improves again.

If there is a fault with the CAN data, the ECM defaults to permanent misfire monitoring.