Circuit/System Description

The engine control module (ECM) determines engine misfire by detecting variations in crankshaft speed. Crankshaft speed variations can also occur when a vehicle is operating over a rough road. The wheel speed sensor produces an alternating current (AC) voltage that increases with wheel speed. The ECM receives a rough road signal from an electronic brake control module (EBCM), if equipped with the anti-lock brake system (ABS). The ABS can determine if the vehicle is on a rough road surface based on the signal from each wheel speed sensor. On vehicles with Non-ABS, a rough road signal is sent from a wheel speed sensor located at the left front wheel to the ECM. The ECM can determine if the vehicle is on a rough road surface based on the signal from the wheel speed sensor. When the ECM detects a rough road condition, misfire diagnostics will be disabled.