Circuit Description
The PCM detects engine misfire by detecting variations in crankshaft
deceleration between firing strokes. For accurate detection of engine misfire
the PCM must distinguish between crankshaft deceleration
caused by actual misfire and deceleration caused by rough
road conditions. The ABS Brake System can detect if the
vehicle is on a rough road based on wheel acceleration/deceleration
data supplied by the wheel speed sensors. If the ABS system
detects rough road above a predetermined threshold, this
information is sent to the PCM via serial data. The 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 sets, this additional DTC also sets
indicating that rough road data was not available during
the misfire calculation due to an ABS System malfunction.
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0121, P0122, P0123, P0335, P0336, P0742
not set. |
| • | The vehicle speed is greater than 10.0 mph. |
| • | The engine load is less than 60 percent. |
| • | The engine misfire is detected (DTC P0300 set). |
| • | The engine speed is less than 3200 RPM. |
Conditions for Setting the DTC
An ABS system malfunction exists preventing the PCM from receiving rough
road detection data.
Action Taken When the DTC Sets
| • | The powertrain control module (PCM) stores the DTC information into memory
when the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The PCM records the operating conditions at the time the diagnostic fails.
The PCM stores this information in the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns OFF the malfunction indicator lamp (MIL) after
3 consecutive ignition cycles that the diagnostic runs and does not
fail. |
| • | A last test failed, or current DTC, clears when the diagnostic
runs and does not fail. |
| • | A history DTC clears after 40 consecutive warm-up cycles,
if no failures are reported by this or any other emission related diagnostic. |
| • | Use a scan tool in order to clear the MIL and the DTC. |
Diagnostic Aids
For an intermittent condition, refer to
Symptoms
.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
Service the ABS system before diagnosing for a misfire, because an actual
engine misfire may or may not exist. The following two examples illustrate
how this DTC can be set with or without an actual engine
misfire:
| • | There was an actual engine misfire during an ABS system malfunction. |
| • | The vehicle operator drove on a rough road, the ABS system could
not detect this due to a malfunction, and the PCM interpreted the variations
in crankshaft speed caused by the rough road as a misfire. |
