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 serial data communication between the ABS System
and the PCM is malfunctioning and the PCM can not receive the rough
road data, the misfire diagnostic continues to run. However, if
a misfire DTC sets, this additional DTC also sets indicating that
the rough road data was not available during the misfire calculation
due to a serial data malfunction.
Conditions for Running the DTC
| • | The vehicle speed is above 10 mph. |
| • | The engine speed is below 3200 RPM. |
| • | The engine load is less than 60%. |
| • | Engine misfire is detected (DTC P0300 set). |
Conditions for Setting the DTC
| • | A serial data malfunction exists preventing the PCM from receiving
rough road detection data. |
| • | The above conditions met for 20 seconds. |
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 DTC
| • | A last test failed, or current DTC, clears when the diagnostic
runs and does not fail. |
| • | A history DTC will clear after 40 consecutive warm-up cycles,
if no failures are reported by this or any other non-emission related diagnostic. |
| • | Use a scan tool in order to clear the DTC. |
Diagnostic Aids
Important: Remove any debris from the PCM connector surfaces before servicing the
PCM. Inspect the PCM connector gaskets when diagnosing/replacing the
PCM. Ensure that the gaskets are installed correctly. The gaskets prevent
water intrusion into the PCM.
Using the Freeze Frame and/or Failure Records data may aid in locating
an intermittent condition. If you cannot duplicate the DTC, the information
included in the Freeze Frame and/or Failure Records data can help
determine how many miles since the DTC set. The Fail Counter and Pass
Counter can also help determine how many ignition cycles the diagnostic
reported a pass and/or a fail. Operate vehicle within the same freeze
frame conditions (RPM, load, vehicle speed, temperature etc.) that
you observed. This will isolate when the DTC failed. 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. |
