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 Setting 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). |
| • | 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 PCM will illuminate the Malfunction Indicator Lamp (MIL) after
two consecutive drive trips that the misfire diagnostic runs and fails. This
DTC can only set if the MIL is already ON for misfire. |
| • | The PCM records the operating conditions at the time the diagnostic
fails. The PCM stores this information in the Freeze Frame and/or Failure
Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn the MIL OFF after three consecutive drive trips
that the misfire diagnostic runs and does not fail. |
| • | If no failures are reported by this or any other non-emission
related diagnostic. |
| • | A last test failed (Current DTC) will clear when the diagnostic
runs and does not fail. |
| • | The PCM battery voltage is interrupted. |
Diagnostic Aids
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 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. |
