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%. |
| • | 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
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 contaminate intrusion into the PCM. |
| • | For any test that requires probing the PCM or a component
harness connector, use the Connector Test Adapter Kit J 35616
. Using this kit prevents damage
to the harness/component terminals. Refer to
Using Connector Test Adapters
in Wiring Systems. |
| • | Using 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 aid
in determining how many miles since the DTC set. The Fail Counter
and Pass Counter can also aid determining how many ignition cycles
the diagnostic reported a pass and/or a fail. Operate the 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. |
