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 percent. |
| • | 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 MIL/DTC
| • | A last test failed, or the current DTC, clears when the diagnostic
runs and does not fail. |
| • | A history DTC clears after 40 consecutive warm-up cycles,
if failures are not reported by this or any other emission related
diagnostic. |
| • | Use a scan tool in order to clear the MIL/DTC. |
Diagnostic Aids
Important:
| • | Remove any debris from the PCM\TAC module connector surfaces before
servicing the PCM\TAC module. Inspect the PCM\TAC module connector gaskets
when diagnosing/replacing the modules. Ensure that the gaskets are installed
correctly. The gaskets prevent contaminate intrusion into the PCM\TAC
modules. |
| • | 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 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. |
