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 antilock brake system (ABS) 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 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 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. |
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 malfunction. |
| • | The vehicle operator drove on a rough road, the ABS may not
detect this due to a malfunction, and the PCM interpreted the variations
in crankshaft speed caused by the rough road as a misfire. |
