Circuit Description
The powertrain control module (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 a
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 the misfire. Even if the serial
data communication between the ABS and the PCM is malfunctioning
and the PCM can not receive the rough road data, the misfire diagnostic
test continues to run. 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 3,200 RPM. |
| • | The engine load is less than 60 percent. |
| • | An engine misfire is detected and DTC P0300 is 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 control module stores the DTC information into memory when
the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The control module records the operating conditions at the time
the diagnostic fails. The control module stores this information in the Failure
Records. |
| • | The driver information center, if equipped, may display a message. |
Conditions for Clearing the DTC
| • | A current DTC Last Test Failed clears when the diagnostic runs
and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles,
if no failures are reported by this or any other non-emission related diagnostic. |
| • | Clear the DTC with a scan tool. |
Diagnostic Aids
Important:
| • | Remove any debris from the powertrain control module/throttle
actuator control module (PCM/TAC) connector surfaces before servicing the
PCM/TAC module. Inspect the PCM/TAC module connector gaskets when
diagnosing or replacing the modules. Verify 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 probing
a component harness connector, use the connector test adapter kit
J 35616
. Using this
kit prevents damage to the harness or component terminals. Refer
to
Using Connector Test Adapters
in Wiring Systems. |
For an intermittent condition, refer to
Intermittent Conditions
.
Test Description
The number below refers to the step number 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 2 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. |
