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
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 test 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
malfunction.
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0335, P0336, P0742, P1120, P1121, P1220,
and P1221 are not set. |
| • | The vehicle speed is above 16 km/h (10 mph). |
| • | The engine load is less than 60 percent. |
| • | An 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 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 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. |
