GM Service Manual Online
For 1990-2009 cars only

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 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 diagnostic trouble code (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 16 km/h (10 mph).
    • The engine speed is below 3,200 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 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 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 water intrusion into the PCM.

Using the Freeze Frame/Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame/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 Intermittent Conditions .

Test Description

The number below refers to the step number on the diagnostic table.

  1. 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:

  2. • There was an actual engine misfire during an ABS malfunction.
    • The vehicle operator drove on a rough road, the ABS 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.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Diagnostic System Check - ABS in Antilock Brake System

Go to Diagnostic System Check - Engine Controls