Malfunction Indicator Light (MIL)
When ignition is first turned on, MIL should come on for approximately 5 seconds to verify bulb and circuit operation, and then go off. The MIL is located on instrument panel, near tachometer.
The Engine Control Module (ECM) monitors several different engine control system circuits. If malfunction occurs which affects vehicle emissions, a Diagnostic Trouble Code (DTC) will be stored in ECM. When DTC is stored in ECM, the ECM will turn on the MIL the next time ignition is turned on and the same malfunction exists. The MIL will remain on steady until DTC is cleared from the ECM.
Note. If a cylinder misfire exists, or excessive time to enter closed loop operation exists, the MIL will be turned on when the first detection of the malfunction exists.
If MIL will not come on with ignition is first turned on, or remains on steady with no DTCs stored in ECM, proceed to trouble symptom inspection table and then perform a specified inspection. Trouble symptom inspection table and trouble symptom inspection procedures are listed after the Diagnostic Trouble Code (DTC) tests.
The ECM will turn off MIL when malfunction is not detected during 3 consecutive engine start-to-stop trips on all applications except an engine misfire or excessive time to enter closed loop. On engine misfire or excessive time to enter closed loop, ECM will turn off MIL when malfunction is not detected during one consecutive engine start-to-stop trip when condition such as engine speed, engine coolant temperature etc. are similar to those when malfunction was first recorded.
On-Board Diagnostics
The Engine Control Module (ECM) monitors several different engine control system circuits. If malfunction occurs, ECM will store a Diagnostic Trouble Code (DTC) when malfunction is detected after engine is restarted on all failures except engine misfire or excessive time to enter closed loop. On engine misfire or excessive time to enter closed loop, ECM will store a DTC when malfunction is first detected. When DTC is stored, ECM will use a substitute value for the failed component in its calculations to continue engine operation. In this condition, commonly know as limp-in mode, the vehicle runs but driveability will be optimum.
The ECM also records and stores engine operating conditions when the malfunction occurred. This information is referred to as freeze frame data. Freeze frame data may be obtained using scan tool, and used for simulating tests for trouble shooting. Freeze frame data is available for
- Engine Coolant Temperature
- Engine Speed
- Vehicle Speed
- Long Term Fuel Compensation
- Short Term Fuel Compensation
- Fuel Control Condition Such As Open Or Closed Loop
- Calculation Load Value
- DTC Data Recording
DTCs may be retrieved for system diagnosis. System malfunctions are identified as either hard failures or intermittent failures.
Hard Failures
Hard failures cause MIL to illuminate and remain on until problem is repaired. If light comes on and remains on during vehicle operation, cause of malfunction must be determined by retrieving Diagnostic Trouble Codes (DTCs). If a sensor fails, ECM will use a substitute value in its calculations to continue engine operation. In this condition, commonly know as limp-in mode, the vehicle runs but driveability may not be optimum.
Intermittent Failures
Intermittent failures may cause MIL to flicker or illuminate and go out after intermittent failure goes away. However, the corresponding diagnostic trouble code will be retained in ECM memory. Intermittent failures may be caused by a sensor, connector or wiring related problems. An intermittent condition may be checked by wiggling wiring to component or moving of a component.
VEHICLE DRIVE CYCLE
- Vehicle drive cycle is performed to drive vehicle at a specified pattern in which the DTC may be set, or to ensure the repair procedure is complete.
- Vehicle drive pattern is performed with vehicle at specified speed, transaxle in proper gear and for a specified amount of time for the system or component that is being monitored. (Scheme 75)
- When checking EGR system, fuel trim or evaporative emission control system, ensure ignition is off for at least 10 seconds before performing first vehicle drive cycle. Perform vehicle drive cycle up to "B" continuously at least twice. (Scheme 75)
- When checking any other systems or components, ensure ignition is off for at least 10 seconds before performing first vehicle drive cycle. Perform vehicle drive cycle up to "A" continuously at least twice. (Scheme 75)
- When performing vehicle drive cycle, ensure the following procedures are followed: With engine stopped, ensure throttle is fully closed, A/C is off, and transaxle is in Neutral. On A/T models, ensure shift lever is in Drive, overdrive switch is in ON position and power/economy switch is in "P" range when driving vehicle. When accelerating, ensure throttle is between 1/4 and 3/4 throttle. When decelerating, ensure vehicle speed decreases to less than 15 MPH or engine RPM is 1000 RPM or less. DO NOT depress brake pedal. DO NOT depress clutch on M/T models. When driving at constant speeds, hold throttle at steady position. Ensure engine coolant temperature when starting engine is at least 50°F (10°C). Ensure intake air temperature when starting engine is 104°F (40°C) or less. Ensure intake air temperature when driving vehicle is 122°F (50°C) or less.
