Malfunction Indicator Lamp: Description and Operation

MALFUNCTION INDICATOR LAMP (MIL)
A Malfunction Indicator Lamp (MIL) is standard equipment on all instrument clusters. The MIL is located near the right edge of the instrument cluster, to the right of the speedometer. The MIL consists of an International Control and Display Symbol icon for "Engine" imprinted on an amber lens. The lens is located behind a cutout in the opaque layer of the instrument cluster overlay The dark outer layer of the overlay prevents the indicator from being clearly visible when it is not illuminated. The icon appears silhouetted against an amber field through the translucent outer layer of the overlay when the indicator is illuminated from behind by a replaceable incandescent bulb and bulb holder unit located on the instrument cluster electronic circuit board. The MIL lens is serviced as a unit with the instrument cluster lens, hood and mask unit.

The Malfunction Indicator Lamp (MIL) gives an indication to the vehicle operator when the Power- train Control Module (PCM) has recorded a Diagnostic Trouble Code (DTC) for an On-Board Diagnostics II (OBDII) emissions-related circuit or component malfunction. The MIL is controlled by a transistor on the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the PCM over the Programmable Communications Interface (PCI) data bus. The MIL bulb is completely controlled by the instrument cluster logic circuit, and that logic will only allow this indicator to operate when the instrument cluster receives a battery current input on the fused ignition switch output (RUN/START) circuit. Therefore, the bulb will always be off when the ignition switch is in any position except On or Start. The bulb only illuminates when it is provided a path to ground by the instrument cluster transistor. The instrument cluster will turn on the MIL for the following reasons:

- Bulb Test - Each time the ignition switch is turned to the On position the MIL is illuminated for about three seconds as a bulb test.

- MIL Lamp-On Message - Each time the cluster receives a MIL lamp-on message from the PCM, the indicator will be illuminated. The indicator can be flashed on and off or illuminated solid, as dictated by the PCM message. For some DTC's, if a problem does not recur, the PCM will send a lamp-off message automatically. Other DTC's may require that a fault be repaired and the PCM be reset before a lamp-off message will be sent. For more information on the PCM and the DTC set and reset parameters.

- Communication Error - If the cluster receives no lamp-on or lamp-off messages from the PCM for twenty consecutive seconds, the MIL is illuminated by the instrument cluster. The indicator remains controlled and illuminated by the cluster until a valid lamp-on or lamp-off message is received from the PCM.

- Actuator Test - Each time the cluster is put through the actuator test, the MIL will be turned on for the duration of the test to confirm the functionality of the bulb and the cluster control circuitry.

The PCM continually monitors the fuel and emissions system circuits and sensors to decide whether the system is in good operating condition. The PCM then sends the proper lamp-on or lamp-off messages to the instrument cluster. If the MIL fails to light during the bulb test, replace the bulb with a known good unit. For further diagnosis of the MIL or the instrument cluster circuitry that controls the indicator, refer to Instrument Cluster Diagnosis and Testing. If the instrument cluster turns on the MIL after the bulb test, it may indicate that a malfunction has occurred and that the fuel and emissions systems may require service. For proper diagnosis of the fuel and emissions systems, the PCM, the PCI data bus, or the electronic message inputs to the instrument cluster that control the MIL, a DRBIII scan tool is required Refer to the appropriate diagnostic information.