Malfunction Indicator Lamp: Description and Operation
MALFUNCTION INDICATOR LAMP MILA Malfunction Indicator Lamp (MIL) is standard equipment on all instrument clusters. The MIL is located near the lower edge of the instrument cluster overlay to the left of center. The MIL consists of a stencilled cutout of the International Control and Display Symbol icon for "Engine" 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. An amber lens behind the cutout in the opaque layer of the overlay causes the icon to appear in amber through the translucent outer layer of the overlay when the indicator is illuminated from behind by a Light Emitting Diode (LED) soldered onto the instrument cluster electronic circuit board. The MIL is serviced as a unit with the instrument cluster.
The Malfunction Indicator Lamp (MIL) gives an indication to the vehicle operator when the Powertrain Control Module (PCM) has recorded a Diagnostic Trouble Code (DTC) for an On-Board Diagnostics II (OBDII) emissions-related circuit or component malfunction. In addition, on models with a diesel engine an Engine Control Module (ECM) supplements the PCM, and can also record an OBDII DTC. 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 or ECM over the Chrysler Collision Detection (CCD) data bus. The MIL Light Emitting Diode (LED) receives battery current on the instrument cluster electronic circuit board through the fused ignition switch output (st-run) circuit whenever the ignition switch is in the ON or Start positions; therefore, the LED will always be OFF when the ignition switch is in any position except ON or Start. The LED 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 indicator is illuminated for about seven seconds as a bulb test.
PCM Lamp-ON Message - Each time the cluster receives a lamp-ON message from the PCM or ECM, the indicator will be illuminated. The indicator can be flashed ON and OFF, or illuminated solid, as dictated by the PCM or ECM message. For some DTC's, if a problem does not recur, the PCM or ECM will send a lamp-OFF message automatically. Other DTC's may require that a fault be repaired and the PCM or ECM be reset before a lamp-OFF message will be sent. For more information on the PCM, the ECM, and the DTC set and reset parameters.
Communication Error - If the cluster receives no lamp-ON message from the PCM or ECM for twenty seconds, the MIL is illuminated by the instrument cluster to indicate a loss of bus communication. The indicator remains controlled and illuminated by the cluster until a valid lamp-ON message is received from the PCM or ECM.
Actuator Test - Each time the cluster is put through the actuator test, the indicator will be turned ON during the bulb check portion of the test to confirm the functionality of the LED and the cluster control circuitry.
The PCM/ECM continually monitor the fuel and emissions system circuits and sensors to decide whether the system is in good operating condition. The PCM or ECM then sends the proper lamp-ON or lamp-OFF messages to the instrument cluster. For further diagnosis of the MIL or the instrument cluster circuitry that controls the LED, (Refer to INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING Testing and Inspection). 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 ECM, the CCD data bus, or the message inputs to the instrument cluster that control the MIL, a DRBIII scan tool is required. Refer to the appropriate diagnostic information.