Malfunction Indicator Lamp: Description and Operation

A 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 right of the odometer/trip odometer Vacuum-Fluorescent Display (VFD). The MIL consists of a stenciled cutout of a graphical representation or icon of an engine in the opaque layer of the instrument cluster overlay. The dark outer layer of the overlay prevents the icon from being clearly visible when the lamp is not illuminated. However, 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 it is illuminated from behind by a replaceable incandescent bulb and bulb holder unit located on 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. This lamp is controlled by the instrument cluster circuit board based upon the cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) over the Chrysler Collision Detection (CCD) data bus. The MIL bulb 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 lamp will always be OFF when the ignition switch is in any position except ON or Start. The lamp bulb only illuminates when it is switched to ground by the instrument cluster circuitry. 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 lamp is illuminated for about seven seconds.
- PCM Lamp-ON Message - Each time the cluster receives a lamp-ON message from the PCM, the lamp will be illuminated. The lamp 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.
- Communication Error - If the cluster receives no engine RPM and MAP message from the PCM for twenty seconds, the MIL lamp is illuminated by the instrument cluster and a "no bus" message will appear in the odometer Vacuum-Fluorescent Display (VFD) to indicate a loss of bus communication. The lamp remains controlled and illuminated by the cluster until five good bus messages are received from the PCM.
- Actuator Test - Each time the cluster is put through the actuator test, the lamp will be turned ON and OFF in a prescribed sequence to confirm the functionality of the lamp and the cluster 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 lamp, (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 system may require service. For proper diagnosis of the fuel and emissions systems, the CCD data bus, or the message inputs to the instrument cluster that control the MIL, a DRB III scan tool and the appropriate diagnostic information are required.