Lamp Out Indicator: Description and Operation

A lamp out indicator is standard equipment on all instrument clusters. The lamp out indicator consists of the words "LAMP OUT", which appear in the lower portion of the odometer/trip odometer Vacuum-Fluorescent Display (VFD) unit.

The odometer/trip odometer VFD unit is soldered onto the cluster electronic circuit board and is visible through a window with a smoked clear lens located on the lower edge of the tachometer gauge dial face of the cluster overlay. The dark lens over the VFD prevents the lamp out indicator from being clearly visible when it is not illuminated. The words "LAMP OUT" appear in an amber color and at the same lighting level as the odometer/trip odometer information when they are illuminated by the instrument cluster electronic circuit board. The lamp out indicator is serviced as a unit with the VFD in the instrument cluster.

The lamp out indicator gives an indication to the vehicle operator when an exterior lamp has failed. This indicator is controlled by the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the Front Control Module (FCM) over the Programmable Communications Interface (PCI) data bus.

The lamp out indicator 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 indicator will always be off when the ignition switch is in any position except On or Start. The indicator only illuminates when it is switched to ground by the instrument cluster circuitry. The instrument cluster will turn on the lamp out indicator for the following reasons:
- Lamp Out Indicator Lamp-On Message - Each time the cluster receives a lamp out indicator lamp-on message from the FCM indicating that an inoperative headlamp (low or high beam), turn signal lamp, or brake lamp (excluding Center High Mounted Stop Lamp [CHMSL]) circuit has been detected with the ignition switch in the On position, the lamp out indicator is illuminated solid. The indicator remains illuminated until the cluster receives a lamp out indicator lamp-off message from the FCM or until the ignition switch is turned to the Off position, whichever occurs first. If the lamp-on message is received when the ignition is first turned to the On position, the lamp out indicator is flashed at a fast rate (1 Hertz) for about thirty seconds before illuminating solid.
- Actuator Test - Each time the cluster is put through the actuator test, the lamp out indicator will be turned on, then off again during the VFD portion of the test to confirm the functionality of the VFD and the cluster control circuitry.

The FCM monitors each of the headlamp, turn signal lamp, and brake lamp (except CHMSL) circuits to determine the condition of these exterior lamps. The FCM then sends the proper lamp out indicator lamp-on and lamp-off messages to the instrument cluster. For further diagnosis of the lamp out indicator or the instrument cluster circuitry that controls the indicator, refer to testing.

For proper diagnosis of the exterior lighting system circuits, the FCM, the PCI data bus, or the electronic message inputs to the instrument cluster that control the lamp out indicator, a diagnostic scan tool is required.