Cargo Lamp Indicator: Operation

WARNING: This page is about a different car, the 2003 Dodge Dakota. However, it is still accessible from the selected car via links, so may be relevant.

The cargo lamp indicator gives an indication to the vehicle operator when the exterior cargo lamp is illuminated. This indicator is controlled by a transistor on the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the Central Timer Module (CTM) over the Programmable Communications Interface (PCI) data bus. The cargo lamp indicator Light Emitting Diode (LED) is completely controlled by the instrument cluster logic circuit, and that logic will only allow this indicator to operate whenever the instrument cluster receives a battery current input on the fused ignition switch output (run-start) circuit. 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 cargo lamp indicator for the following reasons:

Bulb Test -Each time the ignition switch is turned to the On position the cargo lamp indicator is illuminated for about two seconds as a bulb test.
Cargo Lamp-On Message -Each time the cluster receives a lamp-on message from the CTM, the cargo lamp indicator will be illuminated. The indicator remains illuminated until the cluster receives a cargo lamp-off message from the CTM, or until the ignition switch is turned to the Off position, whichever occurs first.
Actuator Test -Each time the cluster is put through the actuator test, the cargo lamp indicator will be turned on, then off again during the bulb check portion of the test to confirm the functionality of the LED and the cluster control circuitry.

The CTM continually monitors the cargo lamp switch circuitry of the headlamp switch through the cargo lamp switch sense circuit to determine the status of the cargo lamps. The CTM then sends the proper cargo lamp-on and lamp-off messages to the instrument cluster. For further diagnosis of the cargo lamp indicator or the instrument cluster circuitry that controls the indicator, (Refer to DIAGNOSIS AND TESTING-INSTRUMENT CLUSTER ). For proper diagnosis of the CTM, the PCI data bus, or the electronic message inputs to the instrument cluster that control the cargo lamp indicator, a DRBIII(R) scan tool is required. Refer to the appropriate diagnostic information.