Principles of Operation

Instrument Cluster and Panel Illumination

Principles of Operation

Dimmable Backlighting

When the parking lamps are on, the Steering Column Control Module (SCCM) monitors the input from the instrument panel dimmer switch. The SCCM sends a message to the Body Control Module (BCM) over the High Speed Controller Area Network (HS-CAN) relaying the instrument panel dimmer switch input. Based on the input from the instrument panel dimmer switch, the BCM sends voltage to the dimmable switches. The BCM sends a message to the Audio Control Module (ACM) (except base AM/FM), the HVAC module, Trailer Brake Control (TBC) module and the Instrument Panel Cluster (IPC) over the Controller Area Network (CAN) to indicate the backlighting intensity level.

If the receiving module receives invalid backlighting data from the BCM for 5 seconds or less, the receiving module defaults the backlighting to the last setting. If the receiving module does not receive the backlighting status message from the BCM or if the data received is deemed invalid for more than 5 seconds, the receiving module sets a missing message related DTC in continuous memory and defaults the backlighting to full nighttime intensity.

Non-Dimmable Backlighting

When the accessory delay relay is energized, switched voltage is supplied to the rear heated seat switches, window control and door lock control switches.

Field-Effect Transistor (FET) Protection

A Field-Effect Transistor (FET) is a type of transistor that, when used with module software, monitors and controls current flow on module outputs. The FET protection strategy prevents module damage in the event of excessive current flow.

The BCM utilizes an FET protective circuit strategy for many of its outputs (for example, a headlamp output circuit). Output loads (current level) are monitored for excessive current (typically short circuits) and are shut down (turns off the voltage or ground provided by the module) when a fault event is detected. A short circuit DTC is stored at the fault event and a cumulative counter is started.

When the demand for the output is no longer present, the module resets the FET protection to allow the circuit to function. The next time the driver requests a circuit to activate that has been shut down by a previous short (FET protection) and the circuit is still shorted, the FET protection shuts off the circuit again and the cumulative counter advances.

When the excessive circuit load occurs often enough, the module shuts down the output until a repair procedure is carried out. Each FET protected circuit has 3 predefined levels of short circuit tolerance based on the harmful effect of each circuit fault on the FET and the ability of the FET to withstand it. A module lifetime level of fault events is established based upon the durability of the FET. If the total tolerance level is determined to be 600 fault events, the 3 predefined levels would be 200, 400 and 600 fault events.

When each tolerance level is reached, the short circuit DTC that was stored on the first failure cannot be cleared by the clear the continuous DTCs command. The module does not allow this code to be cleared or the circuit restored to normal operation until a successful self-test proves the fault has been repaired. After the self-test has successfully completed (no on-demand DTCs present), DTC U1000:00 and the associated DTC (the DTC related to the shorted circuit) automatically clears and the circuit function returns.

When each level is reached, the DTC associated with the short circuit sets along with DTC U1000:00. These DTCs are cleared using the module on-demand self-test, then the Clear DTC operation on the scan tool (if the on-demand test shows the fault corrected). The module never resets the fault event counter to zero and continues to advance the fault event counter as short circuit fault events occur.

If the number of short circuit fault events reach the third level, then DTCs U1000:00 and U3000:49 set along with the associated short circuit DTC. DTC U3000:49 cannot be cleared and the module must be replaced after the repair.

The BCM FET protected output circuit for the instrument cluster and panel illumination system is the dimmable switch output circuit.