Principles Of Operation

Headlamps

Principles of Operation

NOTE: The Smart Junction Box (SJB) is also known as the Generic Electronic Module (GEM).

NOTE: The Headlamp Control Module 2 (HCM-2) is also known as the automatic high beam module.

Exterior Lighting

The SJB monitors the headlamp switch position by sending voltage signals on multiple circuits to the headlamp switch. There is one circuit for each headlamp switch position. At any given time, one of the signal circuits is switched to ground.

If the SJB does not detect any of the inputs to the headlamp switch is active (switched to ground) for 5 seconds, the SJB turns on the parking lamps and headlamps and keeps them on until the battery saver feature times out.

Additionally, if the SJB detects multiple headlamp switch input circuits short to ground, the SJB turns on the parking lamps and headlamps and keeps them on until the battery saver feature times out.

Refer to Exterior Lighting Exterior Lighting for information regarding the battery saver feature.

If either of these situations occur, the SJB cannot be ruled immediately as being at fault. This is normal behavior of the SJB design as it has detected a fault with the inputs from the headlamp switch.

The SJB also monitors the multifunction switch for a flash-to-pass or high beam request. There are 2 voltage signal circuits which monitor this. When the multifunction switch is in the FLASH-TO-PASS or HIGH BEAM position, the voltage signal is routed to ground.

NOTE: The flash-to-pass feature does not require any input from the headlamp switch.

When the SJB receives an input requesting the headlamps on, the SJB supplies voltage to a ballast within each headlamp. The ballasts then provide the necessary high voltage to the High Intensity Discharge (HID) bulbs.

High Beam/Flash-To-Pass Functionality

When the high beams are requested, the ballasts remain powered and a shutter within each headlamp is activated. This changes the headlamp beam pattern to illuminate a greater distance.

If the low beams are off when the flash-to-pass is requested, the ballasts and the shutters are activated for approximately 0.5 second. If the low beams are on when the flash-to-pass is requested, the shutters within the headlamps are activated as long as the multifunction switch is held in the FLASH-TO-PASS position.

Automatic High Beams

The automatic high beam feature is controlled by the HCM-2. GO to Pinpoint Test B, Normal Operation, for further description of the automatic high beam feature and operation. Pinpoint Test B: The Automatic High Beam Feature Is Inoperative

Field-Effect Transistor (FET) Protection

Field-Effect Transistor (FET) is a type of transistor that when used with module software can be used to monitor and control current flow on module outputs. The FET protection strategy is used to prevent module damage in the event of excessive current flow.

The SJB utilizes a 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 continuous 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 circuit 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 remains 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 continuous DTC that was stored on the first failure cannot be cleared by a command to clear the continuous DTCs. The module does not allow this code to be cleared or the circuit restored to normal operation until a successful self-test proves that the fault has been repaired. After the self-test has successfully completed (no on-demand DTCs present), DTC B106E and the associated continuous DTC (the DTC related to the shorted circuit) automatically clear and the circuit function returns.

When the first or second level is reached, the continuous DTC (associated with the short circuit) sets along with DTC B106E. These DTCs can be 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 B106F and B1342 set along with the associated continuous DTC. This DTC cannot be cleared and the module must be replaced.

The SJB FET protected output circuits for the headlamp system are the LH and RH low beam output circuits.

The HCM-2 FET protection utilizes a strategy similar to the SJB. The protected output circuit for the automatic high beam system is the automatic high beam control relay output circuit.