Principles of Operation

Turn Signal and Hazard Lamps

Principles of Operation

The Steering Column Control Module (SCCM) monitors the multifunction switch position. When the multifunction switch is in the LH or RH TURN position, the SCCM (Steering Column Control Module) sends a message over the High Speed Controller Area Network (HS-CAN) to the Body Control Module (BCM) indicating a request for the LH or RH turn signal.

The BCM (Body Control Module) sends a voltage signal to the hazard flasher lamp switch. When the hazard flasher switch (a momentary contact switch) is pressed, the voltage signal is routed to ground, indicating a request to activate or deactivate the hazard lamp function.

When the BCM (Body Control Module) receives a request for a turn signal or the hazard lamps, the BCM (Body Control Module) supplies on/off voltage to the appropriate turn lamps.

The timed on/off cycle for turn lamps is determined by the BCM (Body Control Module) and is set to flash approximately 80 times per minute if both the front and rear turn signal lamps operate correctly. If an individual turn signal lamp is inoperative, the BCM (Body Control Module) fast flashes the remaining turn lamp approximately 160 times per minute to indicate a bulb outage to the driver.

There is no bulb outage indication for the exterior mirror turn lamps.

The timed on/off cycle for the hazard lamps is approximately 80 times per minute regardless of bulb outage.

The BCM (Body Control Module) bulb outage is configurable and can be turned off for the front or rear lamps. This is to accommodate vehicles that have been outfitted with rear LED lamps in place of the traditional incandescent bulbs.

The rear turn signal lamps share functionality with the rear stoplamps.

The multifunction switch is a momentary contact switch, with 2 detents for the LH TURN position and 2 detents for the RH turn position. When placed in the first detent, the corresponding turn signals flash 3 times and turn off. When the multifunction switch is moved to the second detent, the turn signal flashes until the steering wheel is turned in the opposite direction (based on input from the steering wheel rotation sensor), the multifunction switch is moved to the second detent a second time or the vehicle travels farther than 3.2 km (2 miles). The BCM (Body Control Module) monitors the steering wheel rotation sensor and the multifunction switch input to determine when to cancel the turn signals. The BCM (Body Control Module) monitors the distance traveled to determine if the turn signal should be turned off.

Field-Effect Transistor (FET) Protection

A 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 (Field-Effect Transistor) protection strategy prevents module damage in the event of excessive current flow.

The BCM (Body Control Module) utilizes an FET (Field-Effect Transistor) 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 (Field-Effect Transistor) 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 (Field-Effect Transistor) protection) and the circuit is still shorted, the FET (Field-Effect Transistor) 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 (Field-Effect Transistor) protected circuit has 3 predefined levels of short circuit tolerance based on the harmful effect of each circuit fault on the FET (Field-Effect Transistor) and the ability of the FET (Field-Effect Transistor) to withstand it. A module lifetime level of fault events is established based upon the durability of the FET (Field-Effect Transistor). 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 that 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 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 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 (Body Control Module) FET (Field-Effect Transistor) protected output circuits for the turn lamps system are the LH and RH front and the LH and RH exterior mirror turn lamp output circuits.