Turn Signal Indicator - Operation

OPERATION

The turn signal indicators give an indication to the vehicle operator that a turn signal (left or right indicator flashing) or the hazard warning (both left and right indicators flashing) have been selected and are operating. These indicators are controlled by transistors on the instrument cluster electronic circuit board based upon cluster programming, a hardwired multiplex input received by the cluster from the turn signal and hazard warning switch circuitry of the multifunction switch on the turn/hazard switch mux circuit, and electronic messages received from the Front Control Module (FCM) over the Controller Area Network (CAN) data bus.

Each turn signal indicator Light Emitting Diode (LED) is completely controlled by the instrument cluster logic circuit. This logic allows the indicator to operate whenever the instrument cluster receives a battery current input on the fused B(+) circuit. Therefore, each LED can be illuminated regardless of the ignition switch position. The LED only illuminates when it is provided a path to ground by the instrument cluster transistor. The instrument cluster turns on the turn signal indicators for the following reasons:

- Turn Signal-On Input - Each time the cluster detects a turn signal-ON input from the turn signal switch circuitry of the multifunction switch on the turn/hazard switch mux circuit, it sends the appropriate electronic turn signal switch status messages to the FCM over the CAN data bus. The FCM responds to these messages by controlling a battery voltage output and the flash rate for either the right or left turn signal lamps. The FCM also sends the appropriate electronic messages back to the cluster to control the flash rate of the turn signal indicators, as well as to control the click rate of an electromechanical relay soldered onto the cluster electronic circuit board that emulates the sound emitted by a conventional turn signal flasher. The turn signal indicators continue to flash on and off until the cluster receives a turn signal-off input from the multifunction switch, or until the ignition switch is turned to the OFF position, whichever occurs first.
- Hazard Warning-On Input - Each time the cluster detects a hazard warning-ON input from the hazard warning switch circuitry of the multifunction switch on the turn/hazard switch mux circuit, it sends the appropriate electronic hazard switch status messages to the FCM over the CAN data bus. The FCM responds to these messages by controlling a battery voltage output and the flash rate for each of the right and left turn signal lamps. The FCM also sends the appropriate electronic messages back to the cluster to control the flash rate of the turn signal indicators, as well as to control the click rate of an electromechanical relay soldered onto the cluster electronic circuit board that emulates the sound emitted by a conventional hazard warning flasher. The turn signal indicators continue to flash on and off until the cluster receives a hazard warning-off input from the multifunction switch.
- Lamp Out Mode - If the FCM detects an ineffective turn signal lamp or circuit, it increases the flash rate for the remaining operative turn signals and sends an electronic message to the instrument cluster. The instrument cluster then increases the flash rate of the turn signal indicators and the click rate of the electromechanical relay to provide an indication of the problem to the vehicle operator.
- Self Test - Each time the cluster is put through the self test, the turn signal indicators turn on, then off again during the bulb check portion of the test to confirm the functionality of each LED and the cluster control circuitry.

The instrument cluster, also known as the Cab Compartment Node CCN) continually monitors the multifunction switch and electronic messages from the FCM to determine the proper turn signal and hazard warning indicator operation. For further diagnosis of the turn signal indicators or the instrument cluster circuitry that controls the indicators, Testing and Inspection.

Use a diagnostic scan tool for proper diagnosis of the turn signal and hazard warning system, the multifunction switch, the FCM, the CCN, the CAN data bus or the electronic communication related to turn signal indicator operation. Refer to the appropriate diagnostic information.