6.2 Turn Signals and Hazard Systems

6.2 Turn Signals and Hazard Systems

Turn signal and hazard systems are critical safety indicators that notify other drivers of a vehicle's movements. Diagnosing these circuits requires understanding flashers, switches, and computerized control modules.

Flasher and Switching Technologies

• Thermal Flashers: Traditional turn signal circuits use a thermal flasher module consisting of a bimetallic strip wrapped with a high-resistance heating coil. When the turn signal is activated, current flows through closed contacts and the heating coil to the bulbs. The current heats the coil, causing the bimetallic strip to bend and open the contacts, interrupting current. As the coil cools, the strip snaps back, closing the contacts and repeating the cycle. Flash rate depends on current draw. If a bulb burns out, current drops, keeping the strip cool; the turn signal remains illuminated solid or fails to turn on.
• Electronic Flashers: Electronic flashers replace the bimetallic strip with an integrated circuit timer controlling an internal relay. The flash rate remains constant regardless of the circuit's load. However, to meet safety regulations, electronic flashers monitor current draw using an internal shunt resistor. If current drops below a threshold (indicating a burned-out bulb), the timer doubles the flash frequency, known as hyperflashing, to alert the driver.
• BCM-Controlled Flashers: Modern vehicles use the Body Control Module (BCM) to control turn signals and hazards. The switches act as low-current digital inputs to the BCM. The BCM processes inputs and controls lamps using high-side solid-state MOSFET drivers or relays. The BCM monitors circuit current; if a bulb burns out, the drop in current initiates hyperflashing on that side.

Circuit Operations and Bulb Configurations

• Multifunction and Hazard Switches: The turn signal switch is housed within the steering column multifunction switch assembly. It directs battery power to the left or right side turn signal lights. In systems with shared brake and turn signal filaments (two-wire rear lighting), the multifunction switch interrupts the brake light circuit on the active side, allowing that bulb's bright filament to flash while the opposite side remains solid when braking. The hazard warning switch connects all four turn signals to a constant battery power source (B+). The turn signal circuit is powered through the ignition switch (switched), while the hazard switch bypasses the ignition switch to function with the key off.
• LED Retrofits and Bulb Sharing: Rear assemblies often use dual-filament bulbs (e.g., 3157) where the dim filament serves as parking lights and the bright filament is shared between brake and turn signal circuits. LED bulbs draw significantly less current than incandescent bulbs. When replacing incandescent bulbs with LEDs, the BCM or electronic flasher detects the lower current draw and triggers hyperflashing. To correct this, load resistors must be installed in parallel with the LED bulbs to mimic incandescent current draw, or the flasher must be replaced with an LED-compatible unit.

Diagnostic Procedures

• Rapid Flashing (Hyperflashing): Hyperflashing on one side indicates a burned-out bulb, an incorrect bulb type, or high resistance (corrosion) in a socket or wire. Visually check all bulbs on the affected side. If one side is inoperative, check the bulb filament and check the socket for corrosion. Use a test light or DMM to verify pulsed voltage and ground at the socket contacts. If power or ground is missing, trace the circuit back to the switch or BCM. If the hazards work on all corners but turn signals do not, the bulbs and wiring are good; the fault lies in the turn-signal-specific power feed. Check the turn signal fuse, ignition switch accessory circuit, or turn signal contacts in the switch. If turn signals work but hazards do not, check the hazard fuse (unswitched battery power) and hazard switch.

Real-World Tech Scenario

A pickup truck has a hyperflash condition on the left side. Both left lights flash rapidly, but the left rear bulb is dim. The technician checks the left rear socket and finds corrosion. Testing voltage at the socket under load shows 8.2V pulsed power, and a ground-side voltage drop test shows 3.8V pulsed voltage on the socket's ground circuit. Tracing the ground wire to frame splice G302 reveals a loose, rusted mounting bolt. Cleaning the frame connection and securing the ground bolt drops the ground-side voltage drop to 0.05V, and the turn signals flash at the normal rate.

Tech Tips and Exam Reminders

• Tech Tip: If turning on the parking lights causes a turn signal indicator on the dashboard to light up solid, it indicates a lost ground at a front corner light housing. The parking light current backfeeds through the shared turn signal filament and finds a ground path through the dashboard indicator bulb.
• Exam Tip: Turn signals and hazards are powered through separate fuses. The turn signals require ignition voltage (switched), while the hazards require battery voltage (unswitched). This difference helps isolate power supply issues.