How a Blinker Flasher Works: From Bimetal Strips to Smart Electronics

A blinker flasher is a timing device that repeatedly interrupts power to a vehicle’s turn-signal lamps—typically 60–120 times per minute—so they blink. Older systems do this with a self-heating bimetal strip that opens and closes the circuit, while modern cars use electronic modules or the body control module (BCM) to switch the lamps with precise timing, detect bulb failures (often causing “hyperflash”), and sometimes simulate the relay “click.”

The job a flasher performs

At its core, the flasher’s job is simple: create a rhythmic on–off cycle so the turn indicators are highly visible and legally compliant. It must drive either the left or right lamps when the stalk is engaged, or all four when hazards are on, and it often provides a tell-tale in the cluster along with an audible cue.

Main approaches: thermal vs. electronic

Automotive flashers have evolved through two dominant designs, each with distinct behavior and wiring conventions.

• Thermal (bimetal) flashers: use lamp current to heat a bimetal strip that bends to open/close the circuit.
• Electronic flashers and BCM-controlled systems: use solid-state timing and a relay or transistor to switch the lamps, often with current sensing for diagnostics.

Both achieve the same end result—blinking lights—but electronic systems offer steadier cadence, better diagnostics, and compatibility with low-current LED lamps.

How a thermal (bimetal) flasher works

Classic turn-signal units rely on a simple electromechanical cycle driven by the actual current drawn by the bulbs.

1. Power flows through the flasher to the selected turn-signal lamps. The lamp current also passes through (or around) a heater bonded to a bimetal strip inside the flasher.
2. The bimetal heats and bends, opening the circuit. The lamps go dark and current stops flowing.
3. The strip cools, springs back, and closes the circuit again. Lamps relight.
4. This heat–cool cycle repeats, creating the blink, with rate affected by load, battery voltage, and ambient temperature.

Because the timing depends on current, losing a bulb (reduced load) changes the blink rate—often producing a noticeably faster “hyperflash” that alerts the driver to a fault.

How an electronic/BCM-based flasher works

Modern vehicles replace the thermal mechanism with solid-state timing and sophisticated monitoring, often integrating the logic into a body control module.

1. A timing circuit (IC, microcontroller, or BCM software) generates a steady on–off signal at the target rate (commonly 60–120 flashes per minute).
2. A relay or power transistor (MOSFET) switches the lamp circuit on and off according to the timing signal.
3. Current sensing (via a shunt resistor, smart driver, or BCM measurement) verifies expected load. If it’s too low (e.g., a bulb is out), the module speeds up the blink (hyperflash) or posts a warning.
4. Many systems synthesize the “click” through a speaker, and maintain a stable cadence despite voltage or temperature changes.

Electronic flashers are more tolerant of LED retrofits when labeled “LED-compatible,” and BCM systems can be software-calibrated for different lamp types.

What determines blink speed and duty cycle

Blinkers must meet legal timing ranges for visibility. In the United States (FMVSS 108) and Europe (ECE R48), the turn-signal rate typically must fall between about 60 and 120 flashes per minute; duty cycle (the on-time versus total cycle time) is generally near 50% but may vary. Thermal units drift with voltage and temperature, while electronic/BCM systems keep timing precise.

Load sensing, bulb-out warnings, and hyperflash

Because visibility and safety depend on all lamps working, many systems use the blink rate itself as a warning mechanism.

• Hyperflash: a faster blink (often above 120 fpm) indicates a failed or low-load lamp.
• BCM message: some newer vehicles keep the normal cadence but display a “turn-signal lamp out” message instead.
• LED retrofits: LEDs draw less current and can falsely trigger hyperflash unless you use an LED-rated flasher, reprogram the BCM (where supported), or add load resistors.

These strategies ensure drivers quickly notice a fault, reducing the chance of signaling without adequate lighting.

Hazard flashing and tell-tales

Hazard mode ties both sides together so all corner lamps flash. Older vehicles sometimes used a separate hazard flasher can; modern BCM systems simply command all channels. The audible “click” in modern cars is often simulated, not a relay sound, and the instrument cluster tell-tale is controlled by the same timing logic.

Connections and where to find the flasher

Wiring conventions vary. Legacy thermal/electronic cans often use terminals marked X (power), L (load), and P (panel/indicator), or ECE designations 49 (power), 49a (load), 31 (ground), and C/C2 (tell-tales). The unit is typically under the dash near the steering column or within the fuse/relay panel; newer vehicles integrate the function inside the BCM with no separate “can.”

Why flashers behave differently with LEDs

Thermal units require incandescent-level current to heat the bimetal, so low-draw LEDs may not flash at all or may hyperflash. Electronic/BCM solutions maintain timing but still use load sensing for diagnostics, which can be satisfied by LED-compatible modules, coding, or properly rated resistors.

Common issues and quick checks

If your blinkers aren’t behaving, a few systematic checks can save time and guesswork.

• One side flashes fast: likely a burned-out bulb or poor ground on that side.
• No flash, lamps stay on solid: thermal flasher stuck closed or wrong flasher type; in BCM cars, possible relay/driver fault.
• No flash, lamps dark: blown fuse, failed flasher/relay, bad stalk switch, or BCM fault.
• LED retrofit hyperflash: use an LED-compatible flasher, add load resistors, or have the BCM reconfigured.
• Intermittent operation: check bulb sockets, grounds, and connectors for corrosion or looseness.

Starting with bulbs and grounds is often quickest; then move to the flasher module, fuses, and finally wiring or BCM diagnosis.

Safety and legal notes

Any modification that changes blink rate or lamp brightness can affect compliance and insurance. When fitting LEDs, ensure the lamps remain bright, the rate stays within 60–120 fpm, and that your vehicle’s diagnostics (or resistors/LED flasher) keep the system within spec.

Summary

A blinker flasher is a timing device that turns turn-signal lamps on and off at a regulated cadence. Traditional thermal flashers rely on a self-heating bimetal strip whose bending opens and closes the circuit, while modern vehicles use electronic modules or the BCM to switch lamps with precise timing, detect load faults, and provide alerts. Load sensing explains hyperflash and the quirks seen with LED retrofits; using LED-rated hardware or proper reprogramming preserves both function and compliance.

Is a flasher relay the same as a turn signal relay?

Turn Signal vs Hazard Flasher: Understanding the Difference Turn signals failing while hazard lights function; flasher relay wear causing intermittent signal issues. Turn signals and hazard flashers often use separate flasher relays. If hazards work but turn signals don’t, check the turn signal flasher relay first.

How do I tell if my turn signal flasher is bad?

Listen for clicking sounds when signaling; no click indicates a bad relay. Inspect fuses related to turn signals and hazard lights. Replace the flasher relay if faulty. Also, verify wiring connections for corrosion or looseness. Proper relay function ensures correct blinking speed and signal operation.

How to test a blinker flasher?

Testing the flasher unit
To test the conventional type of flasher unit, use a circuit tester between the terminal marked B on the unit and the earth. Turn on the ignition . If the supply side of the unit is working, the bulb should light. If it does not, look for a break in the wiring between the unit and the fuse box.

How does a turn signal flasher work?

The thermal flasher
This completes the circuit, allowing current to flow. Initially, the spring steel does not touch the contact, so the only thing that draws power is the resistor. Current flows through the resistive wire, heating up the smaller piece of spring steel and then continuing on to the turn-signal lights.