Turn signals are controlled by the flasher relay — the device that makes the bulbs blink. In older cars it’s a dedicated thermal flasher; in newer vehicles the function is often handled by an electronic flasher or by the body control module (BCM) within the vehicle’s electrical system.
What component controls turn signals?
Historically, a dedicated flasher relay sits in the fuse/relay box and pulses power to the left and right turn signals. As automotive electronics evolved, many manufacturers moved the function into an electronic flasher module or integrated it into the Body Control Module. That means some cars rely on a separate turn-signal relay, while others use software in the BCM to manage blinking patterns and hazards.
Dedicated relay vs BCM integration
Understanding the difference helps with troubleshooting: a traditional car may have a standalone flasher relay that you can swap like a fuse, whereas a newer vehicle might control blinking through the BCM with no user-serviceable relay accessible behind the dash.
Common types and configurations in use today vary by model and year, but the goal remains the same: create a controlled, regular blink for the turn signals and coordinate hazard operation when activated.
Common types of flasher systems include:
- Thermal (bimetal) flasher relay: uses a heating element to flex a metal strip, breaking and making the circuit to produce blinking. Simple, robust, and common in older vehicles.
- Electronic (solid-state) flasher: uses transistors/ICs to switch current, delivering a steadier blink rate and less heat. Typical in many late 1990s-era to present vehicles.
- BCM-integrated flasher: the body control module handles turn signals and hazards via software, with or without a separate physical relay in the box.
In modern cars, you’ll often find one of these arrangements depending on the design and features of the vehicle. The exact component affecting the blink rate can vary between models, but the function remains the same: to control the blinking of your indicators.
Symptoms of a failing flasher relay include a range of misbehavior in the turn signals or hazards, and they can also hint at switch or wiring issues.
Common signs that the flasher relay may be failing include:
- Turn signals do not blink or stay on continuously instead of flashing.
- Indicators on the dashboard do not reflect the actual signal, or refuse to illuminate.
- Hazards work but individual turn signals don’t, or vice versa.
- Blink rate is too fast or too slow (hyperflash or sluggish flashing).
- Hearing a click when turning signals are activated is noticeably absent or abnormal.
These symptoms can also be caused by a faulty switch, wiring, or bulb issues, so it’s important to test systematically to confirm the relay as the source.
To troubleshoot, follow these steps:
- Locate the flasher relay or the BCM control module in your vehicle’s service manual or fuse/relay diagram.
- Check the relevant fuse(s) for the turn signal circuit to rule out a blown fuse.
- Swap the suspected flasher relay with a known-good unit of the same type (if you have access to a standalone relay) to see if blinking returns.
- If hazards function but turn signals don’t, inspect the turn-signal switch, stalk, and wiring for faults.
- Consider LED bulbs: if you installed LEDs, you may need an LED-compatible electronic flasher or resistor kit to prevent hyperflash.
- Replace with the correct part for your vehicle and re-test all functions (left/right turns and hazards).
When replacing or upgrading, verify compatibility with your car’s electrical system and consider professional help if you’re unsure about identifying the correct component.
Summary
The turn signal system is powered by a flasher relay, which may be a traditional thermal unit, an electronic module, or an integrated function within the body control module. Faults in the relay or its related switch/wiring can cause missing or erratic blinking, and troubleshooting typically involves checking fuses, testing or swapping relays, and considering LED-related needs in modern installations. Proper diagnosis will usually confirm whether the relay itself is at fault or if another part of the circuit is the culprit.