Is there a way to test a flasher relay?

Yes — you can test a flasher relay using practical methods such as bench-testing with a 12V supply and a dummy load, swapping in a known-good relay, and performing on-vehicle diagnostics with a multimeter. These approaches help determine whether the relay itself is faulty or if the issue lies elsewhere in the circuit.

In this article, we explain what a flasher relay does, how failures manifest, and how to perform reliable tests on both traditional thermal and electronic relays. We also cover considerations for LED lighting and modern vehicles where the flasher function may be integrated into a body control module (BCM).

What a flasher relay does

The flasher relay is responsible for the intermittent on-off cycling of turn signal and hazard lights. In older, traditional systems, a thermal flasher uses a heated bimetal strip that bends to break the circuit and then cools to reconnect, creating a blinking pattern. Electronic flashers use solid-state circuits or microcontrollers to generate the pulses. In many modern cars, the flashing is managed by the body control module (BCM) or a similar controller, and a separate, external relay may not exist.

Ways to test a flasher relay

Below are practical methods to verify whether a flasher relay is functioning correctly, covering both bench testing and on-vehicle scenarios.

• Bench test with a 12-volt power source: Remove the relay from the vehicle, connect a stable 12 V supply and ground, and attach a dummy load (such as two automotive bulbs or appropriate resistors) to the relay's output. Observe whether the output switches on and off in a consistent cycle.


• Current draw and switch verification: While the relay is energised, measure the current going through the output circuit and compare it to the expected load for your bulbs or LEDs. Abnormal current (too high or too low) can indicate a faulty relay or bad wiring.


• Substitution with a known-good unit: If available, swap the suspect relay with a confirmed-good relay of the same type and observe whether the signals blink normally. This is often the quickest way to confirm a bad relay.


• On-vehicle inspection and testing: Check fuses and grounds, inspect the bulb sockets and wiring for corrosion or damage, and verify that the hazard switch and any related modules are functioning. If the flasher is integrated into a BCM, diagnosis may require scanning for fault codes or checking controller outputs.

Conclusion: A relay that fails to cycle on the bench or behaves erratically under load is likely faulty. If the bench test passes but the vehicle still exhibits issues, the problem may lie in wiring, bulbs, or control circuitry rather than the relay itself.

LED and low-load considerations

LED turn signals or very low-load bulbs can cause standard flashers to operate improperly, often resulting in rapid or uneven blinking. In such cases, use an LED-compatible flasher relay or add resistors to simulate the electrical load of incandescent bulbs. If your vehicle relies on a BCM, ensure that the incorrect flash pattern isn’t due to a software setting or controller fault rather than a faulty relay.

Common pitfalls and tips

Several issues can mimic a bad relay, including corroded bulb sockets, poor grounds, blown fuses, or faulty bulbs.

• Always disconnect the battery when removing or installing relays to avoid short circuits.


• Label and photograph wiring before removal to ensure correct reassembly.


• Verify the correct type and rating of the relay for your vehicle, especially if upgrading to LED lighting.

Conclusion: Systematic checks of the relay, loads, and wiring help pinpoint the fault. When in doubt, substituting a known-good relay or consulting vehicle-specific repair documentation can save time.

Summary

Testing a flasher relay is a straightforward process that can be done on the bench or in the vehicle. Key steps include bench testing with a proper load, measuring current and switching behavior, swapping with a known-good unit, and conducting on-vehicle diagnostics for fuses, grounds, and wiring. For LED or low-load setups, ensure an LED-compatible flasher or appropriate load resistors are used, and consider BCM-related causes in modern vehicles. A structured approach helps differentiate relay faults from wiring or control-system issues.

What are the symptoms of a bad flasher?

Turn signals or hazards stay on
If the flasher has any sort of internal short it may cause the hazards or signals to stay on when they are activated, instead of flashing. While they may still light up, the lights will not be able to serve their intended purpose of signaling a turn or emergency situation.

How can I test a relay to see if it's bad?

You can check if a relay is bad by first performing a simple "click" test, then using a multimeter to test its coil resistance and contact continuity. The quickest method is to swap a known working relay into the circuit to see if the problem is resolved. 
Method 1: The "Click" test (using a 9V or 12V battery) 

1. Identify the relay's control pins: Locate pins 85 and 86 on the relay, which are typically the control coil pins. 
2. Connect the battery: Connect the positive and negative terminals of a 9V or 12V battery to these pins. Polarity usually doesn't matter for the control coil. 
3. Listen for a click: A good relay will make a distinct "click" sound as the internal contacts switch, indicating the coil is working. If you don't hear a click, the relay is likely bad. 

This video demonstrates how to check if a relay is bad using a 9V battery: 49sWard WrenchingYouTube · May 6, 2023
Method 2: The "Swap" test 

1. Find a similar relay: Locate a relay for a non-essential, similar circuit in your vehicle (e.g., horn or light relay). 
2. Swap them: Remove both the suspected relay and the known good one and swap their positions. 
3. Check the original circuit: See if the original problem is fixed and if the new circuit now fails. If the problem is resolved, the original relay was bad. 

Method 3: Using a multimeter

1. Disconnect the relay: Remove the relay from the circuit. Disconnect the negative terminal of the battery for safety. 
2. Check the coil resistance: Set your multimeter to resistance (ohms) mode and touch the probes to the coil terminals (pins 85 and 86). A good relay's coil will have a resistance between 50 and 200 ohms. An "open line" (infinite resistance) means the coil is broken. 
3. Check the switch continuity: 
   • At rest: At rest (no power to the coil), there should be no continuity between the main contacts (pins 30 and 87). 
   • When energized: When you connect a power source to the coil, there should now be continuity between pins 30 and 87. A bad relay will fail to show continuity when energized or show it when it should be open. 

This video explains how to test a relay using a multimeter: 36sWard WrenchingYouTube · Apr 17, 2023
Additional steps

• Visual inspection: Before any testing, check the relay for any signs of physical damage, like burnt marks or corrosion. 
• Check wiring: If the relay passes a click test but the original problem persists, check the wiring for the relay to ensure it is receiving power and ground. 

How can you tell if a turn signal switch is bad?

Symptoms of a bad turn signal switch include turn signals not responding, blinking too fast or too slow, requiring you to hold the lever for them to work, and other systems like headlights or wipers also malfunctioning. Sometimes, the switch may also have a failure in the canceling mechanism, causing the signal to continue blinking after you've straightened the steering wheel. 
Common symptoms of a bad turn signal switch

• Turn signals don't work: The lights do not turn on at all when you try to use the signal. 
• Intermittent operation: The signals work sometimes but not others, or only work if you hold the lever in a specific position. 
• Failure to cancel: The signal continues to blink after you have straightened the steering wheel, indicating a problem with the canceling mechanism. 
• Blinking speed changes: The lights might blink too quickly or too slowly. 
• Hazard lights affected: Since hazard lights and turn signals often share components, a problem with the switch can also affect the hazard lights. 
• Other systems malfunction: A faulty turn signal switch can sometimes cause other stalk-controlled functions to stop working, such as the headlights, high beams, or windshield wipers. 
• No clicking sound: The characteristic clicking sound of the flasher relay may be absent. 

What to do

• Inspect bulbs and fuses: Before assuming the switch is bad, check if a bulb is burnt out or a fuse has blown, as these are more common and simpler fixes. 
• Check the flasher relay: A failing flasher relay can cause similar symptoms and is often the cause of both turn signals and hazard lights not working. 
• Consult a professional: If the simple checks don't resolve the issue, a bad switch can be tricky to diagnose. It's best to consult a professional mechanic for diagnosis and repair. 

How to test if a flasher relay is working?

To test the turn signal flasher relay, first locate it under the dashboard or in the fuse box. Swap it with a similar relay to check functionality. Use a multimeter to verify continuity and voltage at the relay terminals when signals are activated.