Three-Prong Alternating Flashers

Introduction

If you’re working on vintage emergency lighting, you’ll likely encounter 3-prong alternating flasher relays. The purpose of this page is to explain the types of 3-prong alternating flashers that have been used, how they work, and what’s currently available.

Note: At the time of writing, the Novita EL13A1 and EL13A2 are among the few widely available 3-prong alternating flashers still in regular production. Federal Signal FA3-SC units are still available through select industrial and emergency vehicle suppliers but are typically significantly more expensive.

Thermal Flashers

Thermal flashers like the Wagner and Signal-Stat 537 use an internal bi-metallic strip wrapped in a high-resistance heating wire. When current flows, the wire heats the strip, causing it to bend and break the contact for Circuit A while closing the contact for Circuit B. As it cools, it springs back, reversing the process.

Classic Examples: Wagner 537, NAPA 537, Signal-Stat 537.

Light Capacity: Designed for multiple incandescent or halogen warning lamps. The exact maximum load varies by manufacturer and no official wattage specification could be located.

Flash Rate: Approximately 60–90 flashes per minute, depending on lamp load, battery voltage, and operating temperature.

Pros: Highly durable, simple mechanical design, emits a nostalgic, loud “click” sound.

Cons: Zero LED compatibility. They require the heavy amperage draw of incandescent/halogen bulbs to generate heat. If the current is too low, the lightbar will freeze with one side stuck on.

Behavior: Because the flash rate depends on heat, it naturally varies with temperature, battery voltage, bulb load, and the condition of the internal contacts. It’s common for a thermal flasher to start slowly when cold and gradually speed up as it reaches operating temperature.

Electronic Flashers

Electronic flashers such as the Tridon/Novita EL13A1, EL13A2, and Federal Signal FA3 replace the thermal mechanism with an electronic timer circuit that alternately switches the two output circuits at a fixed rate. These flashers can be used in place of the older thermal flashers.

Classic Examples: Tridon / Novita EL13A1, EL13A2 / Federal Signal FA3

Light Capacity: Novita EL13A1 and EL13A2 are rated for up to 150 watts per side (approximately six 25-watt lamps per side). The Federal Signal FA3-SC is rated for 2–10 amp light circuits.

Flash Rate: Electronically controlled at approximately 90 flashes per minute.

Pros: Fully LED compatible and incandescent compatible.

Cons: More sensitive to extreme voltage spikes or reverse polarity wiring. When they fail, they often stop alternating properly, leaving one or both lamp circuits continuously illuminated.

Behavior: Perfectly consistent, fixed flash rate (typically 60 to 90 flashes per minute) regardless of temperature or vehicle voltage.

Novita EL13A1 vs EL13A2

Novita EL13A1: Comes with a standard, removable flat metal mounting bracket (clip-on ring style). It is designed for flush mounting against a firewall or inside a radio console layout.

Novita EL13A2: Comes with a 90-degree right-angle metal mounting bracket. It is intended for hanging applications, such as securing the canister underneath a vehicle dashboard or inside a classic lightbar frame channel.

Wiring A 3-Prong Alternating Flasher

Regardless of the manufacturer the connections are the same. The flasher will have an ‘X‘ terminal that is the 12V power input, and then two ‘L‘ terminals for each light output. The Federal Signal flashers have ‘L1‘ and ‘L2‘ on them. Older Signal-Stat and Wagner units may use different terminal markings. For example, the Signal-Stat flasher shown below uses L and P instead of L1 and L2. The important thing is to correctly identify the X terminal, which is the 12-volt power input. The remaining two terminals alternate power between the two lamp circuits.

If you’re working with an old brittle plug and creating a new wiring setup, you can order new 3-prong relay plugs from Amazon.

Three Prong Relay Plug

Federal Signal FA3-SC Flasher With A Ground Wire

Federal Signal had two versions of their FA3 flasher. On the early version, if one of your lightbar bulbs burned out, the flasher would freeze and stop blinking entirely. The newer version added a dedicated ground wire to ensure the remaining bulb keeps safely flashing no matter what.

Generation 1: The 3-Prong FA3 (No Ground Wire)

The original FA3 was designed as a direct, plug-and-play replacement for old-fashioned thermal flasher cans.

The Technology: It uses an early electronic hybrid circuit board. Instead of a standard timer chip, it uses low-voltage transistors and a specialized comparator circuit designed to run without a permanent ground.

How it Grounds (Floating Ground): To power its internal electronics, this flasher “steals” a ground path dynamically by letting current bleed backward through the dark filament of whichever lightbulb is turned off at that exact millisecond.

The Fatal Flaw (Single-Bulb Freeze): Because it relies on the lightbulbs to complete its circuit loop, if a single halogen bulb burns out, the electrical path is broken. The internal brain loses power instantly, freezing the flasher and leaving your one remaining “good” bulb stuck permanently turned on without blinking.

Generation 2: The FA3-SC (With Black Ground Wire)

To fix the single-bulb freeze issue, Federal Signal completely redesigned the internal circuit board, creating the FA3-SC model.

The Technology: This version upgrades to a highly reliable, industry-standard 555 integrated circuit (IC) digital micro-timer chip.

How it Grounds (Isolated Ground): A 555 timer chip demands a constant, uninterrupted path to ground to keep its internal digital logic running. Federal Signal engineers wired the chip’s ground pin directly out of the plastic housing, creating the external black wire pigtail.

The Safety Failsafe (Continuous Flashing): Because the digital brain has its own permanent, dedicated path straight to the vehicle’s chassis, it does not care what the lightbulbs are doing. If a bulb burns out on a police car or fire truck, the 555 chip keeps pacing perfectly, ensuring the surviving warning light continues to flash safely at 90 blinks per minute.

Wiring These Flashers

Wiring these flashers is the same as the others above, and you can use the same three-prong plug. The only additional connection is the black ground wire, which must be connected to a good chassis ground.

Federal Signal FA3 Electronic Flasher Video