Flasher units for HGV and trailer electrical systems
The flasher unit controls the pulse rate of indicator and hazard circuits. On older vehicles using thermal flasher relays, the flash rate is governed by a bimetallic strip that heats and cools - which is why fitting a single LED bulb into a conventional indicator circuit causes rapid flashing. The thermal unit detects lower current draw from the LED and accelerates the cycle rate.
Electronic flasher units solve this. They maintain a fixed flash rate regardless of bulb type, making them the correct replacement when converting any part of the indicator or hazard circuit to LED. Most heavy commercial vehicle systems run 24V, so confirm the voltage rating before ordering.
When diagnosing a flasher fault, check the unit itself last. Rapid flash or no flash is more often caused by a failed bulb, a corroded bulb holder, or a wiring fault than a faulty flasher unit.
Frequently asked questions
Why is my indicator flashing too fast after fitting an LED bulb?
A thermal flasher unit relies on current draw to regulate flash rate. LED bulbs draw significantly less current than filament bulbs, so the thermal unit reads the circuit as having a failed bulb and increases the flash rate. The fix is to either fit an electronic flasher unit rated for LED use, or add a load resistor to restore the correct current draw on that circuit.
What voltage flasher unit do I need for an HGV?
Most heavy commercial vehicles run a 24V electrical system. Confirm the vehicle's system voltage before ordering - fitting a 12V unit to a 24V system will cause premature failure. Some trailers run separate 12V circuits for auxiliary lighting; check the circuit voltage at the point of installation.
What is the difference between an indicator flasher and a hazard flasher unit?
On many commercial vehicles, separate units control the indicator circuit and the hazard circuit. On others, a combination unit handles both. Check the vehicle's wiring diagram to confirm which units are fitted before ordering a replacement.
Can I fit an electronic flasher unit in place of a thermal unit?
Yes, in most cases electronic flasher units are a direct-fit replacement for thermal units of the same voltage and pin configuration. Electronic units are more reliable, compatible with LED lighting, and maintain a consistent flash rate regardless of load. Confirm the pin configuration matches before fitting.
