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How to determine the correct resistance when installing LED turn signals
If we replace motorcycle indicators with LED indicators, there is a real chance that the flashing frequency will change. In this blog, we explain why this happens and how you can fix it.
Change in load affects the functioning of the flasher relay
Standard halogen indicators consume significantly more energy than LED indicators. When replacing halogen indicators with LEDs, the load on the electrical circuit decreases, causing the turn signals to flash faster.
The legal blinking frequency is 90 blinks per minute, with a tolerance of 30 blinks (faster or slower). There are two ways to correct this deviation:
Replace the relay with a LED-compatible model
One of the simplest solutions is replacing the original flasher relay with a new relay suitable for LED indicators. This solves the problem of an incorrect flashing speed without adding extra components.
Advantages of an LED relay:
- It eliminates the need to install a resistor
- No heat issues, unlike resistors
- Easy installation
Add the correct resistance to match the original power
If it is not possible to replace the relay, you can add a resistor. This ensures that the circuit is sufficiently loaded, keeping the flashing frequency within legal limits.
Attention: Always mount resistors in parallel, never in series.
Resistors can get very hot (up to 100°C). Therefore, ensure sufficient cooling and do not place them near plastic or other heat-sensitive parts. We recommend placing them on an aluminum plate and giving this a spot in the riding wind, for example near the control unit.
How do you calculate the correct resistance?
Determine the voltage level: Modern motorcycles usually have a 12V system, while older engines sometimes use a 6V system.
Determine the power of the original indicators: You can find this power (in watts) on the lamp itself or in the motorcycle’s manual.
Determine the power of the LED lights: Consult the product description on our product page or the manufacturer’s manual. On average, LED turn signals consume between 0.5W and 3W.
Calculate the power difference: Subtract the power of the LED indicators from that of the original lights to determine the power difference.
Calculate the required resistance: Use the following formula for this:
Whereby:
- R= resistance in ohm
- U = voltage in volt
- P = the difference in power (W)
Example: Replacing a 21W turn signal with a 2W LED turn signal on a 12V circuit
Step 1: Calculate the difference in power
Step 2: Calculate the required resistance
Use the formula:
Where U=12 V
Step 3: Choose a resistance
A resistance of 7.5 ohm with a power of 25 watts is suitable in this case. You will never be able to find the perfect resistance, always choose a value that is as close as possible to the calculated value. It is for this reason that the deviation in blink frequency is permitted.
Common turn signal combinations and resistors
The table below provides an overview of the most common configurations and the required resistors to normalize the flashing frequency. For LED indicators, we calculate 2W in this table. That’s about the average when we compare all available LED indicators at RAD (0.5W – 3W).
| Original bulb | Converted to | Required resistance(s) | |
| 4 x 21W | => | 4 x 2W LED | 4 x 7,5 Ohm |
| 4 x 21W | => | 2 x 21W / 2 x 2W LED | 2 x 7,5 Ohm |
| 4 x 21W | => | 2 x 21W / 2 x 6W | 2 x 10 Ohm |
| 4 x 21W | => | 4 x 6W | 4 x 10 Ohm |
| 4 x 21W | => | 2 x 6W / 2 x 2W LED | 2 x 10 Ohm / 2 x 7,5 Ohm |
| 4 x 21W | => | 4 x 10W | 2 x 7,5 Ohm |
| 4 x 21W | => | 2 x 21W / 2 x 10W | 2 x 10 Ohm |
| 4 x 21W | => | 2 x 6W / 2 x 10W | 4 x 10 Ohm |
| 4 x 21W | => | 2 x 10W / 2 x 2W LED | 2 x 10 Ohm / 2 x 7,5 Ohm |
| 4 x 18W | => | 4 x LED | 4 x 8,2 Ohm |
| 4 x 18W | => | 2 x 18W / 2 x 2W LED | 2 x 8,2 Ohm |
| 4 x 18W | => | 4 x 6W | 2 x 7,5 Ohm |
| 4 x 18W | => | 2 x 6W / 2 x 2W LED | 4 x 10 Ohm |
| 4 x 18W | => | 4 x 10W | 2 x 8,2 Ohm |
| 4 x 18W | => | 2 x 18W / 2 x 10W | 2x 18 Ohm |
| 4 x 18W | => | 2 x 6W / 2 x 10W | 2x 7,5 Ohm |
| 4 x 18W | => | 2 x 10W / 2 x 2W LED | 2 x 18 Ohm / 2 x 8,2 Ohm |
| 4 x 18W | => | 2 x 18W / 2 x 6W | 2 x 10 Ohm |
| 4 x 10W | => | 4 x 2W LED | 2 x 7,5 Ohm |
| 4 x 10W | => | 2 x 10W / 2 x 2W LED | 2 x 16 Ohm |
| 4 x 10W | => | 2 x 10W / 2 x 6W | 2 x 36 Ohm |
| 4 x 10W | => | 2 x 6W / 2 x 2W LED | 2 x 10 Ohm |
| 4 x 10W | => | 4 x 6W | 2 x 18 Ohm |
Pitfalls when installing new turn signals
CE marking: If you want to customize an older motorcycle, which still has to go through the motorcycle inspection. Make sure your turn signals are street legal. Front turn signals must have a CE11 label, while rear lights must be equipped with CE12. Most turn signals are labeled with both.
Minimum distance: At the front, the distance between the indicators must be at least 24 cm, and at the rear at least 18 cm. Use turn signal extenders if necessary.
Shims: With older motorcycles, new, smaller indicators often cannot be directly attached to the existing mounting points. Use shims to solve this problem.
Get started and prepare your motorcycle for the road!
Replacing the relay with an LED-compatible model is the easiest solution as it generates less heat and is reliable. If this is not possible, opt for installing a resistor. Now that you know how to calculate the resistance, you can get started. If you doubt your own abilities, it is wise to call in an experienced mechanic.
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Nathan is our webshop employee who is ready with advice and assistance to help customers. When he is not answering customer questions, he writes helpful texts and blogs for the RAD site.