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Motorcycle tail light set up - LED


New Member

I'm trying to build a good functioning taillight system for the custom bike I'm building, based on a geniusly stupid idea I've had

Basically here is the set up :

I'm using 4 identical sealed LED tail light light units.

3 wires : brake/running light/earth

My idea to use 4 of them is for looks only, because 1 is obviously enough.

But the plan is :

Mounting the 4 lights vertically side by side :

- the 4 units light for the running lights
- the 2 center units also do the brakes
- the 2 outer units do the left and right indicator

So using a multi plug i wired all this together to what i thought was good :

- I joined the 4 grounds together and linked that to the bike earth
- I joined the 4 running lights together and to the bike running light wire
- i joined the center units brake wires to the brake wire
- i joined the outer unit brake wire to each of the indicator wires.

Now for the test :

Running lights on, the 4 go on

Running lights on or off, the center units flash when brakes are applied

Running lights OFF, the indicators each work great, at normal legal flash rythm


With the running lights on, the flashing goes crazy when i put the indicators on...

Any idea what is causing this and how I could solve it ?

Thanks !!!
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Well-Known Member
I think that this is what I have come across before, where the light assemblies share the same LEDs for brake and running lights, and just run them with more current for braking. The issue is that there is a single electronics module, and that runs from whichever of the input is at the highest voltage. The module senses the voltage of the brake light wire, and if that is high enough, the module runs the lights at high brightness. Light assemblies that can run on 12 V or 24 V are more likely to be made like this.

Here is an example of a light fitting that does that:- https://www.ringautomotive.com/en/product/RCV4800 It uses the same LEDs for brake and tail lights. The other lights are separate.

The problem comes when the brake light voltage is high, but slightly less than the running light voltage, then just about no current will be taken from the brake light supply. Then your indicator flasher relay will not sense any current to the rear lights and will flash fast to indicate a blown bulb.

On those lights, this is what happens:-
Running light wire 12 V, brake light wire 0 V. Light dim, low current taken from running light supply.
Running light wire 0 V, brake light wire 12 V. Light bright, larger current taken from brake light supply.
Running light wire 11 V, brake light wire 12 V. Light bright, larger current taken from brake light supply, no current taken from running light wire
Running light wire 12 V, brake light wire 11 V. Light bright, larger current taken from running light supply, very little current taken from brake light wire.
If the wires are both the same voltage, the current could come from either wire.

On your motorbike, I think that the flasher unit may be causing a small voltage drop, so that the brake light wire on the lights running as indicators are running from a slightly lower voltage, and all the current come from the running light supply.

If I'm right, you can put a diode in series with the running light feed to the rear lights, in the direction that keeps them working. Then the voltage to the running lights will be a bit less, and the larger current will come from the indicator supply, as it does now when the running lights are off, and the flasher unit will detect the current and work normally.


New Member
Aaah that is very interesting and would explain a lot. Indeed i never figured that it was the same LEDs running more or less current.

What type of resistor could i test ? Would this be of any help ?

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Well-Known Member
If I'm right about the cause, you just need to get the current to flow from the flasher unit rather than the running light supply.

A diode is good because the voltage drop is about 0.7 V, and does not vary much if the current varies. You could use a resistor, but you need to know what the current is and get the calculation correct. That page is calculating a current limiting resistor. You don't need that because the light assembly has already got the current limiting components.

I would suggest a 1N5401 diode in series with the running lights. If you get it the wrong way round, the running lights won't work.

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