BMW R1200RT/R1250RT Forum

Strat Tuner wrote: ↑Tue Dec 24, 2024 10:28 pmThe Brake light Flashing chip I like most won't work with the BMW system.

Skene Lights may provide a solution, https://skenelights.com/

Compatible, https://skenelights.com/compatible.html

after thoughts...

All the bulbs are now Canbus error free LED...... except one!

There are two brake lights in the rear. One of them is LED, and the other is incandescent!

That's the ONLY combination that is stable and keeps the yellow warning light OFF ALL THE TIME.

putting a bulb in, turning the key on and watching for a canbus error is NOT a reliable testing method.
I've been making two/three mile test rides with different bulbs, and the CanBus yellow caution sign goes OFF and ON.... during the test ride even though there's no error at startup.

Is it worth doing?

I would say yes, as the headlights were very weak before. Adequate, I suppose, but not if you've ever been on a bike with LED headlights.

I hope all this will help someone.

I realize that no one wants to hear, but I continue on...
--There are two bulbs in the rear. I tried putting a resistor on ONE of them with an LED, but the intermittent yellow warning persisted.
-- I presume the single brake wire under the seat controls both.
-- If I put the resistor there, it would be resisting for both bulbs. Seems to me that both bulbs should then be RED LED with now built in resistor?
-- I will FAFO that eventually, but I'ma hoping someone who knows more about electricity will offer advice... Please?

Current = Power / Voltage

For a 21W bulb at 12V, the current would be 1.75A.

Presumably, the LED bulb needs to draw the same current as the incandescent bulb to keep the Can-bus system happy.

Volts / Amps = Resistance (ohms)

So, the LED bulb and/resistor needs to be 12V / 1.75A = 6.86 ohms.

Thank you... Soo.....
Use two resistor for the wire that feeds the two bulbs or just one?

My understanding is that the two rear tail / brake light bulbs are wired in parallel.

Therefore, IMO, the idea would be to match the resistance of each incandescent (filament) bulb with a LED plus (if required) a resistor in each (x2).

I'm sure you're correct.

I'm also sure I don't understand that.

Maybe you mean put a resistor as close as possible to each bulb.??

Right now, the most stable configuration is:
.........a) one LED stop bulb (red) and
.........b) one incandescent, original, bulb (white)

If that's what works, then I'll go with that.

I've given up on getting the brake light to flash on braking. That's just asking too much !

I'm sure BMW is easier to work with in more modern models.

Strat Tuner wrote: ↑Tue Dec 31, 2024 2:21 amMaybe you mean put a resistor as close as possible to each bulb.

In the above video, the resistor if fitted in parallel close to the bulb holder.

When two resistors (bulbs) of the same resistance are wired in parallel, the equivalent resistance is half the value of one of the two resistors.

Using the two 6.86 ohm bulbs as an example, the equivalent resistance is 3.43 ohms.

1/Rt = 1/R1 + 1/R2,

1/6.86 + 1/6.86 = 2/6.86,

So, Rt/1 = 6.86/2 = 3.43 ohms.

This is what the CAN-bus system will see for the tail/brake light circuit.

Perhaps when one filament bulb and one LED (of unknown resistance) are fitted, the equivalent resistance is satisfactory.
However, two LEDs of less resistance than the filament bulbs won't work. Hence the need to fit additional resistors with the LED bulbs.

David. wrote: ↑Tue Dec 31, 2024 7:19 am When two resistors (bulbs) of the same resistance are wired in parallel, the equivalent resistance is half the value of one of the two resistors.

Using the two 6.86 ohm bulbs as an example, the equivalent resistance is 3.43 ohms.

1/Rt = 1/R1 + 1/R2,

1/6.86 + 1/6.86 = 2/6.86,

So, Rt/1 = 6.86/2 = 3.43 ohms.

However, two LEDs of less resistance than the filament bulbs won't work. Hence the need to fit additional resistors with the LED bulbs.

Math and I are lifelong enemies and we are barely on speaking terms. I confess I don't understand your answer much at all... Shouldn't two bulbs, wired in sequence, give DOUBLE the resistance of ONE bulb?

The bulbs I'm using are the "Canbus erorr free" variety that come with the resistor built in to the bulb. (whatever that may mean)

If they work as advertised, then both of the LEDs together should resist the SAME as two filament bulbs.... but they don't.

Canbus is happy with one filament and one LED (resistor built in).

Put in two Filament: works great
put in two LED (with resistors): FAIL

My next plan is to put in two LED (with Resistors) and put one physical resistor on the wire as close as I can get it to the LED bulb.

If that doesn't work, I could move the resistor to the filament bulb.

If that doesn't work, maybe a resistor for each would be next?

Happily, there are a finite number of combinaitons.

You can see I'm just trying every combination looking for one that works. Perhaps the numbers you gave suggest the right combination..., and I'm sure they do, but I'm not smart enough to see it.

I'm guessing that I will end up with the stable combination I have right now when I've done all the swapping: ONE filament, ONE LED (resistor built in), and NO separate resistors

Right now it's STABLE, so I'm reluctant to keep trying new combinations.

Still, I am grateful for your answers. Someone will make sense of them, and, for them, the light will go on! (-:

...

Strat Tuner wrote: ↑Wed Jan 01, 2025 10:28 pmShouldn't two bulbs, wired in sequence, give double the resistance of one bulb.

If the two bulbs are wired in series (sequence), then yes, the resistance will be double that of one bulb.

https://www.bbc.co.uk/bitesize/guides/z ... revision/1

Strat Tuner wrote: ↑Wed Jan 01, 2025 10:28 pmThe bulbs I'm using are the "Canbus erorr free" variety that come with the resistor built in to the bulb.

If they work as advertised, then both of the LEDs together should resist the same as two filament bulbs.... but they don't.

As per the video, with a "super fussy vehicle", sometimes a CAN-bus error free bulb with a resistor built in doesn't cut it and an additional resistor is required to be fitted.

David, you are a trouper. Thank you for continued patience and guidance.

I'll try putting in both LEDs and then add a separate resistor first to one then to both.

Our explorations are not done yet! (-:

Strat Tuner wrote: ↑Tue Dec 31, 2024 2:44 amI've given up on getting the brake light to flash on braking.

It looks like BMW themselves may be able to supply a "plug & play" Supplemental LED Brake Light.

https://forums.bmwmoa.org/threads/suppl ... 0rt.52845/

"As advertised, the LED comes on when the regular tail light comes on, and when the brakes are applied, the LED flashes several times very quickly before becoming even brighter to show the brakes are applied."