Rear LED - Shunt approach rather than resistors

[user17600]

Hi All,

Just got my 1156 LED bulbs in the post, and got the expected "check brake lamps" message.

Having encountered this before when I put in aftermarket headlamps, I pulled the rear brake shunt and drilled it as with the headlamps. The headlamp drilling worked spectacularly (that is, cleared the message on the DIM).

Unfortunately, that doesn't seem to have done the trick for the brake lamps. I pulled all the other shunts in the rear hatch fuse box, and none of the other affect the rear brake lamps.

Has anyone upgraded the rear brake lights and managed to solve the lamp message with just the shunt approach? Or did it require resistors? If the latter, where would one attach the resistors, since the bulbs are socketed into the rear assembly?

Google and site search were not my friend :-(

Cheers!

PS - the lamps were the DDM Tuning S25-WG-60SMD-1156-R - 1156, 60 x 3528 SMD LED, 4.2W, 420 Lumen, 360 Degree, Red. These are as bright as the regular bulbs and have a nice dispersal pattern. And instant on, almost quicker than the center LEDs!

[Allen]

Interesting. I have a brake light issue, too. Right light is intermittent and not blown. Have tightened the connectors, worked for two weeks. Guess I should have another look.

[coflynn]

Unfortunately, that doesn't seem to have done the trick for the brake lamps. I pulled all the other shunts in the rear hatch fuse box, and none of the other affect the rear brake lamps.

Is the problem that you can't find a shunt that corresponds to the brake light circuit, or that drilling the shunt didn't work?

If drilling the shunt: the issue may be that you need to increase the shunt resistance even more. The rear bulbs are 15W or 20W I think (correct me here)? If so, your LED is drawing 4.2W. The current draw will be ~4x lower than expected. When doing the mod for the headlights you are only trying to change the resistance a little bit, since it's just a slightly lower draw (e.g.: as from http://www.xenonvalot.com/xenonfixvolvo.html ), but instead you need it to change drastically.

[user17600]

Is the problem that you can't find a shunt that corresponds to the brake light circuit, or that drilling the shunt didn't work?

The latter. Actually with the headlamps, the drop was in the same ballpark, since I went from 55 > 35 watts and here I've gone from 21 > 4.2, so before I kept drilling, I figured I'd inquire as to whether anyone had success with the rears via shunt.

I'm game to try to open the hole a bit more, but I'd prefer experience to speculation as to diameter.

Thanks for the reply.

[coflynn]

The latter. Actually with the headlamps, the drop was in the same ballpark, since I went from 55 > 35 watts and here I've gone from 21 > 4.2
The difference is in the same ballpark true, but that doesn't matter. The percentage drop is what matters - with the headlights you are drawing 64% of the original current, with the brake lights you are drawing 20% of the original current.

Obviously they don't too carefully measure the current - the setting will be somewhere below the 'planned' 55W draw, since there is naturally some variation in current draw. Thus for the headlights you only need to increase the resistance a little bit, and you can make the message go away. With the brake lights you'll need to increase the resistance a lot more...

You can test how close you are by either adding resistors to waste extra current, or connect two of the LED bulbs in parallel (e.g.: just with some wire) to increase the draw on the circuit.

[user17600]

Ah, no joy. The hole is about twice the size as on the headlamps, and two LEDs don't clear the message.

For those that went the resistor route, where did you place them, since the bulbs are socketed in the assembly? Presumably in the wiring down towards the fuse panel, but where specifically?

Thanks in advance.