Rear LED brake lamps - Shunt approach rather than resistors

[user17600]

Hi all. Asked the below on another forum, got few replies, but figured I would check here too. Model is 2004 V70 AWD.

Here's the issue: Just got my 1156 LED bulbs in the post, and got the expected "check brake lamps" message. (DDM Tuning S25-WG-60SMD-1156-R - 1156, 60 x 3528 SMD LED, 4.2W, 420 Lumen, 360 Degree, Red)

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 shunts in the rear hatch fuse box, and only one affects the rear brake lamps. This is the one I drilled.

The hole is about twice the size as on the headlamps, and two LEDs in series (twice the load) don't clear the message (suggested in the other forum). I don't want to keep drilling and destroy the shunt.

Has anyone upgraded the rear brake lights and managed to solve the lamp message with just the shunt approach? Or did it require resistors?

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.

Cheers!

PS - These are as bright as the regular bulbs and have a nice dispersal pattern. And instant on, almost quicker than the center LEDs! The webpage says 420 lumen, but the receipt says 240, which seems more likely given the brightness (equiv to 20W).

[draser]

Well, basically what you're doing with drilling the shunt is to increase its resistance so the lower current of the LED bulb will still trigger the current sensing device that sets the fault. The correct way to do this is to actually measure the shunt with a digital ohm meter. Then you can calculate the current - that's the bulb wattage devided by 14.4V baterry voltage with engine running. Now you know the current required by the LED bulb (from the bulb specs), the current required by the old incandescent bulb, and the shunt ohms. You can calculate the ohms required for the new/modified shunt: Ohms New Shunt = Ohms Old Shunt x (LED Current/Bulb Current). You then, can modify the shunt by cutting alternating slots in it with a hacksaw - which is more efficient and better controlled than drilling. If the current difference between old/new bulb is too big, then you may not be able to modify the shunt enough to make it work - so you need to use resistors. You'll need to add whatever the formula above shows for the new required shunt. Need to consider the resistor wattage as well, based on the current the LED needs, otherwise the new added resistor will burn out. The way to do that is to add the voltage drop on the shunt and LED bulb itself, and subtract from the 14.4V running voltage. That will be what the resistor will have on it. Then divide that by the LED current and you get the required resistor wattage.
Another trick that may work - depending on what kind of sensing device Volvo uses - is to add small wattage resistors this time to the sensing circuit wires (the small gage wire that goes from shunt to the current sensor), based on the same formula above. If they use something like a transistor circuit to sense the voltage drop on the shunt, by adding a resistor you'll have enough voltage to emulate the old incandescent bulb. However, if they look for current it will not work because the current will be the same. Worth a shot though, since it's a quick test.