Problem with LED Rear Lights on BMW

padraigmyers

Bought an e46 BMW about 2 months ago. It came with aftermarket LED rear lights installed in it. For the first few weeks the indicator on the dash for rear light failure came on for a few minutes every so often (2 or 3 times a week). But lately it has been staying on pretty much all the time. The lights are fine, nothing is blown. There is a unit installed behind each light cluster (in the boot) which, when disconnected, causes the indicator lights to blink very quickly, is it possible that one of these units needs to be replaced (maybe I'm not getting 12v across the test points that the dash indicator is using).

Any idea what this might be and how I can check it?

Its only a small thing but its just annoying to have it on all the time, thanks to anyone who can help.

MercMad

The car uses a CANBus wiring system and it has registered a fault because the current being drawn by the LED's are much less than the system expects with regular bulbs.

There is nothing you can do except change it back to standard. BMW do an LED lamp set also so maybe its possible to programme the upgrade but you would need a very freindly BMW dealer for that..............and there aren't too many of them !!

padraigmyers

MercMad wrote:

The car uses a CANBus wiring system and it has registered a fault because the current being drawn by the LED's are much less than the system expects with regular bulbs.

There is nothing you can do except change it back to standard. BMW do an LED lamp set also so maybe its possible to programme the upgrade but you would need a very freindly BMW dealer for that..............and there aren't too many of them !!

But why would this have only stated happening lately? If that was the problem it would not be intermitent. There are 2 aftermarket resistors (6 Ohm, 50W) wired into the light circuit, perhaps they were controling the current. I'll have a look and see if one of these are broken, must find a multimeter now! Is it definetly the current that the CANBus checks or is it the voltage? It its the voltage then the resistor makes sense, increase the resistance becasuse the current is low should fix the voltage.

kdevitt

What year is your car?

Alun

I'm pretty sure it's the current that's measured, as the resistors are wired in parallel with the LED lights. They can be prone to failure though as they can get pretty hot, which is why they're usually encased in a heatsink. There are a number of websites that do these kinds of bulb sets that do replacement resistors.

http://www.eautoworks.com/html/ord-Light~bulbs-Signal,Corner,Tail~and~accessories~bulbs-1-33226.htm

MercMad

CANBus is actually a signalling system. The consumers require voltage and current obviously, which is sent but its actually worked by the control sending a signal to a specific unit to "operate now". This is how you can have 3 cables operating dozens of components instead of seperate wires for each item !

The resistor though could be causing the problems though as the lads describe !

padraigmyers

Alun wrote:

I'm pretty sure it's the current that's measured, as the resistors are wired in parallel with the LED lights. They can be prone to failure though as they can get pretty hot, which is why they're usually encased in a heatsink. There are a number of websites that do these kinds of bulb sets that do replacement resistors.

http://www.eautoworks.com/html/ord-Light~bulbs-Signal,Corner,Tail~and~accessories~bulbs-1-33226.htm

Thats the exact same one as are wired into the LED, looks like thats all that needs to be replaced so. I did notice as well, that as the weather has got hotter the indicator on the dash came on more frequently. All points towards one of those resistors failing then.

Thanks for all the help guys, I'll let ye know if I get it sorted.

Alun

If you've got access to a multimeter you should be able to tell pretty quickly whether it's the resistor that's gone. Possibly, just possibly, if it's weather / temperature dependent it could be something as simple as a dodgy connector or solder joint somewhere where the resistor is wired in. Might be worth wiggling the connections a bit to see before shelling out for a new resistor.

padraigmyers

Alun wrote:

If you've got access to a multimeter you should be able to tell pretty quickly whether it's the resistor that's gone. Possibly, just possibly, if it's weather / temperature dependent it could be something as simple as a dodgy connector or solder joint somewhere where the resistor is wired in. Might be worth wiggling the connections a bit to see before shelling out for a new resistor.

Wiggling the wires about was the first thing I tried becasue I figured it was just a dodgy connection. Yeah, I have a multimeter, but the resistance across both seams fine, 20.1 across one 6.2 across the other. Will a failed resistor be showing infinite resistance?

Alun

padraigmyers wrote:

Wiggling the wires about was the first thing I tried becasue I figured it was just a dodgy connection. Yeah, I have a multimeter, but the resistance across both seams fine, 20.1 across one 6.2 across the other. Will a failed resistor be showing infinite resistance?

Probably, yes.

david powell

I'd say some has fitted the resistors to fool the electronic module driving the lamp cluster that the correct blub is fitted...

The electronic module will usually monitor for the following things:-
1) short to Vbat
2) short to Ground
3) bulb of correct value fitted / present ( usually done with bulb off )
4) over current when driving the bulb.

I'm not sure of the exact configuration, but if there is a 6 Omh resistor across the LED lamp cluster:-
I = 13.5 / 6 = 2.25A
P = 2.25 * 2.25 * 6 = 30W!!

These resistors will need alot of heat sinking, ie need to be bolted to something! I would say a low value of resistance ( 6ohms ) was used, as this would be similar to the resistance of a blub when it's in the off state, it's resistance will increase as soon as the blub tunrs on fully...

LED lamp clusters are very hard to do diagnostics on, as the LEDs can be interconnected in a number of different ways.

Hope this is of some help...