• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Car LED rear light circuit bulbs

Bigkris06

New Member
Hi guys.
I didn't know where else to turn!
I have tried the BMW forums with no success but now it's time to ask the experts.
My daughter has a BMW X3 which she has spend every penny of her saving on only to discover now it's getting darker at night that the rear lights are only partially working and lowland behold... The seller can't be contacted!
I have managed to strip the sealed units of the rear lights down to find that some if the leds on the board are not working (both rear lights have the same issue)
We are getting power to the two first led's on the side, then it runs to the next three which don't work. When I cross the two legs of the next led the final two work so I'm assuming that one of the led's have blown.
I have removed the led I suspect has blown but no way of identifying it and don't understand the formulas on how to work out what we need.
I have taken a few pictures and am really hoping that someone may be able to help is out as to replace the rear lights on this car would be nearly £300 which she can't afford.
I would appreciate any help on this, I am confident with soldering etc but I am purely a remove on and replace with another kinda guy
I would also like to thank in advance any help any person may be able to give.
Thanks again.
 

Attachments

Diver300

Well-Known Member
Most Helpful Member
When you say you "cross the two legs", do you mean that you connect them together?

If so, I think that you may be correct that the LED itself it blown.

Have you got a multimeter that reads current? I guess that is one that can be seen in the first picture. Can you use the multimeter, set to read current, to link the two legs of the LED and make the others light? When you do that, what is the current?

The choice of LED isn't critical. You need the right colour, but most likely any red LED will do. BWM will match the colours very carefully, to make sure that it is exactly the correct shade in the showroom. If it looks red to the drivers behind, and you are happy with it, that's all that's needed.

You need an LED that fits in place. I think that is a 3mm LED. You measure the diameter on main part, not the wider flange. If you can fit a larger LED, that's fine. The individual LEDs can't be seen when the car is in use, so it doesn't matter if they aren't the same size as the original. Obviously BMW will have one size for mass production.

You need an LED that can take the current that the circuit provides, or more. If you measure 35 mA, then any LED that is rated at 35 mA or more is fine. A 50 mA or 100 mA LED will be work. LEDs will degrade quickly if driven at too much current, but they are actually more efficient at lower currents than their rating. There is only a problem at tiny current, like less than 1% of their rating, but that won't happen for you as you don't have enough space for an LED with a far larger rating than needed.

You don't need to worry about the working voltage of the LED. It will be around 2 V for a red LED, and the control circuit will take care of any variation.

It's a good idea to get an LED with a similar viewing angle to the ones you have. Just look and see how close to straight on you have to have the axix of the LEDs to your eye to be bright. Search for "led viewing angle" if you're not sure, but it's not an exact measure, and again you can be less fussy than BMW will be, and if the rear light looks OK, that's all that's needed. You do need the rear lights of the car to be a similar brightness when the viewer is off to one side of the rear of the car.

You need to fit the LED the correct way round. The circuit has two LEDs in series, the working ones, with one driver circuit and three LEDs in series with a separate driver circuit. You can see the top of the pins, inside the clear plastic, and the larger one of those is negative. You can test the polarity with the multimeter on continuity or a low Ohms setting. Red LEDs usually light up dimly when tested the right way round with a multimeter.

I guess that the LED that blew was the top one, that ran hottest. Getting larger LEDs will help stop that happening again. You might change all the LEDs, as the others may be degraded.

It's a good idea to compare the brightness of the new LEDs with old at some point. LEDs do vary in efficiency, and if you get it badly wrong, the new LEDs could be too dim, or less likely, too bright. You don't want the car tail lights to be nearly as bright as the brake lights.

You can get LEDs from Farnell, RS, Mouser or Digikey. They can be searched by diameter, colour, viewing angle etc.

It's quite likely that other BMW lights use the same LEDs, so you could salvage from them. You could just salvage LEDs from both sets of lights to get one working light and then buy a new light for the other side. I did something similar with a BMW Mini, where the springs on the door check strap rust out, but only on the top where the water can't drain. I bought a new check strap for one side, and used the bottom spring to replace the top on the other side.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, basically look in RS etc. for LED's of a similar shape and size - I repaired a Mercedes third brake light a number of years ago - I had to replace quite a few SM LED's on it, but as they came in a roll of 50 it wasn't a problem. I seem to recall a new light from Mercedes was £300?, and it was only a couple of pounds for 50 LED's.
 

Bigkris06

New Member
Hi, thank you for your replies, that is exactly the information I require.
OK, so I'm still clueless as to what Led I should buy, I'm not particularly fussed about them matching as long as they are bright enough as I intend on replacing all three anyway.
Yes, when I connect them together, the other two light up.
I do have a multi meter but not 100% sure where i need to measure and the best setting to use.
On the actual board I am getting 8.9v to the required led but less then 2v to another working led on the same board but if it has two driver circuits on the same board that may explain that.
Would I need to take a reading from one of the working led's AMD if so what would be the best setting to use.
I have attached a picture of my meter below.
Please excuse my lack of knowledge, I am able to follow instructions but in honesty the maths behind it all I struggle with.
Many many thanks in advancebfor everybody's help, it really is appreciated.
 

Attachments

Bigkris06

New Member
Another thing, I don't think the angle etc matter to much as they fit in three holes which power a reflective strip so it looks more like three rails of light rather than the three leds being the light sort.
Many thanks again.
 

alec_t

Well-Known Member
Most Helpful Member
We are getting power to the two first led's on the side, then it runs to the next three which don't work.
Are you sure all 5 LEDs are supposed to come on at the same time? Could 2 LEDs be for the continuous rear light and the other 3 for the intermittent brake light?
How many wires go to the light module?
 

Bigkris06

New Member
Thank you for your reply.
I'm sure they are all supposed to come on together as I have looked at another working one (on a neighbours car) it has two wires going to it and the brake light is a standard 12v bulb.
 

Diver300

Well-Known Member
Most Helpful Member
I do have a multi meter but not 100% sure where i need to measure and the best setting to use.
Move the red lead of the multimeter to the socket marked mA. Turn the knob to the 200 mA setting. Then connect the two leads across the failed LED with the assembly powered up. The reading may be negative, it doesn't matter. Tell us the current reading.
 

Diver300

Well-Known Member
Most Helpful Member
On the actual board I am getting 8.9v to the required led but less then 2v to another working led on the same board but if it has two driver circuits on the same board that may explain that.
Would I need to take a reading from one of the working led's AMD if so what would be the best setting to use.
The voltage of 8.9 V across the failed LED isn't important. I would expect more than 2 V, but depending on the exact circuit, it could be a lot more than 2 V or a little more than 2 V.
There is no need to take a reading from one of the working LEDs. The three are in series, so the current through all of them is the same, and it is easiest to take the reading across the failed one.
 

Bigkris06

New Member
Would this be the correct setting and would I be able to put the probe in the holes where I removed the led from or do I have to solder the blown one back in?
Many thanks for your help up to now. It is much appreciated
 

Attachments

JimB

Super Moderator
Most Helpful Member
does it matter which was around the terminals go for the measurements?
No, if the meter is connected the wrong way round, the displayed numbers will just be prefixed by a minus sign.

JimB
 

Bigkris06

New Member
I think I may have caused some damage to the board. Now none of the leds work at all and I am getting no readings from the meter when I try to test the points. I know I am getting the appropriate voltage to the board but nothing to any of the leds.
I am going to try again on the other side and will post results here soon.
Thanks guys
 

audioguru

Well-Known Member
Most Helpful Member
I think that crossing the wires of the blown LED or replacing it with the current meter increased the current so much that the "good" LED also is blown or the driver IC is blown now.
The pcb has two red things and three possible LEDs in white holders but on an angle. Which one is blown?
 

Bigkris06

New Member
Sorry, I didn't use any power to cross the wires. I just used a single probe to make the connection. When I check the continuity of the led's there is no continuity so I think I have blown them all on that side.
Moving over to the other side which is exactly the same but Iay have the meter set up incorrectly or my meter may be broken as when intro to measure the leds nothing happens on the meter.
 

Diver300

Well-Known Member
Most Helpful Member
The multimeter is fused, so if too much current flows, the fuse will blow.

If you connect the multimeter, when set for mA, across a working LED, and the LED continues to work, the fuse is probably blown. If the fuse is OK, the LED will go out, and the multimeter will show the current in the remaining LED(s).
 

alec_t

Well-Known Member
Most Helpful Member
When I check the continuity of the led's
How did you do that check? Depending on the DMM it may not provide enough voltage on its continuity setting to make a LED conduct.
 

Latest threads

EE World Online Articles

Loading
Top