Problem with Rear Right Taillight – Possible LCM Issue?
 

Hey guys,

I was hoping someone might have some insight or have had a similar issue to what I am currently seeing with the rear taillight on my X5. I have had the problem before with either a melted plastic housing or a burn through hole in the metal but this is a new one for me. From what I can tell the light housing is working correctly and when bench tested all the lights work but when it is installed in the vehicle the bottom light refuses to light up. There is no corrosion on the connector to the light and we have not had any water issues. Has anyone had their Light Control Module fail in this manner? Is there a possible fray point for the wires? My understanding is that the LCM has to ‘detect’ the light otherwise it will not turn it on and throw a warning but the contacts are clean, the bulb is new and it works on the bench.

I am going to check the continuity of the line going to the taillight from the LCM this week but I wanted to see if anyone had seen this before. I thought it was going to be a quick fix but it looks like it isn’t. Any help would be great, thanks.

2001 X5 5 speed, 200k+ miles and going strong!

you should check where the male and female connector plugs into each other and check for continuity there. if that makes sense.

Are you talking at the LCM or the plug into the light itself?

the plug into the light itself, but ya at the lcm would be good to check too.the only issue I had with my tailights is a burnt spot where the bulb sits, which I just soldered up and been good for over a year now.

If it test o.k.on the bench but doesn't light up when you plug it in, you should verify that you have voltage at the connector. No voltage, trace the wiring back to see if you have voltage at the LKM. You could have a break in the wire.

So I just unplugged the A connector on the LCM and the lights went into safety mode, lighting up all the marker lights EXCEPT the rear right taillight. This makes me thing that the issue is not the LCM but a wiring problem. The investigation continues.....

keep us updated . thx

Finally got some time at home to look into this. The wiring tested ok so I moved onto the LCM and found this. Anyone know where to get an Infineon BTS736L2 chip? Looks like they are out of production.

Can't help with the sourcing the I.C. but assuming you do find one (or replace the entire LKM) the next question is... What caused the excessive current?

How are you at soldering SMDs? ;)

BTS736 BTS736L2 Smart 2 Channel Highside Power Switch Infineon Genuine | eBay

Well, 200k miles of using a crappy tail light design are what I think did it in. Current draw from the LCM to the tail light was not excessive when I tested the wiring. I have had to repair the tail lights a couple of times because of melted plastic and burns/pits in the metal. They really went cheap on these things but BMW is not alone....

I already ordered some chips off ebay, thanks for the link! I spent an extra $20 on shipping to get it here faster. Kinda pissed that I could not find a local source (USA) that could ship it faster. This is the first time Avnet has let me down.

I have done this type of work before, it does not look too bad. I have not had to deal with a waterproof coating before so that should keep things interesting. I will post more pics when I am done and let you guys know how it goes.

Well, I received the replacement chip and soldered it in. Not my best work but the waterproof protective coating was a pain to deal with. I had to do a lot of tip cleaning to make sure the joints were clean. I also damaged the trace at pin 6 so I had to add a wire to make up for it. It is one of the chip grounds.

Unfortunately this didn't 100% repair the issue and I think when this chip died it damaged the chip that controls the setting of the taillight between marker and brake. Right now the repair is working intermittently, about 50% of the time. On several occasions I have heard an electric humming like a transformer by the taillight right before it throws the warning light. This makes me think that there is a high-frequency switching part of the assembly that has been damaged. Is anyone familiar with the basic circuity of the LCM and what part I should look for next?

Well done with the repair. :thumbup:

The "buzzing" is something often you get when the driver circuit is overloaded, or at least working VERY hard. :confused:

You don't have an intermittent short to ground in your loom somewhere near the rear tails? That would fit both the cause of the original chip failure and the new symptoms... :dunno:

Thanks! I though it looked kinda sloppy from the pic and I was waiting for someone to hate on the job I did. Thanks for the positive encouragement!

There might be something going on with the light housing, I have repaired it a couple of times and it looks like there might be some rust/corrosion under where it connects to the wiring harness. Unless someone has a better idea I will try to isolate the light socket and see if that fixes the intermittent issue. Otherwise I will run a temp wire from the LCM to see if that helps.

Since the circuit trace got black and it wasn't just the chip that burnt I would say there must have been a short and still could be at times.

That is a good point. Does anyone know the layout of these boards better or know how the brake/marker light dimming is controlled? I am going to have to do more work to isolate the circuit.

Full disclosure - I don't know how this circuit works - this is guess work.

But, given that the IC driving the lamps doesn't have a heat sink or look to be able to dissipate much excess heat, any dimming of the lamp circuit must be via PWM (yes, same PWM as the control of the aux. fan - although used in a different way in this case). The noise you hear when it's faulty supports this theory. I'm guessing the LCM varies the mark/space ratio of a square wave going to the tail lamp to modulate the brightness. The PWM used for lamp dimming is usually in the 1-2kHz range, right smack in the middle of our hearing range. When your tail lamp assembly is faulting (short or similar to ground etc.) the IC is delivering a high current at 1-2kHz and you can hear this, via the protesting components. :yikes:

I should drag my oscilloscope out into the car park and check the tail lamp circuit- this would confirm the PWM operation. But it's cold and dark outside! :confused:

This fault just HAS to be an intermittent fault path to ground in the lamp assembly somewhere...

I'm assuming that the light circuit is connected to the solder pad at the end of the burnt trace ? These chips must have a really low on resistance, I recently replaced the chip on my rear wiper board and it was very similar to what your working on. The tiny SMD can carry 30 amps, albeit over two pins (just like yours) and had no heat sink! Has one now though ;)

Looking at the picture, is that a burnt resistor in lower RH corner ?

The picture does look like it is damaged but it is just the circuit board coating and the picture quality.

So the chip I installed died a horrible death as you can see from the picture. I agree wpoll and I think what was happening is that when the signal was being pulsed for the tail running lights it was causing repeated arcing beneath where the connector inserts into the tail light housing which would account for what I was hearing and why I could not see any shorts. This would also explain why when I tested the light for shorts I couldn't find one. I installed one of the replacement repair Mercedes light sockets along with another chip and everything seems to be working fine. I will update thread if it breaks again but it has been a few weeks so I think problem solved!

Thank you everyone for your input!

Great follow-up - thanks. :thumbup:

And it's a warning to us all - not attending to ATLS (arcing tail lights syndrome) can result in more than just ruined tail lights - it seems it can actually damage the LCM too. :(

if you post it, they will respond!
brilliant find wpoll! my soldering technique is a bit rusty, (especially for Surface mounted stuff!) hopefully the op gets his x sorted out!