acuteperformance

Hi Everyone

I've been doing a ton of research since this afternoon basically brushing up on my high school physics and learning about BMW's use of the CAN Bus and how it affects our lighting. Essentially I wanted to answer the question "Why the heck when I change my bulbs to LED or Xenon does the damn error code come on?!"

What is the CAN Bus?
According to Wikipedia: "Controller-area network (CAN or CAN-bus) is a computer network protocol and bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer." So what the heck does that mean exactly? Basically through this system, varies signals are sent through the cars electrical system which is perfect for detecting the health of accessories attached to the car. In our case, the lighting system.'

Okay So Why Does Installing Xenon Fog Lights Trigger an Error Code?
This requires a little background info on the differences between Xenon bulbs and Halogen bulbs. When a halogen bulb is turned on, the bulb draws high current because the filament is cold and hence has low resistance. Within 100ms, the current decreases and eventually stabilizes at about 4.5 amps. If everything works fine, CAN is happy and doesn't throw an error code. Now with a Xenon bulb the ballast does all the grunt work in lighting the bulb. However, the characteristics of the ballast and how it lights the bulb is different. Most ballasts do not immediately draw current after switched on. After a short while the current consumption looks quite unpredictable and eventually the system stabilizes around 3 Amps. However in many cases CAN has already decided that something is wrong and disconnects the power.

Another issue is that ballasts, by design, have a lot more electrical noise than a halogen bulb. Because CAN sees this noise it will throw an error code.

So How Do You Prevent the Error Code When Switching from Halogen to Xenon?
You throw a capacitor (aka Canceler Device) in the mix. The canceler is actually a very simple device that uses a capacitor to remove the electrical noise. This is where I had to brush up on my HS physics. A capacitor is kind of like a battery in that it stores energy. However, unlike a battery, the capacitor doesn't have to generate energy and it can release stored energy very quickly. For those who've dealt with car audio equipment, it's often recommended to get a farad cap (ie capacitor) to reduce noise and maintain a clean current.

So basically, by throwing in a canceler device (ie capacitor) it's tricking the computer into thinking it has the same noise pattern as a halogen bulb. That's it, nothing more and nothing less.

But You Talked About Resistance Earlier
Just like Xenon bulbs, switching a halogen bulb to an LED bulb causes the computer to throw a code as well. But the code isn't due to noise in the current but the amount of voltage an LED draws a regular bulb. (LEDs draw considerably less than a regular bulb). That is why if you looked at fixes for LED bulbs most say to wire a resister in parallel and the computer will no longer throw an error code.

So What About The Rear LED Tail Lights?
THIS IS ONLY SPECULATION. I COULD BE WRONG, I'M ONLY MAKING AN ASSUMPTION. I'm guessing that those who do the taillight conversions to LED get an error code because (1) it either requires a resistor to lower the voltage or (2) requires a capacitor to help get rid of the electrical noise. Unfortunately I don't have LED taillights to test with so I can't know for sure.

Michael

Brit_in_NJ

Good post, just wanted to correct a couple of things.

The LED or bulb doesn't generate voltage but it does cause a current draw. You might have been thinking voltage drop. An LED draws much less current than a regular bulb so a resistor is placed in parallel to the LED to create a larger current draw, that prevents the error codes.

A single LED usually only has a 1.7v voltage drop so to connect it to a 12volt supply you would need a series resistor however most licence plate LEDs contain multiple LEDs and have a combined voltage drop of 12v across the package.

acuteperformance

Thanks for the correction. I'll update the original post.

Brit_in_NJ

no prob. but I just updated my correction!

acuteperformance

So why do you think this happens? The LED trunk lid lights that have seem to throw the most error codes w/ 08 LCI, any idea why?

Michael

Brit_in_NJ

I have Umnitza LEDs on my '05 and without resistors they give error codes some of the time, I can ignore the error codes for now until I can think of a more elegant solution since I don't want resistors in the circuit getting hot if I can avoid it.

Don't the LCIs have LEDs already? I have a 328i loaner with about 1300 miles on it that has OEM leds I just assumed that the LCIs were the same. If not are the error codes on the '08 LCI caused by LEDs used with resistors or without?

I think on the older cars it was purely a case of putting in a resistor that created a curent draw over a threashold. I think the 08 might be more picky and check for too much current as well as too little. Also what might be happening is as the resistors get hotter the resistance increases which in turn drops the current drawn...so triggering an error.

I'm no expert on car electronics but I have a degree in electronic engineering...that I don't use much anymore.