Disabling light bulb failure error by modding ECU firmware?

[WeekendDriver]

As any LED enthusiast knows, replacing some of the light bulbs with LED counterparts is a pain on our B6s due to "bulb failure" errors that aren't easy to circumvent (esp. if you, like me, don't consider load resistor to be a good option). So I've been searching the Net for better solutions, and there appears to be a hardware mod for the electric switch unit (not sure about the specific unit name, but it's a separate box that contains solid-state switches for various bulb lines). Sadly, I didn't save the link and can't find it ATM, but it was something like cutting one lead on the PCB and soldering it to ground.
But recently I've been told that it's also possible to prevent any specific error from displaying through editing the ECU firmware. So, the way I see it, that's doesn't actually disable the bulb integrity check routine, but it prevents the error from being displayed on the screen.

Can someone confirm it's possible? If so, does someone has an instruction on how to properly edit the firmware to that end?

[Charles.waite]

Pretty sure the Bulb out stuff is on the cluster, not the ECU. And the cluster is generally even more locked down than the ECU, at least as far as changing things (because of odometer tampering, theft, etc)...

[maccalo]

If you disable DIS (mpg, trip etc.) you will be left with only miles to empty and no bulb-out warning. I know as I did the other way around to enable the features.

[WeekendDriver]

Pretty sure the Bulb out stuff is on the cluster, not the ECU.

Makes sense. I had a feeling programmatic solution (as opposed to hardware mods) is too good to be true...

- - - Updated - - -

If you disable DIS (mpg, trip etc.) you will be left with only miles to empty and no bulb-out warning. I know as I did the other way around to enable the features.

Certainly not worth it to lose all the functions, inc. other, useful warnings, to disable the bulb warning.

[ADCS]

Here is a link for the B5. I imagine you could do something similar on the B6 if you can decipher the CCEM. The B6 system is quite a bit more complex than the B5.

http://www.audizine.com/forum/showth...control-module

[WeekendDriver]

Yes, that's it, except the guides for B5 do indeed stress that these guides are not directly applicable to B6. But I did see one for B6, just can't find it. Pretty sure it was on the UK forum http://www.audi-sport.net

[diagnosticator]

The lamp failure warning/bulb check functions are provided by the Central Electric Control Module, that monitors the exterior lighting circuits. When a lamp fault occurs, the CECM sends the fault data via CAN bus to the IC for display on the DIS.

LED lamps that are rated for 12 volts DC are equipped with internal resistors to increase the applied voltage value above the default voltage of a typical LED that is from 1 to 2 volts. The use of load resistors causes the bypass of the LED of power from the power supply circuit to ground, increasing the measured current draw of the LED as seen by the CECM. The Load resistor is wired in parallel connection with the LED. Using Ohms Law, a equivalent resistance of two resistances in parallel calculated as: 1 / {[1/R1] + [1/R2} = Req.

R1 is the LED, and R2 is the parallel resistor placed in the LED circuit from + > ground. The added resistance of R2 must be low enough for Req to equal the resistance of an incandescent lamp, but higher values can be used up to the low current limit of the lamp check functions in the CECM when a lamp fault warning is initiated.

Someone with intimate knowledge of the CECM design and firmware may be able to modify the firmware for different detection values for failed lamps. I have never seen this information posted anywhere and I don't know how that could be done as a hardware hack.

[WeekendDriver]

The lamp failure warning/bulb check functions are provided by the Central Electric Control Module, that monitors the exterior lighting circuits. When a lamp fault occurs, the CECM sends the fault data via CAN bus to the IC for display on the DIS.

So I was misinformed and the ECU has nothing to do with it. Thanks for the competent answer.

However, if I implemented a CAN bus filter (think a microcontroller that not only connects to the bus, but breaks it in order to relay all the CAN messages from the input to the output programmatically), I could simply filter these messages out, right? And I could filter by specific bulbs, too? A CAN interface MCU is something I've been thinking for a long time for things like connecting the multi-wheel buttons to a car PC and patching custom text messages from PC to the DIS. I'm confident the software part is manageable, even if tedious as far as decrypting the CAN message IDs and data. But the hardware part (connecting the MCU to the bus, esp. since the bus must be cut) might be a bit too much for me to handle...

[diagnosticator]

So I was misinformed and the ECU has nothing to do with it. Thanks for the competent answer.

However, if I implemented a CAN bus filter (think a microcontroller that not only connects to the bus, but breaks it in order to relay all the CAN messages from the input to the output programmatically), I could simply filter these messages out, right? And I could filter by specific bulbs, too? A CAN interface MCU is something I've been thinking for a long time for things like connecting the multi-wheel buttons to a car PC and patching custom text messages from PC to the DIS. I'm confident the software part is manageable, even if tedious as far as decrypting the CAN message IDs and data. But the hardware part (connecting the MCU to the bus, esp. since the bus must be cut) might be a bit too much for me to handle...

See my amended post above. I suppose a CAN filter would work, that would disable the bulb check function completely. Whereas a load resistor would still allow detection of a faulty LED if the parallel resistor was valued right.

The load resistor is a practical and simple remedy IMO. I am skeptical the much more difficult scheme you are thinking about has any useful merit in comparison.

There are add-on modules available that will custom display any text and graphics on the DIS.

[WeekendDriver]

I suppose a CAN filter would work, that would disable the bulb check function completely. Whereas a load resistor would still allow detection of a faulty LED if the parallel resistor was valued right.

You're right in that a load resistor would allow for faulty LED detection if valued just right. Since the LED current is small, there's not much margin here. I haven't seen the specific current threshold of the fault detector circuit for each type of a bulb, but I have seen mentions that ~1/5th the current of the stock bulb is still enough to not trigger a warning.
If you're certain that the warning messages are sent to DIS via CAN, then you can definitely filter out just the specific bulbs, not all of them, since the faulty bulb location must be encoded in the CAN message so that the DIS could tell you which bulb went out.

The problem with load resistors is heat. It's manageable for 5W LEDs (although it still shortens the lifespan of LED), but imagine a resistor for a P21W bulb (brake / reverse / front and rear turn light). Even if 1/5th the power would suffice (3-4 watts dissipated at the resistor), it's still nothing to sneeze at. Let alone - full 21 watts. Turn lights is a different problem entirely, though, because even if we deal with the warning one way or another, they will still flash with incorrect frequency because of the stupid thermo-relay (100 years old technology).
Front position lights often burn out, literally (unsolder, even) because the load resistor heat + heat from the headlight bulb is too much to withstand in the closed headlight compartment.

[Charles.waite]

Interesting ideas for sure.

I've been interested in LED bulbs for a bit now but I sort of view running resistors inline as a hack and frankly it's pretty inefficient to have resistors burning heat and energy for no purpose.

If there were some some way to reprogram the CAN signals and manipulate them so that the CECM interprets the small 2-5watt loads as the bulb being good then that would be great.

I have some LED city light in my b7 and when I removed the original bulb there was some heat damage to the housing which is apparently fairly common. The inline resistors make me nervous though since they're sitting in an unventilated headlight housing.

[WeekendDriver]

Yep, I totally agree, load resistors are inefficient and often problematic.

Does anyone know exactly what kind of a pulse (period and duration) is sent by the CECM to test the bulbs? It might just be possible to fool the test using a capacitor instead of a resistor. Unlike resistor, a capacitor would draw near 0 current when the bulb is on.

[diagnosticator]

You're right in that a load resistor would allow for faulty LED detection if valued just right. Since the LED current is small, there's not much margin here. I haven't seen the specific current threshold of the fault detector circuit for each type of a bulb, but I have seen mentions that ~1/5th the current of the stock bulb is still enough to not trigger a warning.
If you're certain that the warning messages are sent to DIS via CAN, then you can definitely filter out just the specific bulbs, not all of them, since the faulty bulb location must be encoded in the CAN message so that the DIS could tell you which bulb went out.

The problem with load resistors is heat. It's manageable for 5W LEDs (although it still shortens the lifespan of LED), but imagine a resistor for a P21W bulb (brake / reverse / front and rear turn light). Even if 1/5th the power would suffice (3-4 watts dissipated at the resistor), it's still nothing to sneeze at. Let alone - full 21 watts. Turn lights is a different problem entirely, though, because even if we deal with the warning one way or another, they will still flash with incorrect frequency because of the stupid thermo-relay (100 years old technology).
Front position lights often burn out, literally (unsolder, even) because the load resistor heat + heat from the headlight bulb is too much to withstand in the closed headlight compartment.

The load resistor values don't need to be low enough to equal the power of the incandescent lamp. Higher values can be used within limits Using a higher ohm valued resistor limits the power draw to less than the original lamp but is still enough to remain within the current limits of the bulb check system. The load resistors don't have to be inside the headlight, they can be external with the same effect on the circuit. Placing the resistors remotely from the headlight is easier than installing them inside the headlight.

[Charles.waite]

Yea the most elegant solution is to reroute the headlight wiring through a metal plate with a raft of resistors and place the plate somewhere in the engine bay (likely near the ABS pump to minimize the wiring).

That way all the components are in one place and have plenty of ventilation.

However I'll admit i know very little about circuits and resistors.

[WeekendDriver]

Yea the most elegant solution is to reroute the headlight wiring through a metal plate with a raft of resistors and place the plate somewhere in the engine bay (likely near the ABS pump to minimize the wiring).

Great idea to place all the resistors on a single heat-dissipating metal plate, but I still hate adding weird stuff under the hood. I'll investigate other options first. I have high hopes for a capacitor.

[f1torrents]

You can easily find CAN compatible LED bulbs on the market these days. They have integrated circuitry to prevent the warnings.

[Charles.waite]

Almost none of those work in practice though.

[WeekendDriver]

You can easily find CAN compatible LED bulbs on the market these days. They have integrated circuitry to prevent the warnings.

Just to clarify, the circuitry is a single resistor and it has nothing to do with CAN. And the problem with the resistor, as discussed above, is that the resistor a source of heat and LEDs don't like heat. The die because of it.

[diagnosticator]

Just to clarify, the circuitry is a single resistor and it has nothing to do with CAN. And the problem with the resistor, as discussed above, is that the resistor a source of heat and LEDs don't like heat. The die because of it.

If heat really concerns you, just mount remotely. A parallel connection with the LED can be done anywhere between the CECM and the lamp. I don't know what CAN compatible LEDs do different, but our B6s don't use CAN in the power supply circuits.

Using load resistors still provides the desirable characteristics of LED lamps like instant illumination even if there is some power wasted, it is not much since the lamps that are on all the time are a few low power lamps and the higher power lamps are intermittent, with cool down time while off.

[WeekendDriver]

I know. But as long as it must be in the car (which, you know, it must one way or another) it does indeed concern me.

[f1torrents]

Put stock bulbs back in, no concerns..

[WeekendDriver]

I hear you, but halogen bulbs kinda suck in a light with LEDs for position, brake and fog:



I'll be installing these instead:


Still not perfect, but better. And they're brighter than the stock bulbs. Turn light is much brighter, in fact. Bright turn light = better safety, I'm all in on that:

[Let it snow]

I think that light assembly belongs on a Mitsubishi. Sorry had to.

[WeekendDriver]

Well, it's not THE best aftermarket tail light for B6 I've seen, but it's the second best. And at $25 for new unused pair how exactly do you suppose I was gonna pass on? :)

[ADCS]

I dont know if this helps but my lowline B7 does NOT report dead bulbs on the DIS. At least not for fogs, city lights, corner lights or tailights. It might for lowbeams and brake lights I am not sure. It does however show up as an error when scanned with VCDS so it does recognize that the bulbs are out it just does not report it on the DIS.

Lowline: halogens and no auto low beams.

[diagnosticator]

I know. But as long as it must be in the car (which, you know, it must one way or another) it does indeed concern me.

I don't believe load resistors mounted remotely develop as much heat as you imagine. For the lamps that are higher power, they are driven by PwM voltage so are not actually full power lamps,. This is done to control the lamp brightness.

[diagnosticator]

I dont know if this helps but my lowline B7 does NOT report dead bulbs on the DIS. At least not for fogs, city lights, corner lights or tailights. It might for lowbeams and brake lights I am not sure. It does however show up as an error when scanned with VCDS so it does recognize that the bulbs are out it just does not report it on the DIS.

Lowline: halogens and no auto low beams.

All A4s here in NA get high line electrics, That is when the exterior lighting is powered from and controlled by the Central Electric Control Module. Lowline electrics are only available in Europe, and with that system, the headlights are powered by fuses from the fuse panel. It does not matter if the headlights are halogen or HID xenon, or auto lowbeams

[diagnosticator]

Yep, I totally agree, load resistors are inefficient and often problematic.

Does anyone know exactly what kind of a pulse (period and duration) is sent by the CECM to test the bulbs? It might just be possible to fool the test using a capacitor instead of a resistor. Unlike resistor, a capacitor would draw near 0 current when the bulb is on.

The power supplied to the exterior lamps is a PWM voltage of fixed frequency and pulse width. The B6/B7 A4 product introduction SSPs describes this functionality, but does not detail the frequency or pulse width supplied to the lamps. The PWM voltage for the brake lamps and tail lights is adjustable for percent dimming in the CECM adaptation channels. A lamp dimming value of 20 percent would have a power supply voltage Duty cycle of 80 percent. . Dimming can be selected in percent by entering a different value in the adaptation channels for the lamps. Using a DVM that has Duty Cycle % and frequency in HZ for measuring PWM frequency to determine DC percent and pulse width period would provide the specific values of PWM frequency and pulse width period implemented. Period is 1/PWM duty cycle %.

LED lamps that replace the incandecsent lamps, would have a different brightness response to applied power PWM duty cycle % compared to incandescent lamps.

[WeekendDriver]

Interesting points, diagnosticator, thanks.
I suspect my car is also lowline. It does report number plate bulb fault on the DIS.

[Charles.waite]

The power supplied to the exterior lamps is a PWM voltage of fixed frequency and pulse width. The B6/B7 A4 product introduction SSPs describes this functionality, but does not detail the frequency or pulse width supplied to the lamps. The PWM voltage for the brake lamps and tail lights is adjustable for percent dimming in the CECM adaptation channels. A lamp dimming value of 20 percent would have a power supply voltage Duty cycle of 80 percent. . Dimming can be selected in percent by entering a different value in the adaptation channels for the lamps. Using a DVM that has Duty Cycle % and frequency in HZ for measuring PWM frequency to determine DC percent and pulse width period would provide the specific values of PWM frequency and pulse width period implemented. Period is 1/PWM duty cycle %.

LED lamps that replace the incandecsent lamps, would have a different brightness response to applied power PWM duty cycle % compared to incandescent lamps.

I'm not sure I completely follow what you're saying by I think I get the gist. From personal experience doing some minor electrical work on my house (yes I know, bear with me for a moment) when installing dimmer switches on incandescent bulbs you can use whatever you want. But with LED and Fluorescent you need special dimmers that have a special pwn sine wave profile in order to properly dim the bulb without flickering or buzzing. And with low voltage track lighting you need an even more specialized (aka expensive) dimmer geared towards the specific type of 120->12V transformer your track lights have. The dimmer switch for my living room cost $90. For a damn switch.

Anyway my point being, and Diagnosticator you post jogged my though process on this, is that unless the CECM sends the proper power pulse wrt duration and sine vs square wave it may or may not work with LEDs. Or at least it might not work well.

Also what exterior lights actually dim on our cars?

[WeekendDriver]

You're correct, Charles. Just one note: PWM would work with the simplest, driver-less car LEDs. That is, an LED and a series resistor connected directly to the 12V power grid. That'd work. But if the LED has a driver (voltage stabilizer or current stabilizer circuitry, which is good for the LED's life span in an automotive application), then we'll have a problem if the input voltage is PWM-modulated.

I also wonder what purpose it's done for. Does it, like, dim the exterior lights (brake etc.) in the dark (e. g. when city lights are on or smth.)?

[diagnosticator]

The illumination level of the brake and tail lights,(B7) and the headlights when used for DRLs,(B7) is a fixed percent dimming value by the PWM power supplied to the lamps. The dimmed output is effective whenever the lamps are energized. The amount of dimming can be changed in the adaptation channels of the CECM. A very low PWM voitage is applied to the various lamps for bulb check diagnostics also, The B7 CECM in my B6 does not display lamp faults unless the lights are turned on or the lamp check button is pressed.

B6:

A002,0,Brake Light Dimming
A002,5,Brake Light Dimming
A002,6,Range: 0...100 %
A002,6,Standard (Limousine/Sedan): 24 %
A002,7,Standard (Wagon/Avant): 0 %


B7:

A002,0,Brake Light Dimming
A002,5,Brake Light Dimming
A002,6,Standard: 30 %
A002,7,Range: 13...40 %
;
A003,0,Tail Light Dimming
A003,5,Tail Light Dimming
A003,6,Standard: 32 %
A003,7,Range: 13...40 %
;
A004,0,Rear Fog Light Dimming
A004,5,Rear Fog Light Dimming
A004,6,Standard: 28 %
A004,7,Range: 13...40 %
;
A005,0,Daytime Running Lights Dimming (Low Beam)
A005,5,Daytime Running Lights Dimming (Low Beam)
A005,6,Affective with Halogen only.
A005,7,
A005,8,Standard: 92 %
A005,9,Range: 51...100 %
;
A006,0,Daytime Running Lights Dimming (High Beam)
A006,5,Daytime Running Lights Dimming (High Beam)
A006,6,Affective with Bi-Xenon only.
A006,7,
A006,8,Standard: 70 %
A006,9,Range: 51...100 %
;
A007,0,Daytime Running Lights Dimming (Fog Lights)
A007,5,Daytime Running Lights Dimming (Fog Lights)
A007,6,Affective for Canada only.
A007,7,
A007,8,Standard: 92 %
A007,9,Range: 51...100 %


Ref: VCDS label file.

[Kevin C]

You're correct, Charles. Just one note: PWM would work with the simplest, driver-less car LEDs. That is, an LED and a series resistor connected directly to the 12V power grid. That'd work. But if the LED has a driver (voltage stabilizer or current stabilizer circuitry, which is good for the LED's life span in an automotive application), then we'll have a problem if the input voltage is PWM-modulated.

I also wonder what purpose it's done for. Does it, like, dim the exterior lights (brake etc.) in the dark (e. g. when city lights are on or smth.)?

This was news to me... From what I read it allows one filament to perform multiple functions. Running light= dimmed, turn signal and brake lights are run close to full brightness.

http://forums.vwvortex.com/showthrea...ives-the-bulbs

A simple circuit could decode the PWM and correct drive multiple lights. Companies that make trailer wiring kits have boxes to decode the signals for trailer wiring. A micro controller and a handful of lines of code would be one way. There are analog ways to do the same thing.

[WeekendDriver]

Makes sense. I thought there's simply a dual-filament P21W/5W bulb for running light and brake, but you can totally do it with a single filament as well. And if your turn light is red, the same filament can be used for it as well. EU cars (like mine) have a separate yellow turn light bulb. Could be then that my car doesn't use PWM to drive the tail lights at all.

The question still remains about the nature of pulses that the CCEM issues for bulb testing when the corresponding light is turned off. THAT is what we need to fool in order to get rid of the "light out" warning. For instance, I have LED number plate lights, and I get a warning, but it goes away and never reoccurs once I turn exterior lights on.

[Kevin C]

The question still remains about the nature of pulses that the CCEM issues for bulb testing when the corresponding light is turned off. THAT is what we need to fool in order to get rid of the "light out" warning. For instance, I have LED number plate lights, and I get a warning, but it goes away and never reoccurs once I turn exterior lights on.

As time allows, thats something I can look into. I have a scope and an A4... Kind of curious.

[WeekendDriver]

That'd be awesome! I guess there's only 3 parameters: pulse width (duration), voltage, and period (frequency). That's assuming the pulses are square, ofc.

[arankaspar1]

As time allows, thats something I can look into. I have a scope and an A4... Kind of curious.

Did you scope the brake light signal?

[Kevin C]

Did you scope the brake light signal?

No, but wow that five years went by fast....

[WeekendDriver]

No, but wow that five years went by fast....

I feel the same way!

[Charles.waite]

Well, what’s holding you back now?!