Double-Flash Hazard Lights

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While at a recent Autocross event, I noticed something. The normal custom while instructors are driving your car, is to drive the course with the hazard lights on. Something caught my eye when I noticed a new 1 series with double-flashing hazards instead of the ON, OFF, ON, OFF of normal hazard lights.

I thought it looked kinda cool. And so began my investigation on the Google. Some reports are that newer Euro-spec cars are programmed to double-flash the hazards (can any of my european friends confirm this?). There was also some information indicating that a double-flashing hazard is a signal to emergency response teams that an airbag has been deployed.

Soon I found out these guys with late model BMWs in the US are able to reprogram their late model BMW’s Light Modules using NCS Expert and a serial cable with a laptop. If you’re not familiar with this software, it’s basically the software used to program all sorts of things in cars that have reprogrammable modules. If you want your LKM to turn on ALL lights when you hit the high beams the way they operate in Europe, you would need to reprogram the LKM.

Would it be possible (I thought in my naiveté) that the US-spec E31 LKM could be reprogrammed to produce this double-flash hazard effect? If so, it would be real cool. In our E31 there are a few programmable modules, but being built in the late nineties, there’s not nearly the options the newer cars have.

Eventually, with the help of our most experienced electronics board member Revtor, I found out, no, the E31’s hazard lights are controlled by a standard relay and the pulse of that relay is not adaptable or controllable without adding something.

Not to be deterred, in short order I was searching the webernet for a pre-built programmable light module that could produce the effect and be installed inline somewhere in the car. First was to find a black box that could do it and second was figuring out where exactly to interrupt the hazard signal.

Finding a place to splice in the module was the easy part. Originally we thought it was the hazard relay that controlled the pulse rate and nothing else. However with visual alarm capability installed (like with most alarm systems) arming the alarm produces a quick double-flash on its own along with the audio indicator. So we knew there had to be a way to double-pulse the hazard lights. We just had to find the correct interception point.

Examination of the ETM revealed that in the E31 you need to intercept the wired leading FROM the hazard switch TO the N17 Crash Control Module (pin 7 brown/blue wire) where the module can drive input to the Crash Control Module.

Now we just had to find an off the shelf module that could do it. However, there were a few requirements:

  • The module must have a fairly short programmable period, like in the hundreds of milliseconds range. Preferable even better to be able to adjust the timing as close as possible to the real BMW double-impulse timing.
  • Power supply demands: Since the hazard lights can be used at any time (even when the ignition is off), the module has to be powered permanently. But as you know the E31 tends to be a bit power hungry, so any extra drain on the batteries is to be avoided. The power consumption should be as low as possible when the module is idle. The typical current draw of the E31 electronics in sleep is 50 mA. The new module should add no more than a few milliamperes to that. Modules built for home applications usually are not very power efficient (equals cheaper for the manufacturer). Such a module could easily double the car’s power consumption while sleeping, so off-the-shelf household modules are to be avoided.
  • The second issue is that modules for home applications are usually not designed to operate on the hostile on-board power net of a car. In a car the voltages can go up to 14.5 V and even higher spikes are possible. This requires special precautions for the power circuit of the module.

On finding an off the shelf module, long story short, there’s nothing on the market that can do the job – it was going to have to be built custom.

With the generous help of guru Revtor to point me in the right direction, he designed a prototype custom module. As you can see there aren’t many components on the board. The board is only 29 x 24 mm (1.1″ x 0.9″). He designed it so all logic is inside the Atmel ATtiny25 microcontroller. The other components are the power supply and an open collector output. The power supply is built around the LM2931Z-5 – a popular voltage regulator for microcontrollers in automotive applications. It’s designed to handle the abuse of the on-board power net.

While idle, the module draws only 0.2 mA. The effect on the batteries is negligable, so it’s perfectly safe to have it powered permanently. If you wake up one day and the car has dead batteries, rest assured this module is not the culprit.

He included a jumper in the design that enables/disables the double-impulse blinking. That way you can revert to the original behavior without changing the wiring. The jumper can be removed/installed while powered-on.

Finally, Revtor programmed the module so it will now always complete a double-impulse cycle, regardless of when you release the hazard switch. Cosmetic, but he thought it would be nice. Kudos Revtor the electronics God!

In typical Revtor thoroughness, he created a picture showing what each terminal of the module should connect to. There are only four wires:

  1. The brown/blue wire between the S18 hazard switch X516 pin 2 and N17 crash control module X65 pin 7 should be interrupted and rerouted through the new module. If you don’t like the idea of cutting the original wiring harness, you’re free to open/disassemble connector X516 on the hazard switch, remove the wire from pin 2 and run a new one from it. That way, you could always restore the situation without a visibly cut wire. Not sure that’s possible though.
  2. With the connector open you could also simply solder an extra wire on pin 3 (brown wire) for the module ground.
  3. The permanent +12 V is a bit more difficult. It’s not connected to the hazard switch. Revtor had a quick look at the electrical troubleshooting manual (ETM) for a permanent 12 V near the hazard switch and found one that is easily accessible: The DWA alarm LED next to the gear lever. Its 4-pin connector (X514) has a permanent 12 V at pin 1 (red/black wire) that can be spliced into.
  4. Ground can be spliced either to the hazard switch’s ground (#2 above) or any readily available ground location. One note though: avoid grounding to anything on the radio harness. To avoid interference it’s best to leave that wire for audio devices only.

There’s no need to disconnect the batteries while installing the module but if you don’t, just temporarily remove fuse F8 (front power distribution box) and F31 (rear power distribution box). That will remove power from the crash control module (& hazard switch) and the DWA alarm LED.

After hooking this little baby up – it worked on the first try! I am stoked to have this unique, classy and modern addition to my CSi and I want to earnestly thank Revtor for his patience and guidance helping me determine what would and would not work as well as designing a perfect little module. Click on the videos and let me know what you think!

Double Flash Hazards 1

Double Flash Hazards 2

This goes very well with my Single Stroke Comfort Blinker already installed to modernize the car. The double-flash hazard feature makes the E31 seem… ‘otherworldly’ and even more unique – at least until double-flashing hazards become all the rage!

So the next time I’m at the Autocross, those lucky late model BMW code monkeys may be impressed this ol’ gal is now in the Euro double-flash hazard club.

Happy trails…


If you’re looking to make yourself one of these, Revtor tells me he might post the parts list and instructions at as soon as he gets some time and I will cross link to it then.


About the Author:

I am an 8 Series nut. In 2009 I restored a '92 850i to peak condition and subsequently sold it - which left me depressed. So I embarked on catching my White Whale: this '95 850CSi Chassis #CD00166. I’m opinionated and swear constantly, but I have friends in spite of myself. I am funnier than my wife realizes. Please tell her. Cheers!
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