At Three Grand A Tail Light, There’s An Opportunity For A Hacker

It can be amusing sometimes, to read an incredulous reaction from outside our community to something that would be bread-and-butter in most hackerspaces. Take the sorry saga of the Cadillac XLR tail light, as reported by Jalopnik. This car was a more-expensive Corvette with a bit of lard around its midriff, and could appear a tempting pick for a bit of inexpensive luxury rubber-burning were it not for the revelation that a replacement second-hand tail light for one of these roadsters can set you back as much as three grand. The trusty auto on the drive outside where this is being written cost around a tenth that sum, so what on earth is up? Is it because a Caddy carries some cachet, or is something else at play?

It appears that the problem lies in the light’s design. It’s an LED unit, with surface mount parts and a set of fragile internal PCBs that are coated in something that makes reworking them a challenge. On top of that, the unit is bonded together, and instead of being a traditional on-off tail light it’s a microprocessor-controlled device that gets its orders digitally. This is all too much for XLR owners and for the Jalopnik hacks, who castigate General Motors for woefully inadequate design and bemoan the lack of alternatives to the crazy-expensive lights, but can’t offer an alternative.

Reading about the problem from a hardware hacker perspective they are right to censure the motor manufacturer for an appalling product, but is there really nothing that can be done? Making off-the-shelf microcontroller boards light up LEDs is an elementary introduction project for our community, and having the same boards talk to a car’s computer via CAN is something of a done deal. Add in LED strips and 3D printing to create a new backing for the tail light lens, and instead of something impossibly futuristic, you’re doing nothing that couldn’t be found in hackerspaces five years ago.

So what’s to be learned from the Cadillac XLR tail light? First of all, there’s scope for an enterprising hacker to make a killing on a repair kit for owners faced with a three grand bill. Then, there’s another opportunity for us to be acquainted with the reality that the rest of the world hasn’t quite caught up with repair culture as we might imagine. And finally there’s the hope that a badly designed automotive component might just be the hook by which the issue of designed-in obsolescence moves up the agenda in the public consciousness. After all, there will be other similar stories to come, and only bad publicity is likely to produce a change in behavior.

Of course, to get it really right you need a car that’s hackable in the first place. Or better still, one designed by and for hackers.

Thanks [str-alorman] for the tip.

Cadillac XLR header image:Rudolf Stricker [CC BY-SA 3.0].

65 thoughts on “At Three Grand A Tail Light, There’s An Opportunity For A Hacker

  1. It’s not microprocessor nor CAN/LIN bus controlled.

    The Jalopnik author failed hard at reading the wiring diagram.

    Read the article *and* the comments (there’s a few in there pointing out the author’s failures, including mine).

    There’s a ground and three positives for parking, stopping, and turning lamps. It’s easily retrofitted for anyone willing to do some splicing.

    1. It does have a microprocessor in it and significant amount of other circuitry, plus however the failed-light signal output is decided on.

      I suspect a PWM circuit to modulate for a constant illumination over the wide voltages a car can produce and to produce the correct brightness for each function.

      1. Cadillac absolutely uses PWM running lights. I can see them flickering and it drives me nuts. A few other manufacturers have different models with issues but Cadillac is the worst by far.

        1. Yes, and their absolutely obnoxious. There are regulations on brightness and color, why not throw in maximum flicker amount. Wasn’t necessary until PWM became common… Spend the extra $0.03 and smooth the PWM to give it a steady brightness, rather than making a strobe light leaving dotted trails if you swipe your field of view past one at night, especially at close range. Or at least crank the PWM up by a factor of 10, but I’d bet it’ll still be noticeable even then and still an annoyance to follow.

          1. I have a video rear view in my vehicle and so does my brother, we both have noticed PWM lights on quite a few newer vehicles.

            GM and Ford most notably and it is pretty striking when you notice the rate differences in the video review view.

            In many cases I have mistakenly thought there was an emergency vehicle behind me and when I check the side mirrors I see that it is a standard vehicle and I am just seeing the PWM frequency of their lights in the video feed.

            This is particularly true of the newer models with LED headlights.

          2. Seems like anyone who has done some arduino code think the world is as simple as digital.out method. Well the reality is not as simple as the geniuses here would like us to believe. I happen to have done research on this type of lighting back in the day – increasing PWM will causes interference and EMC throughout the vehicle where your FM radio will not work well.

            Also, there are speakers in the back, fuel pump, heat and many multiple problems to deal with. It is not a $0.03 as you might have suggested. These problems takes half a mil to solve and retests are expensive (a simple EMC test is $100K) after a simple change of the PWM from 100Hz to 120Hz. I did the research for DaimlerChrysler Pacifica.

            GM designed the type of circuits to be smart enough to know if light LEDs were burned out –> non-functional brake lamps are the causes of many rear end accidents. GM wanted to avoid read end collisions by cause of brake lamp outage.

            You guys think you’re so smart –> go ahead and apply for an entry level position at GM or others and see if you can pass their first level interview questions. Go back to your Arduino and hack your code – but don’t mistake that you’re qualified to make sound judgment on production level part.

      2. There are no connections indicating or requiring a uC controlled device. If it were a uC, it wouldn’t have 3 separate *supply* voltages for each function; it would be CAN_H, CAN_L, +12V, GND with the specific functions transmitted via CAN (or possibly LIN bus, not sure if GM uses that or not).

        There may be LED driver ICs and passives to go with the drivers, but it’s not nearly as complex as it’s being made out to be.

        The mechanical aspect of mounting replacement lamps/strips is much more complicated than actually getting them to illuminate when they’re supposed. That and figuring out what the BCM is expecting on the lamp out signal are the most complicated parts of a retrofit.

        (and pardon the double top level post, looked like the spam filter had blocked this first one)

          1. You mean the same lawyers that people run to when their loved ones are crippled for life by the negligence of another ? (DUI for example, or perhaps debris falling on your mama’s skull when she was just minding her own business walking down the street, and a clueless building owner ignored warnings about crumbling ledges 7 stories up)….yes those same lawyers … kind of like cops… no one wants to deal with them – until they need one…..

  2. There’s no microprocessor or CAN/LIN bus involved.

    The Jalopnik author failed hard at reading the wiring diagram.

    Read the article *and* the comments. There’s a few comments pointing out the author’s failures (including my own).

    There’s a ground there and 3 supply voltages for parking, stopping, and turning lights. The fix is as easy as splicing and mounting new lamp holders or LED strips.

    1. I couldn’t see the comments, but I agree that the author likely saw the electronics and jumped to an incorrect conclusion that the assembly was CAN bus controlled. I saw inputs for 1) stop light, 2) turn signal light, and 3) running/parking light. There was an output signal suggesting an assembly failure, but I’d have to do some circuit tracing (or see a working light in action) before I could say for sure.

      I know there’s at least one guy who repairs instrument clusters, and he’s doing little more than a visual inspection for troubleshooting and repair of VFD / LCD displays.

  3. I agree on that a tail light shouldnt cost 3k.. i suspect some of that amount goes into certification of the light and making it failsafe. You don’t want it to fail when you break hard and cause an accident.

    Are you insured with this modifications?

    1. That’s not the case. These parts are expensive because GM is not producing replacement parts and their design is repair-unfriendly, so remaining ones are incredibly expensive on the second hand market.

    2. It just cost 3k because it’s all about milking customers dry. You have no choice that replacing it or write off the car, so manufacturer can and will charge insane price if there is no OEM part supplier (which is almost always the case for low production car like this).

      Btw are you insured for replacing light bulbs?

      1. If I replace the bulb with another (same rating as old one) I am insured, if I mess with the CAN bus it is a grey area (which I wouldn’t touch if my life depends on it).

        From the article there is the suggestion (which is not the case if you check the linked post) the rear light is controlled through CAN instead of just a pair of wires and a simple bulb. If this it the case the tail light is a magnitude more complex and lots of safety features needs to be built into the microcontroller (and circuit) to make it work reliable and failsafe; you can’t do this with a couple of Arduino code lines (reliable!).

        If you decide to go the ‘arduino’ route a lot can go wrong and probably will..

    3. “you don’t want it to fail when you break hard and cause an accident.”

      First of all it’s BRAKE *not* “break”… learn the Queens English !
      Secondly, if a driver is minding their own business and gets rear ended by some numbskull who shouldn’t be operating a motor vehicle, it’s *that* driver’s fault – for TAILGATING and “following too closely”. Prudent drivers are able to keep a safe following distance and reacting accordingly, independently of the stop lights… they “should” be able to adequately determine that object-in-front-is-getting-bigger-and-not-moving.

      Unfortunately tailgating is a national cancer that is widespread and evidently not enforced. Witness the 70 car pileups on interstates that is blamed on weather – instead of properly being put in the lap of the twits who think they’re “drafting” on a NASCAR track…. yes, there’s fog and ice ahead.. let’s hurry up and ride the rear bumper of the car in front at 65+ mph ! ….. ” smh “……

      1. Everywhere I go, there’s zero traffic enforcement by local police. ZERO. Whenever I see someone pulled over, it is either the State or County police and usually thats’s for speeding +20 over the limit. Being a scofflaw has gone mainstream because of it.

        I’ve asked local cops at the gym, which is just a small sample of course, and they’re collectively against monitoring the driving public for tailgating, cell phone use, or texting and driving. If they pull someone over for another offense, they’ll generally ticket for safety issues like bad headlights or taillights.

        There’s a small amount of intersection red-light policing, but it’s so infrequent per month that people will run red lights constantly.

        Probably the “safest”way to run a red light is to full stop, and then “go anyways” when there’s nobody around but what people are REALLY doing is flooring it (do you want to t-bone someone doing 70mph? I don’t) or by tailgating so close there’s no gap to pull out when you get a green light (and without a camera, hitting THEM will be “your fault”).

    4. ” i suspect some of that amount goes into certification of the light and making it failsafe. ”
      Every car sold around the world manages to have brake lights and the vast super majority of these have managed to do it without this kind of cost. If they need some sort of massive new certification to do the job that everyone else has managed for nearly 100 years that in itself is a failure.

    5. The issue is a total of 15,000 of these cars were sold in the US and the tail lights are unique to the car. The only option is to buy one of the few used ones on the market. GM used the same technology in other cars and you can buy used replacements for those cars for less than $50.

      There’s worse out there – try and find a good used battery for a 2010 Mercedes S400 Hybrid!

      Here’s a unreturnable used one on eBay for $3593.

      Here’s a new one for a little over $12,000.

    6. “i suspect some of that amount goes into certification of the light and making it failsafe.”

      We’re talking about General Motors here…
      Failprone is the correct term.

  4. its $3K because no US mechanic has a first clue about fixing components, its all reading codes and replacing lego pieces until red light on the dash goes off
    anyway there is a dude (vettronics) rebuilding them at $500 a pop

    1. Nope, the problem is that GM never kept a inventory of spare parts like they are legally required to by law so the only spare tail lights are found on wrecks which are few and far between.

  5. Autospark here, while we dont get cadilacs here inthe uk, we have plenty cars with LED lighting. Standard operating procedure is to either heat the unit in an oven and split it , or dremmel out the back where it cant be seen when installed, access and reapair the circuitry, glue back together.

    done plenty, probably dont chanrge enough lol. Id be suprised if there isnt many folk in the US doing similar.

    also dint have access to caddilac wiring diagrams, but most all LED style rear lights ive repaired are not canbus but infact accept the regular switched 12v signals, some even go further and still use the current draw on that 12v line to detect a faulty “blown” bulb.

  6. I went for a ride in a convertible Cadillac XLR once. On the way back from the place we went to, we decided to put the top down. When the roof was fully extended up in the air, like a sail, it got inexplicably stuck. There was no manual override, and the buttons did nothing. We couldn’t go anywhere, and it looked stupid having the roof up like that. We had to wait a long time for an authorized Cadillac person to arrive at our location and get it fixed.

  7. Several years ago, and for a time, I actually worked in a GM foundry and ran one of a couple of the die-casting machines that made the North Star engine blocks for the caddy in the picture. You can see the North Star badge on the left rump.
    Interesting casting as it was all aluminum except for the cylinder sleeves which are cast iron, placed on mandrels in the die by robot and then cast over with around 90 pounds of aluminum at extremely high pressure. After trimming all the excess aluminum off – such as the runner, it was around 70 pounds – ready for machining and to be turned into a working engine.

    As for you all spouting off about no replacement parts. That couldn’t be any further from the truth… They actually run parts for sometimes years after a model is no longer “produced” and warehouse them. Which is actually why those parts cost so much for the end user to replace. Think about all the cars on the road, think about how many parts are in those cars, think about storing all the spare parts to replace failures that you have to fix under warranty, think about all the parts you’re storing for down the road, for years of support beyond warranties… It all adds up to an immense amount of overhead that gets paid for when someone has a failed tail lamp for an expensive luxury car of a limited number produced. It actually makes sense that these parts are pricey. They have to plan all of this out based on projections of how many cars will end up being produced, purchased, how many will be lost to accidents, how many will come back under warranty, how long they project a certain number will remain on the road for a certain number of years… A lot of thought goes into these things.
    You think a car company, who has to be innovating, making new cars that fit current tastes are going to keep the tooling in their factories around to make stuff for your 1978 Toyota Camry when it fails??? Just for you!?!
    Sure there are contract and aftermarket companies that might make or warehouse spare parts, but they only do so if it is lucrative enough to make a profit in the case of your run of the mill production cars. They ain’t doing it out of love and certainly not because you feel entitled to keep your Camry on the road forever. Now if a million of you have 1978 Camry’s and you live fast and loose with your pocketbooks, then sure they’ll help you out.

    1. “They actually run parts for sometimes years after a model is no longer “produced” and warehouse them. ”
      All manufacturers do this. Thats the major consideration when buying a car thats no longer produced AND from a manufacturer that is also gone like Daihatsu. The taillight from an A3 VW Jetta which hasn’t been made since ’99 costs under $100 new. A mkiv VW New Beetle tailight which the last model came out in 2012 runs $175 new. The beetle is also a low number car that isnt their ‘run of the mill production’. GM is just failing to compete here.

      1. A front brake disc for a MK2 Golf/Rabbit costs about $10. Brand new. Almost every part is still available for a ridiculously low price. For Mercedes Benz of the early and late 60s it is the same. You can get parts like the steel/rubber mountings for the front axle for $20 a pop from the OEM.

  8. These are the @#&%ers that flicker at about 15Hz at low intensity, too, I’ll bet. The ones that, if it happens to be in your field of view when you move your eyes, explodes into a stroboscopic cloud of uncertain location. I despise these things with a passion. At first it was just caddy that did it but last couple of years it’s become more common. I mean come on, how fsckn hard is it to do your PWM at 100Hz? And to find out they bill you 3000 for one of these horrid things? The difference between PWM-ing at sub 20 Hz and say 75 or more is probably nothing more than an initialization constant in the flipping micro! Why they choose to inflict such low rate PWM is baffling, given that in driving situations the last thing you’d want to do is make it hard to figure out where something actually IS.

    That said, let’s just take a step back and sanity check this. A microcontroller? To handle a light? A light that has just three jobs: off, dim, bright! This has to be on a CAN bus? Really? Honest to whatever gods there may be, is this really better than just sending power or not sending power to a dumb light? I mean come on, if you have to reinvent the wheel, or in this case, the tail light, and you just can’t resist there urge to throw active components into it, could it not be done with a 555?

  9. I try to purchase used cars that are popular and have inexpensive replacement parts (including body parts) available. I also try hard to repair auto parts I can’t purchase. If I was faced with this particular tail light situation, assuming the red plastic “lens” was intact, I would likely find a reasonable LED assembly to illuminate the lens like the OEM part, run a wire to my newly installed brake pedal switch that I bought new for $1.80, and pick up the “parking light” voltage from the other side if possible. Seems workable even if some home brew circuitry was required. If the plastic lens was damaged – it could be fixed. I’m sure to the naked eye, this could be done so it looked identical to OEM. I’ve seen plenty of aftermarket tail light assemblies for trailers and the like that pass our state safety inspections. I’m sure my repair would also pass.

    1. Sure, in 1980. But the ECM in any modern car isn’t going to take kindly to you just making up your own wiring scheme. I don’t know any state that’s going to pass a vehicle when the computer is throwing fault codes.

      1. All 2000’s GM cars I’ve had separate the emission related ECM’s (for engine & transmission control) from other functions. Typically there is a BCM (body control module) that handles dashboard, switches, radio/infotainment, and related accessories – also usually anti-theft and sometimes ABS/traction control/etc. Lights on my GM junkers, have been controlled by the BCM. This module is not scanned via the OBD-II port during emissions related inspections. One of my cars had an ABS fault due to water damage in the BCU. Had zero emissions issues (no check engine light), and so passed our state’s strict emission and safety checks. Curiously ABS faults do not fail a car for inspection in our state. Having said all that, the water damaged BCU in my GM junker was randomly tripping anti-theft measures which would disable the infotainment and occasionally disable the car from starting. So yes depending on the fault, a BCU fault can cause usability issues in the car. Once I discovered the corrosion damage on the potted PCB, and fixed it, the random misbehavior stopped. It’s why and how I got the car cheap – previous owner got stranded too many times by the random BCU faults. My most reliable vehicle is a 1979 Datsun – zero computers – but tough to find parts for.

  10. It only “cost” $3000 because they can. China could most likely do a functionally perfectly equivalent part for $10, shipped, if they wanted to. And if I didn’t tell you which is which, you could probably never tell as long as they’d be on the car.

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