The software in your car

Readers may recall that some months ago I blogged that I had replaced the (incandescent) brake lights in my Subaru car with LEDs.  Readers will also recall that Volkswagen is in the news for having included software in the engine computer of some diesel cars that would detect when an emissions test was going on, and at such times would adjust the engine to greatly reduce the emissions.  All of this reminds me of the DMCA (Digital Millenium Copyright Act) which generally forbids reverse engineering of software in consumer electronic products.  So how do my LED brake lights fit into this story?

The dashboard of my car has a couple of dozen idiot lights — icons that light up if there is something wrong that I need to know about.  Three times in the past six months, what would occasionally happen is that three of the idiot lights would light up simultaneously.  These were (1) the “AT Temp” light would light up, (2) the icon with an outline of a car with skid marks underneath would light up, and (3) the “BRAKE” light would blink.

  • The AT Temp light means the temperature of the automatic transmission fluid.
  • The “skid marks” icon means the stability augmentation system.
  • The “BRAKE” light means an imbalance between the pressure in the two brake systems.

This was of course a very scary thing to see.  The user manual for the car says that generally an idiot light merely being “on” means that something needs to get fixed but the car can safely be driven from here to the repair location.  The manual says further that a blinking idiot light means you cannot drive the car further.

One of the oddities about this is that there is no single point of failure that would affect all three systems simultaneously.  The temperature of the fluid in the transmission, for example, rises and falls at various times, but there is absolutely nothing about this that would lead to a three-times coincidence of a mismatch of brake fluid pressures at exactly the same instant as the transmission fluid temperature crossing some temperature threshold.

Each time that this constellation of three idiot lights lit up, I would stop using the car and park it overnight and the next day, the lights would be off again.  But after three cycles of this odd behavior of the idiot lights, I decided I had better get it looked at.  I took it to a Subaru dealer.

I was astonished to hear that supposedly the constellation of idiot lights was caused by my having replaced the incandescent brake lights with LEDs.  Apparently the dealer was able to interrogate the engine computer to find that it was unhappy about the amount of current being supplied to the brake lights.  My astonishment doubled when the dealer said the problem was that the LEDs had been drawing more current than the stock incandescent brake lights.

Today I had a chance to measure the current drawn by the various lights.  The incandescent bulbs draw 1.52 amperes (which is about 18 watts).  (That is not too far off from the rated 21 watts for these bulbs.)  The LEDs, on the other hand, draw only 210 milliamperes, or about one-seventh of the current drawn by the incandescent bulbs.

Maybe the engine computer does indeed monitor the current drawn by the brake lights.  I know that some cars actually do this, with a goal of warning you when a brake light burns out.  The usual way to do this would be to measure the total current to the brake lights, which would be a nominal 3 amperes, and to annunciate a trouble condition when the current drops below, say, 1 ampere.  The two LED brake lights would together draw only about 420 milliamperes, which is well below a typical threshold of 1 ampere.

But the engine computer did not behave in a way that makes sense for this to be the explanation.  If the problem was that the engine computer thought that maybe an incandescent brake light had burned out, then the correct behavior would not include griping about the temperature of the automatic transmission fluid.  The correct behavior would also not include griping about a supposed failure of the stability augmentation system.

A further point is that nothing — not the “BRAKE” icon itself, not the discussion in the user manual of the “BRAKE” icon — gave any hint (a) that the system monitors the current drawn by the brake lights or (b) that the flashing of the “BRAKE” icon meant that there was some real or imagined discrepancy in the magnitude of the current drawn.

Still another point is that if the explanation for the constellation of idiot lights was the problem of the LEDs drawing a significantly different current than the incandescent bulbs would have drawn (in this case a current only about one-seventh of the expected value), then why did this constellation only light up three times in six months?  And why would the constellation go away by itself overnight?

There is finally the astonishing problem of the dealer saying the problem was the LEDs drawing too much current when in fact they draw much less current than the incandescent bulbs.

Which gets us back to the VW diesel emissions scandal in which the software of the engine computer contains a special program that detects when an emissions test is going on, and that adjusts the engine at such times to generate far smaller emissions than usual.

Which in turn reminds me of the hotels where I have stayed over the years where I could do a speed test to test the speed of the hotel’s internet service, and it would show blindingly fast speeds.  Yet other activity (visiting web sites, checking email, streaming a movie) would be much slower than was suggested by the super-fast result of the speed test.  I have often been suspicious that maybe the hotel’s internet service provider watches to see if the user is visiting an IP address of a known speed test, and when this is observed, maybe the user is momentarily nearly the entire bandwidth of the hotel’s data line.

Which gets me back to the constellation of idiot lights that gave a completely message about the actual problem (which was not really a problem) in the car.  If only a car owner were to have access to the software in the car, the car owner could study the software, find the mistakes, and correct them.  I used to be an assembly language coder, years ago, and it would be a fun challenge to fix this idiot light problem.

3 Replies to “The software in your car”

  1. Also keep in mind that the current draw from LEDs increases quite significantly as they (and their heat sink) warm up. (Coming up with a heat sink design which effectively dissipates heat so as not to deplete the battery, but which also does not have exposed areas of hot metal that can burn the user, is one of the critical design challenges of caving helmet lights.) As a result, current dissipation per second of being “on” could vary quite a bit depending on whether you’re sitting in stop-and-go-traffic or driving on the highway.

  2. You should look into getting that flasher module restored. I tried one of those “For LED” flashers on my BRZ and it caused weird behaviors, such as unrelated indicator dimly blinking when hazards were on. The flasher I bought from had 9V relays in it. Riight, for an automobile? I ended up paying the big bucks for a “tap turn” module (Google it) and have not looked back. On my Impreza, I just opened up the factory flasher and used a dremel tool to thin out (increase resistance) of the wire shunt resistor so that it is happy with the reduced current draw from the LED signal bulbs. Works beautifully. What if I screwed up? I have an extra flasher module. In fact, the modified one is from the BRZ. Exact same part. My Impreza is using LED brake light as well, the DOT approved Philips 7443 replacement, which works with both standard

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