Why our modern(ish) cars don’t like LEDs?

A week ago, just before I was going to set off on a short one-day trip in my Citroen C4, I decided to do a full light check and verify if all of the car’s lights work (because I got a message for a faulty front side lamp). I knew that the stop lamps were working fine, since a few minutes ago I had seen their reflection in the windscreen of a bus waiting behind me on a row of traffic lights. So, I only had to walk around the car and check if all of the lights besides the brake ones were working.

Unfortunately for me, I really had a blown front side lamp on the driver’s side. Fortunately for me, the C4 makes use of simple W5W (T10 in the USA) bulbs for the side lamps and I always have spares (they are really cheap, the regular ones, contrary to the enhanced colored versions such as the Philips X-treme Vision LED W5W T10 which cost around 20 euros for the pair, giving the single bulb a price tag of 10 euros).

C4 Headlamp

The light streak is beautiful…or used to be, 11 years ago.

I changed the bulb and I was happy…sort of. If you have a look at the C4’s headlamp photo, you will see that it uses a semi-transparent piece of plastic so that the light from the side lamp is transformed into a long lighting line (very beautiful to me). But, with years having accumulated (11 for my car), this semi-transparent glass had become less transparent (or, in other words, it had darkened) and the effect of this light streak was just not happening. I needed some brighter bulbs to make these side lamps shine as they were designed to 11 years ago.

However, I was adamant that I wouldn’t give 20 euros for two side bulbs, no way. So I decided to opt for some kind of cheap LED bulbs and I found a simple W5W/T10 LED solution for around 40 euro cents the piece. That’s less than a euro for both!

I was happy and quickly installed the new bulbs and, from that moment on, the funny part began!

As you probably know, modern(ish) cars, or the cars of age less than, let’s say, around 20 years, use CAN bus standard for microcontroller and device communication between the car’s electronic components, which provides high speed of data transfer and is really suitable for controlling actuators or receiving events and data from sensors.

Dimmed Side Lamp

The current look of the side lamp streak on my actual car – as you see, it is really dimmed due to the aged plastic. And in the dark, there is no much difference.

In other words, CAN bus helps the engine control unit (ECU) to know what is happening to each of your car’s electronic components all the time and allows it to send commands to different electronic sub-systems and devices depending on the received sensor data (a simple example – in case of a crash the airbags need to open in, I don’t really know, a fraction of a second, a matter of several tens of milliseconds and CAN bus provides the speed to allow the ECU make the airbags deploy really at the time of sensor triggering). Since the ECU wants to know what is the state of all electronic components throughout the car, this includes bulbs as well. No matter if your car is running or idle, if you have turned the key so that electronically the car is alive or not, the ECU sends constant CAN bus requests to all bulbs to verify they are not blown. On a deeper level these requests operate with electricity – the ECU detects the power consumption of the bulb. If the consumption of a given bulb is less than a predefined expected value, a bulb fault message is displayed, warning the user there is a lamp that needs to be changed.

LED Replacement Bulb

The 0,4€ LED replacement bulb I used. Don’t pay attention to the minion behind.

Now, guess what – the LED bulbs in general consume much less power than their non-LED equivalents. What happens then when we replace a regular bulb with a LED one? Two things, actually. On the one hand, we get a CAN bus error message for a blown blub (because the ECU detects low power consumption value which is less than expected). On the other hand, the electrical current which is shortly sent by the car’s computer to the bulb for testing and measuring the consumption is enough to light up the LED bulb to, let’s say, 1/3 of its light capacity even when the button for the lights is on the OFF position. The relief here is that this electrical testing current is going through the lamp sockets for around 30 seconds each time you invoke the car even if you have regular bulbs fitted as well, but it is not enough to light them up. So, in terms in power consumption, the oftentimes-running LEDs won’t make your battery go flat.

The solution? There are many cheap CAN-free LED bulbs on the market (which are supplied with an extra resistance so that the ECU does not complain about low wattage). The pricey Philips bulbs are CAN-free as well. So now I am left with three options – to use regular bulbs, cheap CAN-free LEDs or the expensive (but never problematic) Philips LEDs. I have still not decided what to do, so for now I have installed back the original W5W bulbs.



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