Why are P21/5w used as DRL?

[onix45]

Hello,

my 2013 C4 has a P21/5W lamp as the DRL. As you know, these lamps are dual filament.
My question is, why are they dual filament?!
So far i have tried turning on every possible light combination and the higher power filament never turns on!

[GiveMeABreak]

This is because they are supposed to be fully powered in the daytime and switch to the single lower powered filament at night or when the main headlamps are on so as not to dazzle oncoming traffic.

This will be down to the operating mode that is set in the BSI, specifically for the DRLs. There are several settings there that determine the mode of operation and some may be country-specific settings.

[xantia_v6]

My experience is that the 21W filament does not last very long when rum continuously. The first place I retrofitted LED bulbs.

[GiveMeABreak]

That's a good point, though I would add that the LED option might not be suitable for all vehicles that have a 'dual mode' for the DRL. On the C5 X7, (pre-LED DRL version) the bulb is a 24W HP24W, but this has only one filament so is either on or off. The DRL settings in the BSI then determine whether the DRLs are activated, Deactivated or come on with the sidelights too. If the DRL is designed for a low and high power setting, depending on day or night driving, then a typical aftermarket LED may not operate properly - or may operate at full light output at night causing dazzling.

The P21/5W bulbs are thankfully cheap enough to replace, unlike the HP24W at anything up to £50 a bulb!

[xantia_v6]

On the 308 (which had the 21/5 bulbs) I fitted some relatively cheap white LED bulbs from a Chinese supplier, the low power 'filament' is noticeably dimmer than the high power, so OK for night time.

[van ordinaire]

I thought that DRL's were supposed to extinguish when lights were turned on (although there are clearly many manufacturers who do not realise that), or is this a single unit that doubles as a sidelight?

[GiveMeABreak]

Yes, they are designed to be seen in the daytime, so have to be bright enough during the day to counter natural sunlight, so would therefore be far too bright at night. So some types of DRLs are designed to shut off completely once the headlights are activated (usually the older single filament bulbs). But if you have a dual type filament they switch to the lower powered filament giving a much dimmer light at night.

The side lights are pretty much obsolete with DRLs as they really don't serve much purpose unless parked up where you want to be seen without leaving your headlights on - now there's another topic for the ranters - seeing vehicles parked up facing you on the same side of the road on a kerb at night with their headlights blasting at you.

The newer LED DRLs have 'drivers' that maintain full power for the day and switch to a low power mode at night. It has been noted by many that even this 'lower' power output can still be too bright at night on some vehicles - especially on higher SUVs, where the light cluster tends to be more directly in your line of sight.

I changed my HP24W DRLs for LEDs because I hated the yellowish output - but mine are rightly set to completely extinguish themselves when the headlights are on or at night, where the Xenons do their stuff.

[onix45]

Does anyone know, what would happen if i reversed the terminals on my p21/5w sockets so that the other filament would be on all the time? Will the car start giving me errors?

[GiveMeABreak]

No, and I wouldn't recommend it.