What’s this Warning light?

[mantianak]

I don't think the number plate light is counted on the bulb check? at least not on my car.
An indicator of an indicator bulb failure (ho-ho) is it flickers very fast on the side missing the light. Again, it has done on my car anyway.

[Jürgen]

Originally Posted by JohnDotCom

The bulb sometimes do that before they permanently go for some reason, I would change it any way or the pair.

I experienced this, too. I recommend to change both bulbs at the same time, for two reasons. First, mostly the second one will blow up also soon, second, the new bulb can be considerably brighter than the old one. Some time ago a front sidelight was gone. As it isn't really easy to change these, I said to myself "OK, this time one changed bulb is enough". But when I checked the function I noticed quite a difference in brightness. So I took the other 5 year old bulb out and noticed a black coating inside the bulb, hence the lesser brightness.

[Jürgen]

Originally Posted by mantianak

I don't think the number plate light is counted on the bulb check? at least not on my car.

Yes, it is! But only if both bulbs have failed.

[stocktake]

Originally Posted by mantianak

I don't think the number plate light is counted on the bulb check? at least not on my car.
An indicator of an indicator bulb failure (ho-ho) is it flickers very fast on the side missing the light. Again, it has done on my car anyway.

Number plate light will show as bulb out only if both bulbs have blown

[mantianak]

Ah, that's why then, only 1 had died on mine. Actually the bulb hadn't died but fell apart. I put it back together again and works better than ever now.

[black olive]

the ends of the filament will have vibrated and touched together whilst current was flowing, and they will have temporarily fused together, but will burn hotter and soon burn out.
as an aside we used to "calibrate" the bench lamps at work by whacking them, they went very bright nut not for long.
the other trick we did was take the innards out of the plug fuse and put a diode in. this ran the bench lamps on pulsed DC and they were dim and pulsing- was very confusing for the lads on the bench

[lightpainter]

That’s not bad then, carried on for another 11 days before the warning light came on again, have changed both stop lamp bulbs today!

[BigJacko]

Originally Posted by stocktake

Number plate light will show as bulb out only if both bulbs have blown

I read on another forum that this is because the number-plate bulb is a low wattage, and only draws a small amount of current, which isn't enough to trigger the 'bulb-dead' detection threshold when it blows. When the second one goes, because they are paired (presumably in parallel), the combined unused current does trigger the dashboard warning light into realising something is 'amiss'.

Same story for the indicator side-repeaters, apparently.

This also explains why (apparently) it's common to find both number plate lights have gone (or possibly both indicator-repeaters, or one of each, but that's a personal guess), when you get the dashboard warning light. It's not that they both actually blew at the same time - you just didn't notice the first one go (and got no warning), but the second is enough to trigger the warning and make you look, and then of course, you find both blown.

Makes sense, if you think about it.

[Fille]

My Tourer has the LCD screen info thing in the dashboard and it started talking about failing brake lights a couple of days ago. Sure enough, the left one was dead. Changed the bulb, turned on the ignition and what does it say... bulb failure, left rbake light. Had the tail light assembly still hanging in its thread beside th car so as i pushed the brake pedal I could actually see the brake light bulb lighting up. It took three turns of the key on-off and a lot of pushing the brake pedal until the warning went out! Wonder if it doesn't check the lights permanently?

I know on my Citroen C5 that the on board computer checks the lights on a regular basis and not always. This can be seen when locking the car at night, the rear high brake light diodes glow slightly. Apparently the car's computer sets off a small current which isn't enough to light up the light bulbs but the diodes in the stop light do...

[CDTi Auto]

Originally Posted by BigJacko

I read on another forum that this is because the number-plate bulb is a low wattage, and only draws a small amount of current, which isn't enough to trigger the 'bulb-dead' detection threshold when it blows. When the second one goes, because they are paired (presumably in parallel), the combined unused current does trigger the dashboard warning light into realising something is 'amiss'.

Same story for the indicator side-repeaters, apparently.

This also explains why (apparently) it's common to find both number plate lights have gone (or possibly both indicator-repeaters, or one of each, but that's a personal guess), when you get the dashboard warning light. It's not that they both actually blew at the same time - you just didn't notice the first one go (and got no warning), but the second is enough to trigger the warning and make you look, and then of course, you find both blown.

Makes sense, if you think about it.

I thought that would be the case but my warning light has a mind of it's own when it comes to the rear number plate bulbs. Warning light comes on if they are both out - or just one is out - or not sometimes! Totally screwy but sounds more like a wiring/connection fault as i've changed bulbs numerous times! Is there a common failure point in the wiring or connections though the bootlid area for this (saloon, not tourer)

Thanks