C5 x7 Break number plate lights intermittent fault

[aerodynamica]

Hello all, I noticed today the number plate lights were not on with the headlights. All other rear lights were fine and the powered rear hatch working too. I couldn't see any fuses listed in the driver's manual for any of the rear lights either. When I got home they seem to have come back to life. They're fairly recent LED replacement units.

Any known fault that might cause this? I suspect they'll play up again.

[MattBLancs]

Are they full lamp units or LED T10 / W5W?

If the latter, and are similar to this:

Rather posh OSRAM LEDs https://www.amazon.co.uk/OSRAM-LEDriving%C2%AE-signal-Off-road-Blister/dp/B08PKSK1GD/ref=asc_df_B08PKSK1GD/?tag=googshopuk-21&linkCode=df0&hvadid=499266141253&hvpos=&hvnetw=g&hvrand=6838408397603968418&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=1006873&hvtargid=pla-1118325976785&psc=1&th=1&psc=1

Then I have found it very easy to end up with the bent wire contact to be "next to" rather than "trapped under" the bulb holder's contact. Given the low current it tends to work ok, until any tiny vibration causes continuity to be broken. This is often fixed, or shown to fail, under a bit of "percussive maintenance".

Note I've no direct experience of the OSRAM LEDs shown above, just a suitable picture with bulb base looking like the cheap eBay versions from China I tend to buy

I'd take the LED bulb out again and see if you are suffering the same issue,

Matt

[Lenny]

At the risk of sounding pessimistic, when mine did this it was down to broken wires in the rubber tube between the hatch and body.
As a temporary measure I patched a feed in from the tail lights (and delayed sorting the job properly until the weather cheered up).

[Jay-Bruce]

You might have the same problem I do, whereby I believe the cars bulb check procedure sends an AC ripple across the LEDs and stalls the led number plate lights. Easy workaround / test is to start the car with lights on auto, check number plate lights, and they probably aren't on, so turn the lights off, and put them back on, should work now..I believe the answer is going to be to wire in diodes on the lines supplying the LEDs.

[aerodynamica]

Hi folks, they're full led units - a full lamp fitting and not only the bulb.

Yea, I had a fear it might be the wires going through the rubber tube .....

But I'm gonna try Jay's suggesting first!

[aerodynamica]

Hm, cannot seem to get it to repeat the problem now..

[Jay-Bruce]

Well that's a result - take the win and enjoythe easy fix. But should it rear it's ugly head again, try what I said about turning the lights off then back on again.

When my offshore escapades, periods of good health, and a lull in the duchess's schedule line up in such a way that I can ground her car, our X7, for a few days, I'm going to have fun with it. [Health is nothing major, but last leave I had COVID, this leave I've had a cold that became a stubborn chest infection that I cannae shift]

To start with I've got that silly little hydraulics job, you know the wee job I've been rabbiting on about for ages now, fitting the almost new rear firmness regulator, the new suspension cylinders, and remaking the fore to aft hp line with my experimental fittings, nothing too major. But while I've got the car, I'm going to try putting two diodes inline to the number plate lights, maybes with a resistor in parallel to give the bulb check something to measure, if that doesn't nail it I'm likely to try a timer spurred onto the feed for the tail lights, so the LED Lamp modules will only be exposed to power a few seconds after the tail lights come on, thus keeping them offline for the duration of the HF/AC "ripple" bulb test that I believe to be "stalling" the diodes.

I'll chime back in on this thread and let y'all know what if any of those interventions nails it.

[aerodynamica]

To start with I've got that silly little hydraulics job, you know the wee job I've been rabbiting on about for ages now, fitting the almost new rear firmness regulator, the new suspension cylinders, and remaking the fore to aft hp line with my experimental fittings, nothing too major. But while I've got the car, I'm going to try putting two diodes inline to the number plate lights, maybes with a resistor in parallel to give the bulb check something to measure, if that doesn't nail it I'm likely to try a timer spurred onto the feed for the tail lights, so the LED Lamp modules will only be exposed to power a few seconds after the tail lights come on, thus keeping them offline for the duration of the HF/AC "ripple" bulb test that I believe to be "stalling" the diodes.

..so, not much then

[KennyW]

Graeme,

I tried the led bulbs in mine and at first they worked fine but the started to flicker and then turn on and off. Just replaced them with normal bulbs and have not had any problems since.

Kenny

[aerodynamica]

These are the ones I got:

They're full light fittings (my old ones were in some state..) and to be fair, they've been really good - proper bright and never flicker. They are supposed to function with the part CANbus system. I had originally fitted only LED bulbs in the old bulb holders and I got the same as you, flickering. I transferred them to the boot inner lights and they have been great. They only flickered when the engine went off and the lights remained on for 10 sec or so. I assume the voltage drops for a period just before they extinguish.

I would have thought that these led fittings would not upset the test lamp feature (not sure my car actually has this anyway because it didn't know the brake lights were out or the old number plate lights) because they have the correcting diodes in them already.

Knowing my luck it'll be the wires through the hinge of the hatch.

I recall on my CXs (all saloons) that the rear number plate lamps were actually earthed through the boot door itself and grounded to the body via the hinges. These seemed to suffer from bad earths when the hinges wore over the decades - you could watch the lamps go off then on again at the boot was closed. Sometimes they'd be on then go off when it was closed... that was annoying in the end I added an earth wire back through the harness itself (why citroen didn't do that is anyone's guess) This was a series 1 CX phenomenon - I think (hope) they improved it on the CX2

[aerodynamica]

When my offshore escapades, periods of good health, and a lull in the duchess's schedule line up in such a way that I can ground her car, our X7, for a few days, I'm going to have fun with it. [Health is nothing major, but last leave I had COVID, this leave I've had a cold that became a stubborn chest infection that I cannae shift]

Get well soon Jay, as a sufferer of long covid, I've had nearly 3 years of a bad chest. I might be a car fanatic but like Jim of this Parish, I'm a cycler and commute most of the week (only drive on the occasional Thursday) clocking up a good 60 miles on a good week.. but on the recovery from what the virus has done to my induction system....

..only just began felling back on (almost) form this last 6 months... wouldn't wish it on anyone

[KennyW]

Graeme,

Do the all in one led light units work fine ie plug and play ?

Kenny

[Jay-Bruce]

In general terms yes, with the caveat that I explained before where ours sometimes seems to "stall" but work if lights are turned off and on again.

[aerodynamica]

Yea, I honestly chose those ones specifically because all the reviews reported they worked well. They're seemingly still working OK - not sure what happened the other day. They might have staked but the trouble is I don't know when in my journey that day - I only noticed they were not lit when I stopped and got out with the engine still running. Gonna try the off-on test next time

[aerodynamica]

Graeme,

Do the all in one led light units work fine ie plug and play ?

Kenny

Also, they're purpose-built for the C5 and other PSA cards that use that shape of lights. Just plug straight in. They do seem to be decent quality. My car's original lihht fittings were in a bad state so really needed to be replaced anyway