Number plate lights on when car is off

[Ezz]

Hello.

My xc90 2005 d5 is being odd. Ages ago i had a message on the dash saying i need to check the brake lights. Normally this means a bulb is out. After checking all bulbs i couldn't find any that were not working.

Car had mot and passed. Garage said about the message, but all bulbs were working so must be a fault else where. Maybe a relay. I left it at that.

I have just seen tonight that when the car is parked up, the number plate lights on the boot are slighly on. Not enough to do there job but the is a glow from them. Are these 2 things connected. If not, what the hell is going on.

Any suggestions welcome.

Cheers
Dave

[Harley Dave]

If the numberplate lamps are LED replacements, they may be round the wrong way.

Try taking them out and reversing how they are inserted and see what happens.

Cheers

Dave

[SwissXC90]

Originally Posted by Ezz

My xc90 2005 d5 is being odd. Ages ago i had a message on the dash saying i need to check the brake lights. Normally this means a bulb is out. After checking all bulbs i couldn't find any that were not working.

Car had mot and passed. Garage said about the message, but all bulbs were working so must be a fault else where. Maybe a relay. I left it at that.

I have just seen tonight that when the car is parked up, the number plate lights on the boot are slighly on. Not enough to do there job but the is a glow from them. Are these 2 things connected. If not, what the hell is going on.

I've also had Tail Lamp bulb failure warnings and found no bulbs that were not working.
Happened more and more frequently and then I got stuck in to analyse the fault.
I found the reason: a Tail Lamp bulb was faulty. Exactly what the message said.

What happens is that a small break in the filament occurs that sometimes separates and stops the bulb working, and then when it cools down it works again. And vibration / road bumps / heat can cause it to separate when driving. The failure detection circuitry correctly detects and reports the fault, but when you stop and check the lamps, often then the filament rejoins and you cannot tell which it is.
I diagnosed mine by tapping each bulb until I found the one that went off when I tapped it.

So your very first step to the brake lamp failure warning is: fit 2 x new bulbs.

Don't even bother trying to determine which bulb is faulty.
The bulbs cost very little. Just fit 2 x new bulbs.

This may also resolve the number plate lamp issue as well, as the REM module that switches the bulbs on and off can do some funny things with residual voltage when one of the bulbs isn't working.

Try and report back.

[Ezz]

Hi.

The number plate lights are led and i just tried swapping them around. They are still on.

When i first got the brake light fault message, i did buy 2 bew bulbs. I got led lights but message persisted. I ended up with the normal bulbs back in position. And the led bulbs in the boot.

I shall try again at the weekend when i have someone to press on the brake pedel

I don't know if this means anything but.. the message appears every time i press the brake pedel.
We went on holiday in august and so we had a long drive. When we were on the motorway, if i needed to brake, sometimes the message would not appear. 🤔

[Harley Dave]

The brake light error message is due to the LED brake lamps, which are much higher resistance than incandescent lamps and do not draw enough current to satisfy the lamp check circuit in the car which "thinks" a lamp is blown.

One fix is to install suitable resistors in parallel with the LED lamps to fool the car into "thinking" it has the proper lamps in place.

This is covered extensively elsewhere on the forum, so I won't spell it out - have a search for LED Brake Lights.

You will prob get loads of "advice" telling you not to do it but it's your car and your money.

Another fix is to put proper Volvo long life lamps in the brake lights... (This may well resolve the numberplate lamps issue too)

Cheers

Dave

[cheshired5]

I would just fit standard bulbs and if the problem goes away, you know it was the LEDs causing the problem.

The fact that the LED bulbs light whichever way they're installed suggest they're faulty anyway given that they're diodes so should only allow current one way.

[Moose Test]

I have to agree with Swiss and Cheshire on this. You should replace both the brake lights and the number plate lights with new, incandescent bulbs and see where you are, as a baseline. You've made too many independent exchanges from original equipment to be able to isolate the fault.

Swiss could well be correct with regards to the original problem, with intermittent filament breaks but then you possibly masked that with the same error message but different cause by using LED bulbs without the appropriate resistor load.

A clean slate would be my approach, re-introducing (correct) LED bulbs then if you wish.

As for the number plate light specifically, I guess the LED bulb could have failed but I think it's also likely that they are manufactured with two or more LED chips in an anti-parallel configuration exactly so that it doesn't matter which way around a user fits them.