Licence plate bulbs canbus error. Possible broken canbus system?

[pinman]

Hi, both licence/number plate bulbs are out on my 1.9 tid A4 2002. I've got the warning light on the dash and neither bulb work.

I've replaced them and still no good.
I've tried them all of them in a 12volt battery and all bulbs, new and old, work fine.
I've tried scratching up the contacts on the bulb holders and bulb connections as in the photos, still no good.
I've checked voltage and I'm getting about 6 volts maybe but it varies between the contacts.

Now this all started when I left the boot lid open in heavy rain (don't ask 🫣) and water pooled between the number plate and the boot latch assembly/bulb holders so I assumed it just blew out the bulbs. But as the bulbs are all working something else it going on.

Is there a chance the water has broken the canbus system? The fact I'm getting voltage across the fuse holder means canbus is trying to see if there is a bulb before it sends full voltage to the bulb? I'm not sure what to check next?

I have had a problem with the whole boot loom broken wires issue a year or 2 ago. I had to solder a new bit of wires to fix broken ones but it's been fine since. Also the boot latch switch is detected by the car as the dash warns the boot is open when I open it. And the electric boot release switch works. Any help would be appreciated. If I can't fix it it's a fail on the MOT and scarp as it won't pass with warning lights on the dashboard in UK. Thanks.

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[pinman]

Here are photos I forgot to add

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[codemode]

You couldn’t have broken the can bus since there is none. Your car simply senses whether there is a load attached and throws a light accordingly. You shouldn’t be getting 6V. If your battery is fully charged you should be getting around 12V. This tells me you have a corroded wire somewhere that acts as a resistor and sinks all the voltage. Or you have a partially broken wire somewhere. Taking it apart further would tell us more. There is a screw, numerous clips and a plastic lock guard held by friction. Do a visual inspection.
Sometimes those springy contacts holding the bulb get bent inwards resulting in poor contact.

[Kevin C]

The error is generated by sensing the current to the bulb. The sensing is done by the body control module, if it sees an error it does send a message via canbus, but at the bulb itself its an analog sensing system that applies a load to the bulb (even if its off) to verify it is in the circuit. As Codmode pointed out, you should be seeing about 12V at the bulb. That is with the bulb in the circuit (you want a load when you measure the voltage).

I would measure the hot side (12V) to a know good ground and then measure the bulbs ground to a know good ground (again with the bulb in the circuit). That will tell you what side of the circuit the problem is on (supply or ground). On mine the root cause was broken , intermittent wires in the flex harness to the lid.

https://www.audizine.com/forum/showt...unk-wiring-fix

[Danisson]

The error is generated by sensing the current to the bulb. The sensing is done by the body control module, if it sees an error it does send a message via canbus, but at the bulb itself its an analog sensing system that applies a load to the bulb (even if its off) to verify it is in the circuit. As Codmode pointed out, you should be seeing about 12V at the bulb. That is with the bulb in the circuit (you want a load when you measure the voltage).

I would measure the hot side (12V) to a know good ground and then measure the bulbs ground to a know good ground (again with the bulb in the circuit). That will tell you what side of the circuit the problem is on (supply or ground). On mine the root cause was broken , intermittent wires in the flex harness to the lid.

https://www.audizine.com/forum/showt...unk-wiring-fix

Mine is from '02 and I had the thrunk wiring harness all twisted and broken. However, no bulb error in the dash from it... despite that if I'd take any other bulb out it would show up. Who knows...

[pinman]

You couldn’t have broken the can bus since there is none. Your car simply senses whether there is a load attached and throws a light accordingly. You shouldn’t be getting 6V. If your battery is fully charged you should be getting around 12V. This tells me you have a corroded wire somewhere that acts as a resistor and sinks all the voltage. Or you have a partially broken wire somewhere. Taking it apart further would tell us more. There is a screw, numerous clips and a plastic lock guard held by friction. Do a visual inspection.
Sometimes those springy contacts holding the bulb get bent inwards resulting in poor contact.

Ok so I've taken everything apart and I'm a little confused by What's I've found.

Firstly the plastic guard that carries the contacts for the bulbs doesn't have any wires. It just has the contacts the bulb carriers sit against so I'm guessing embedded in the plastic is a circuit? I've added the first picture of this. It's not the best picture I now realise.

The micro switch that controls the boot release is in the second picture. This works absolutely fine and is on the same circuit as the bulbs as its on the same connector although there are 4 cables going into it so not necessarily on the same circuit I suppose?

I've checked the voltage into the connector and found that on the 3rd picture I'm getting constant 12 volts on those 2 pins (11 on my voltage meter but it could be because is well worn). I've pushed a bit of wire into them to make the contact.

The 4th picture I'm getting 0 volts when the lights are off and 12 volts (11 volts again) when the lights are on so I'm hoping that means no break in the wires.

So what's next to check. I'm really not sure of the next steps or where to look. The fact that there is voltage means the plastic assembly is broken and I need a new one? Really not sure what to do from here. Any help would be appreciated.

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[pinman]

The error is generated by sensing the current to the bulb. The sensing is done by the body control module, if it sees an error it does send a message via canbus, but at the bulb itself its an analog sensing system that applies a load to the bulb (even if its off) to verify it is in the circuit. As Codmode pointed out, you should be seeing about 12V at the bulb. That is with the bulb in the circuit (you want a load when you measure the voltage).

I would measure the hot side (12V) to a know good ground and then measure the bulbs ground to a know good ground (again with the bulb in the circuit). That will tell you what side of the circuit the problem is on (supply or ground). On mine the root cause was broken , intermittent wires in the flex harness to the lid.

https://www.audizine.com/forum/showt...unk-wiring-fix

I've done my checks for the voltage and I'm getting 12 volts at the connector for the light assembly. Actually 0 volts lights off, 12 volts lights on. That's if I've actually put a bit of wire down the correct holes in the connector? My other reply has a bit more information.

I'll try and take a better photo of the plastic circuit holder tomorrow as well.

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[pinman]

I've done my checks for the voltage and I'm getting 12 volts at the connector for the light assembly. Actually 0 volts lights off, 12 volts lights on. That's if I've actually put a bit of wire down the correct holes in the connector? My other reply has a bit more information.

I'll try and take a better photo of the plastic circuit holder tomorrow as well.

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Looks like I could be in the same boat as all of these used parts I can see on eBay. They all must of had problems like I have as in the pictures you can see repairs have all been done to them with makeshift wires soldered into the assemblies. Second or third picture in this eBay link.

https://www.ebay.co.uk/itm/405146847...mis&media=COPY

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[codemode]

Looking at your pictures, where are you bulb wires at? I only see two golden contacts.

[pinman]

Looking at your pictures, where are you bulb wires at? I only see two golden contacts.

exactly. How in the hell does the voltage go to the bulbs. It looks like the circuit is embedded in the plastic frame that screws in to the assembly. I'm only guessing but it can be the only way it works right?

I've just taken another photo of the connector and I'm getting between 2 and 4 volts in the following pins I've got it plugged in to.

I've taken a better picture of the plastic frame that screws in to the assembly.

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[pinman]

Looking at your pictures, where are you bulb wires at? I only see two golden contacts.

I've just checked and there is continuity between the negative of each side and positive of each side. So left and right negatives and left and right positives must share the same circuit that lead to the connector.

As for the connector do you know or can you find out on your car which wire is for the 12volt bulb feed? I'm not sure if it's the wiring or the assembly that's the problem now as I'm getting 12volts from some of the as my other posts above mention.

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[codemode]

That is wild! I took my assembly out just yesterday and there were physical wires with crimp terminals in there, no through the plastic stuff to be seen.

There still has to be a female plug, where is it?
Also why did you put two resistors in there, I guess your probes weren’t thin enough.

If the two negatives are connected together, that means that the other two contact are your positives.

I can get you a wiring diagram.

[pinman]

That is wild! I took my assembly out just yesterday and there were physical wires with crimp terminals in there, no through the plastic stuff to be seen.

There still has to be a female plug, where is it?
Also why did you put two resistors in there, I guess your probes weren’t thin enough.

If the two negatives are connected together, that means that the other two contact are your positives.

I can get you a wiring diagram.

Yes pretty much what I realise now. The negatives and positives share the same circuit which powers the bulbs. And the plug powering the bulbs is the part I've been plugging the resisters into.

As you say the probes are to small so I've plugged a resistor in to make the contact. In my other posts I've said that with the lights on I get 12 volts and lights off I get zero volts so it's got to be through that plug that goes in to the boot latch micro switch.

If I check the 2 connections on the left I only get between 2 and 6 volts when lights are on but just not sure which wires are bulbs and which is boot latch.

Wiring diagram would be good

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[pinman]

That is wild! I took my assembly out just yesterday and there were physical wires with crimp terminals in there, no through the plastic stuff to be seen.

There still has to be a female plug, where is it?
Also why did you put two resistors in there, I guess your probes weren’t thin enough.

If the two negatives are connected together, that means that the other two contact are your positives.

I can get you a wiring diagram.

Well looking closer at the plastic circuit thingy someone has been working on this before for a repair it looks like. From the picture it seems like been in there with a soldering iron fixing a connection and covered it back up with semi transparent glue. But I think the fix may of held up as I'm getting continuity in the circuit.

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[codemode]

Pin 1 Black/Brown - T16/10 CCM Trunk lid switch

Pin 2 Brown - Trunk lid switch earth connection, rear wiring harness

Pin 3 Gray/Yellow - T32b/6 BCM Number plate light positive

Pin 4 Brown - Number plate light earth connection, rear lid wiring harness

So your only concerns are pins 3 and 4. No need to measure anything else.

[Gunnarrrrr]

Still think you should probably double check your trunk wiring harness.

[pinman]

Pin 1 Black/Brown - T16/10 CCM Trunk lid switch

Pin 2 Brown - Trunk lid switch earth connection, rear wiring harness

Pin 3 Gray/Yellow - T32b/6 BCM Number plate light positive

Pin 4 Brown - Number plate light earth connection, rear lid wiring harness

So your only concerns are pins 3 and 4. No need to measure anything else.

Hi, thanks for this. I did test the pins by holding the probes against pairs of pins then press the micro switch. Got continuity between 1 and 2 when the switch was pressed.

Well I put it all back together and noticed one of the lights was on briefly but when I starting messing around with it it went off!

So turns out it is the wiring to the trunk! After doing a bit more wiggle testing one would flicker on temporarily so I'll need to check all wiring. I'm sure I already carried out comprehensive wiggle testing of the wiring before I took the whole thing apart . Damn it. Never mind. Thanks for your help. I'll have to trace the wiring back and find the break.

The last time I fixed it I put hot glue over the wiring to try to stop flexing of the cable in that area 🫣. I'll try add a photo when I get around to doing it so you can see how much of a shit show it is

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[pinman]

Still think you should probably double check your trunk wiring harness.

Yep, you're 100% right

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[Danisson]

When I did mine 2 years ago I discovered that these blue and red wires leading to the golden "plates" where the bulb housings make contact were also very brittle so I replaced them along with the small switch for the trunk opening. Yours also looks as if it'd seen better days so yeah... I'd check the wires in the loom.