A3 saloon - Left rear inner sidelight not working

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mattknight1984

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Hi all, hoping someone can help.
A3 8v saloon 2015. The rear left side light (on tailgate only) has stopped working. All other lights work fine in the same cluster, and so do the 3 other clusters. No faults on dash.
Removed light and confirmed sidelight works fine when power applied directly to it.
Checked all wires for breakages (apparently common on saloons) but all good. Oddly, the wire/pin for sidelights on the broken one only reads 2v with a meter, whereas the other side is 10+v.
All fuses look ok.
Has anyone seen this before? Not sure what else to try!
 
Sounds like there is not a complete brake or a section of wire that is very high resistance, I would guess. The simplest thing to do is run a new wire alongside the bundle tapping the intermittent wire somewhere further up the loom before where the fault usually happens.
 
Thanks.
I checked for continuity and the wiring seems fine between the plug for the light, and where the cables enter the boot (accessed from the passenger side rear door). I did this for all 5 cables in the plug. Surely this means the wiring is ok? If I measure voltage of the sidelight wire, its only 2v before it enters the boot too.
 
mattknight1984 said:
Thanks.
I checked for continuity and the wiring seems fine between the plug for the light, and where the cables enter the boot (accessed from the passenger side rear door). I did this for all 5 cables in the plug. Surely this means the wiring is ok? If I measure voltage of the sidelight wire, its only 2v before it enters the boot too.
Click to expand...
You may have continuity but the resistance maybe too high because there is not enough cable left connected at the break etc or there is corrosion. This is probably the case as you are measuring 10v on the good side and 2v on the side that is not working, and you have tested the lamp removed from the car and know it is not faulty .

You could quite easily have a problem with the cable that you can't see ie the wrap or the insulation hasn't broken but the copper has .
 
Does the wire that reads 2V do that with both the bulb in and out of circuit? If it goes up to 12V with the bulb out of the socket, it does sound like a high resistance in the wire. If it stays at 2V, it may well indicate a failure of the lamp driver in the lighting module.
 
steverobertsbbc said:
Does the wire that reads 2V do that with both the bulb in and out of circuit? If it goes up to 12V with the bulb out of the socket, it does sound like a high resistance in the wire. If it stays at 2V, it may well indicate a failure of the lamp driver in the lighting module.
Click to expand...
He says further up that it works out of the car on the bench.....

These are LED rear lights ?
 
steverobertsbbc said:
Does the wire that reads 2V do that with both the bulb in and out of circuit? If it goes up to 12V with the bulb out of the socket, it does sound like a high resistance in the wire. If it stays at 2V, it may well indicate a failure of the lamp driver in the lighting module.
Click to expand...
It reads 2v (nearer to 2.5v) whether the tail light is connected or not. Do you have any more information about the lamp driver/lighting module?
 
jrumball said:
He says further up that it works out of the car on the bench.....

These are LED rear lights ?
Click to expand...

Yes, PFL factory LEDs. Pic below shows the part that isn't working.
IMG 9615
 
The lamp works if you have it on the bench ? I am not entirely sure how the rear lights work but I assume there are a self contained unit ie apply 12-14v and they light up, I assume the LED driver is built into the light cluster then, so if the cable that powers that cluster is reading 2v when you measure to Ground and works on the bench then there is a cable fault somewhere which is a pain to trace, so easier to just replace it .

I would also guess without seeing the wiring diagram that the inside and outside are fed from the same place and that the fault is some where before they split to go to the inside and outside cluster
 
mattknight1984 said:
It reads 2v (nearer to 2.5v) whether the tail light is connected or not. Do you have any more information about the lamp driver/lighting module?
Click to expand...
I don't, I'm afraid - I was just basing my comments on basic electronic fault finding. However, as you've just noticed that another element is also out, you need to discount the possibility that it's this LED that has gone short and is pulling the other down with it.
 
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If the inner lamp is fed from the outer, then it could be wiring or an outer lamp fault, if you've bench tested the outer lamp as working on the LEDs that aren't functioning, then it's pointing towards wiring & you need to run a test wire from the supplying module to the light, bypassing the internal existing wiring, if this then works, you've found your issue, if still not working, then the supplying module could very well be faulty, it should be straight forward fault finding tbh.
 
Thanks everyone, this gives me more options. I removed the outer light and noticed it was full of condensation and water. I also tested this outer lamp off the car and its the same as on the car, one section of it doesn't work.
 
I'm confused you said in your 1st post it was ok of the car now you're saying it's not ok, however with condensation & water it's probably either killed the PCB partially or the contacts are corroded, either way it's water damaged by sounds of it.
 
NHN said:
I'm confused you said in your 1st post it was ok of the car now you're saying it's not ok, however with condensation & water it's probably either killed the PCB partially or the contacts are corroded, either way it's water damaged by sounds of it.
Click to expand...
Apologies, I should have been more clear. The inner light works perfectly off the car, but the sidelight part of it doesn't work on the car. Hadn't tried the outer one off the car until today - and its this one that is water damaged.
 
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