Cracking Rear Lights
 

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I recently came back from the USA and the car had been at Manchester Airport for a few weeks.
As soon as I got to the back of the car I saw the rear lights' cracks had got much worse and even had a 1" vertical crack in the middle too.
These lights were made in 2010. I see 20 year old cars without this problem.

I have removed them and split the red lenses off, they are not meant to come off but I split them off the glue is brittle like cyanocrylate.
I have flooded the reverse lens with Ultraclear UV-resistant epoxy on the inside, regrettably, it take 72 hours to cure.
The whole lens is covered in crazing when held up to the light, millions of cracks in all directions. The lens is made of a clear outer layer and a thicker red inner layer which has the strip cut out for the reversing light. The inner red material is not cracked at all, it seems to be in the clear outer layer. Sadly I damaged one on the corner.

This also happened to some perspex/acrylic stuff in our kitchen, a small microwave pan and a lemon juicer after they were put in the dishwasher, first the miniscule crazing and eventually a big stress relief crack. It seems Audi used the wrong kind of plastic, this affects the R8 and A6 too.

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Clearly a flaw in the production:(. For future I would recommend ppl to use some UV protectant every month on the lights.

I have been researching this online - there are whole research papers on the subject and seems that solvents are one major cause on clear polymers such as acrylic (PMMA), and polycarbonate, so don't clean your lights with alcohol/polish with petroleum distillates etc...

So this only happens to lights with a clear outer layer of specific materials such as that favoured by Audi, it doesnt affect the brittle red material usually used on most cars' rear lights.

It is a shame.

Anyway, I left my light lenses in the oven overnight on platewarmer setting and they have cured nicely so will be gluing them back together this morning and fit them this afternoon if the rain stops. If not, will just leave them off until another day and tape over the cable hole, the outer lights seem sufficient/legal.

Another interesting point is that the direction of the cracks always comes from the small circular dot. It is in the middle of the reverse lights on ours and look on the next phot, comes from a similar dot on the lower right.

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Did you notice a lot of condensation in each unit before they cracked?

No, there wasn't any. Since this story, the lights clouded up due to the curing of the cyanocrylate, so I bought another 'perfect' one off ebay that was also cracked but got a refund and kept the light which I sealed from the outside.

I also bought a new ULO one for £185 from carparts for less. Now that is perfect+++

ULO is Odelo GmbH's aftermarket branch and the ULO is clearly off the same line as the Audi originals, therefore theoriginals are Odelo.

I emailed them about the issue, but I'm yet to receive a reply.
I took apart the old ones, they are really quite beautifully made inside. It's a shame about this issue.

The reason I ask is because I fitted a new pair from Spareto a year ago and they soon had condensation appearing in the clear, reverse light section.

It's got to be detrimental to their longevity but I'm not sure how to prevent it, or resolve it

Both the inner lights on my 2004 pre facelift D3 are cracked, but the outers are still perfect.

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Here is my old light after splitting (for the second time before binning).
The outer shell/lens is brittle red plastic which has a 'letterbox' aperture in it for the clear reverse light. This red plastic has no cracks, not even hairline ones. This is fused with the clear plastic on the outside surface which is covered in spiderweb fissures culminating in a stress crack across the reverse light aperture. Whatever has affected this is coming from the outside (such as car polish or UV light) I think.

The inner workings are plastichrome and these also have their own clear plastic fresnels lenses across them. The LEDs do not shine outwards, they are mounted 90˚ in rows pointing down or up and get shone out 45˚ to the red lens.

The question has been asked in the past if there are orange indicator LEDs present in the inner lights even though not in use. There are not, but there is room for a PCB if they wanted them or maybe they have them on USA models? Where the PCB would go is just a metal strip to stop the light leaking from adjascent segments.