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Rear Light Assembly Ground Fault ~ Non-OEM Repair

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72K views 213 replies 59 participants last post by  Henrik964  
#1 ·
Various topics appeared on this forum already regarding the rear light cluster packing up completely.
As we've seen in the other topics, it's mostly due to a corroded earth connector in the plug/socket.

In my case however the connectors were still reasonably clean and no signs of overheating. Hopefully I now have a solution to the problem as it appeared in my September 2006 TT.

First of all, I noticed that the resistance from plug to the car battery was 0.6 Ohm and not 0.0 Ohm as should be the case. The root cause appeared to be at the other end of the wire, in the boot corner of the car. I disconnected it, used a fine grid sand paper to clean the bolt, the 'O' connector and the body work. Reinstalled the connector and result: 0.0 Ohm now. No resistance any more from plug 'all the way' to the battery.

Next point of attention: the actual connection from plug onto the bulb holder. Two issues here:

1) the metal used as wire on the holder corrodes. This corrosion turns into electrical resistance. Current and resistance create heat. From there things just go pear shaped.

2) the actual connector for earth imho is too small really. When brake lights, indicator lights and reverse light are on all at the same time, we're looking at 2*21 + 2*16 + 1*16 Watts = 90 Watts, close to 7A. That is a serious current.

I think this is a bit of a design flaw and replacing the bulb holder will only help if Audi uses a more suitable metal for the current ones. I don't know if they do.

This week I thought hard of a solution that did not involve running off to Audi and that had to be mostly reversible just in case it wouldn't work out. I think I've got it now.

First I looked for a suitable point for creating an additional earth connector, without the need of drilling a hole in the body. Found it:

Image


This is one of the 4 bolts holding the beam behind the rear bumper on to the bodywork. I connected a 50cm wire with a soldered O ring to it to this point. Applied some wax afterwards, protecting the bolt and connector from dirt and water.

Next I looked up a suitable place to drill a hole in the bulb holder plate:

Image


I cleaned the 'wire' locally a bit (after taking the picture) and put a screw in with washers at both sides:

Image


And the other side:

Image


Now the mounting on the car. Used a wing nut so I can undo the additional connector (and the whole light unit for that matter) without the need for any other tools than those supplied with the car. Here you see the light unit turned upside down, with the additional earthing connector connected.

Image


The location of the connector was chosen in such a way that the likely hood of it getting into contact with water is quite small. Still after making the connection, I applied again a bit of wax.

So that's it: an additional earth connector to the light unit. It's installed on the right hand side now. When comparing the units left and right, I can see that the intensity of the rear lights and the reverse lights is more stable with the indicator switched on than on the left hand side. A clear sign that on the left, the unit is still suffering from a poor earth connection. Hope this solution will prove itself for years to come. Took about an hour to install and a couple of hours of thinking. If it doesn't work, it's all reversible except for the 3mm hole in the bulb holder. But that holder would have to be replaced anyhow in case this fix doesn't work long term.

For the sake of completeness: this is the right hand side unit on a left hand drive car. When applying the same 'upgrade' to your own car, ensure that it's the correct 'wire' in the unit that you're connecting to earth. Otherwise you'll create a short circuit. And at all at own risk. Feedback is welcome.

Edit: restored the links of the pictures. Thanks to Brittan for copying them for the knowledge base
Edit2: restored the links once more. Pictures are now hosted by the forum itself
 
Discussion starter · #9 ·
peter-ss said:
Pre modification, with the engine running, the car in reverse, the brake lights on and the indicator flashing the voltage on my reverse light was dipping to just over 10V and you could see the intensity alter as the indicator flashed.

Post modification the voltage only dips to 12v and the intensity of the reverse light doesn't seem to alter at all.
I did the exact same test afterwards.

peter-ss said:
I'm guessing that the reason you don't get the Ford Escort style disco lights is due to the lights being electronically controlled, rather than all being physically connected together, so they can't earth through each other.
It's either that, or the drivers inside the control unit that provides power to the various bulbs (based on a canbus signal) can't provide enough current for two bulbs.
For the rear lights/break lights the individual connectors are also there for switching on just 1 bulb in case of using the parking light: indicator to one side, ignition off.

Perhaps if one of the bulbs is actually broken, the VAGCOM may tell exactly which one it is.

By the way: the earth connectors inside the boot space had a bit of resistance too on mine. They are in the corners close the the light cluster, underneath the carpet.
 
Discussion starter · #15 ·
Hi and welcome, as I see this is your first post here on the TT forum :)

I'm not exactly sure which bolt you are referring to (a picture would help), but any bolt that makes proper connection to the metal of the body itself should provide for a proper earth connection. As long as the earth point doesn't get wet it should be OK for a long time.
 
Discussion starter · #17 ·
As long as there is proper metal contact towards the body, it'll be OK. Reason why I used the bumper frame bolt is that I didn't need to damage any paint, plus I wouldn't be connecting to aluminium. Aluminium combined with another metal can cause electrolytic corrosion, something that you'd want to avoid when it comes to an earth connection.

For protecting I'd use a spray on anti corrosion wax, just like the one they use nowadays to protect the body work from rusting on the inner side. I think the bumper bolts are high enough behind the bumper to stop them from catching dirt and mud from the rear wheels. A spray of wax will take care of protection for many years to come.
 
Discussion starter · #25 ·
Both inner indicators giving up seems too much of an coincidence. Try the rear lights again without the additional earth connectors, check the bulbs and check how they sit in the bulb holder.

I don't see a connection with the fog light. Perhaps it is broken?
 
Discussion starter · #27 ·
:)
So you've done the right thing then, lights are brighter so we're sure the earth connection is working.
You didn't accidentally forgot to re-attach the fog-light earth connector? I'd expect that one to be shared with the rear light units.
 
Discussion starter · #29 ·
Each unit has its own earth connector. So with yours the left one needs to be fixed. I'd fix the right side too as most likely that one will fail when it's cold and wet, if you see what I mean.
 
Discussion starter · #37 ·
The connectors and pins are available separately through the dealers according to various posts on various forums. I'm afraid I don't have the part numbers though.
 
Discussion starter · #42 ·
Guess who's here? :wink:

My reason for typing this

"For the sake of completeness: this is the right hand side unit on a left hand drive car. When applying the same 'upgrade' to your own car, ensure that it's the correct 'wire' in the unit that you're connecting to earth. Otherwise you'll create a short circuit. And at all at own risk. Feedback is welcome.


is that I have a LHD just like yourself. I'm not in a position where I can check all ins en outs on RHD cars. Most things are the same, some are surprisingly different. Peter-ss did great work on working out the details for RHD cars and turning this topic into an article for the TT owners magazine. As far as I'm aware it all turned out to be exactly the same.
However as I cannot foresee the skills of anyone reading this topic I had to put in the usual warnings. Come to think of that, the internet if full of far more stupid things to do that don't come with a warning.

Anyhow, do try this yourself (that's an encouragement :) ) and do pay attention that you're indeed adding an additional earth wire to the earth wire in the unit. The earth wire in the loom is brown, so follow that lead and if you have some measurement equipment you can be a 1000% sure if 100% is not enough.

I do recommend doing both sides. It'll keep the brightness of the bulbs the same and when done right you can put this whole issue behind you for a long time. Next time you see a bulb warning, it'll probably be indeed a broken bulb and not some vague issue in a socket. :D

Edit: Time flies. It's almost 3 years since I applied this fix. My light units are still working fine. 8)
 
Discussion starter · #49 ·
Fair question. I think the answer would be no. At least I can't remember seeing a post that all was repaired free of charge and that the customer was provided with a gift to compensate for the inconvenience. :lol:

It also wasn't my intention of this topic in the first place. Sometimes you have to work around Audi instead of with Audi. I feel this rear light issue is such an occasion.

Please do feel free to start a new topic with a poll regarding the earthing issue of the rear light. I'd prefer keeping this topic focussed on the workaround itself. I always keep an eye on this one when it pops up again in order to see if more assistance is required and so on.

:D
 
Discussion starter · #54 ·
imac said:
Hi All, I am about to fix this problem on my 2007 TT in Australia.
I have bought the new left & right housings and new connectors and it comes with some wires in the kit. Does anyone have instructions on how/what needs doing to the wiring using the OEM products?
Hi and welcome!

Complete wire looms? If that's the case then you'll have to replace the wire loom and probably assemble the connector once the wire is pulled through the hole behind the light unit. The wires are colour coded, so its rebuild following the original, I'd guess.
 
Discussion starter · #58 ·
tttony said:
imac, you probably don't want to hear this, but if you are going to add the new additional earth wires anyway, you didn't need to buy the new bulb housings, connectors and wires.
Yup. That's why I started this topic in the first place: how to avoid paying Audi twice for the same cr@p.
 
Discussion starter · #59 ·
imac said:
Thanks for the welcome Driver,

I bought the 2 light housings, 2 connectors, new lamps and the connectors came with 6 new yellow wires with sockets crimped on both ends.
I am assuming these are to replace the burnt sockets and splice into the old brown ground wire.

My main question would be, do I need to do the described Mod on the new housings to stop this from re-occurring? Or have these new light housings been fixed to remove the faulty design?
The ground pin on the new housings look the same as the old ones to me except they are they still clean.
Given the stuff you've bought, you can now opt for the OEM repair. Replace the burned pin in the socket and install the new bulb holders. My suggested repair is the bushmen repair using stuff from the local hardware store. Now should you install the additional wires? Well apparently down under the climate circumstances do trigger the same fault. You could try and install all goodies plus using some sort of grease to protect the connectors from oxidising. And then hope for the best. Or install the additional wire while your at it. And since you'll have to cut into the original wire anyway, I'd connect the additional wire to the original wire and effectively only doubling the earth connector on the bulb holder. Takes less effort. Do solder the connections. Don't use crimp tools or other methods. Up to you.
 
Discussion starter · #63 ·
you're welcome.

The only reason why I used different earth points is because I wanted to avoid cutting into the original wires. You could double check the connection between the wires and the body. The brown wires are connected to the body quite close to the light units, underneath the boot carpet. You may need to remove the Styrofoam inserts. A bit of sand paper on both the body and the connector and it should be 100%.

The main issue is indeed the overloaded pin in the socket. Bypass that pin and the problem should be solved for a long time to come. Like said earlier: since you have to splice the wire anyway, by all means start the bypass from there. It's easier than undoing the bumper bolts and starting a new earth connection from there.
 
Discussion starter · #65 ·
:D You're welcome. I'm glad this fix is helping so many fellow TT owners.

The front light units have their own mass connectors. They can start playing up too. I found a bit of corrosion a couple of years ago between the connector and the sill on the earth connectors close to where the gearbox sits. A spanner and some sandpaper is required to clean them. A bit of anti rust spray will protect them from rusting any time soon again. I'm not 100% sure though if those earth points are the points for the head lights too. Checking them won't do any harm though. If I recall correctly another forum member managed to fix one of his front bulb error messages by checking those connectors.