Rear Light Assembly Ground Fault ~ Non-OEM Repair

TT-driver · Mar 2, 2012

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:

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:

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

And the other side:

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.

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

igotone · Mar 2, 2012

Excellent - should be very helpful as this seems to be a common issue.

wja96 · Mar 2, 2012

That's a really useful post. Definitely should be sticked or added to the KB.

Thanks for posting this.

IC_HOTT · Mar 3, 2012

cheers man, very informative and useful :wink:

Sifu-TT · Mar 4, 2012

Nice, just had mine repaired at the dealer under warranty after buying the car a week ago.

peter-ss · Dec 8, 2013

I too now have this issue - My outer offside light has become intermittent and on inspection I have very slight distortion of the plastic around the earth pin.

For now, I have just cleaned up the connector but will be carrying out your modification in the very near future.

The only thing that intrigues me is why a dodgy earth connection manifests itself in just the one bulb going out viewtopic.php?f=19&t=162142&hilit=rear+light+fault. 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.

I drew the layout of the rear light cluster and the pinout of the connector (offside) and, interestingly found that both indicators are fed separately as are the two Brake/Tail lights - Maybe this is for the bulb failure warning system?

Thanks for the write up.

ZephyR2 · Dec 8, 2013

These were the kind of problems I was dealing with on old Fiats and Alphas 20 years ago. How can a premium car manufacturer like Audi still be getting it wrong?
FFS :x

peter-ss · Dec 21, 2013

I have now carried out the supplementary earth modification and the difference in my rear lights is visible.

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 now feel a bit happier knowing that my rear light fittings aren't likely to melt!

TT-driver · Dec 22, 2013

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.

peter-ss · Dec 22, 2013

TT-driver said:
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.

I hadn't noticed that this was the case and it explains why they're not just paralleled up.

I wonder if there's a reason for the indicators being fed separately too?

TT-driver · Dec 23, 2013

peter-ss said:
I wonder if there's a reason for the indicators being fed separately too?

I looked at the wiring diagram and the J519 unit descriptions. But I can't find the reason...

V11TNO · Dec 23, 2013

I have just carried out this modification and its worked a treat!

Thankyou very much for the 'How to'

UMZ · Mar 13, 2014

I'm no technician and although this guide seems spot on I went to my local Audi garage in Stockport. They wanted £120 to resolve 'the known issue'. I got them down to £80 as they could clearly see I've been through 2 new rear bulb cluster that have been burnt through because of the earthing issue. For this they will replace the cluster £27rrp, change the pin £5 RRP, solder in fix the earthing issue plus labour. I think that's reasonable but still pissed off. Been stopped 3 time by the police in 1week due to no rear indicator and the fog this morning. Annoying.

Stolberg · Mar 22, 2014

Hi, I have had a go at doing this earth bipass mod, as I too was getting the intermittent tail light failure on my 08 TT.
I had tried the cleaning of spades and connectors to no avail.
My question is: I have placed the additional earth connector to a bolt on the chassis/bumper (metal) adjacent to the light fitting, rather than down and underneath the car like shown in your photographs.
Will this work? The lights are working. Is this a worthy earth point???
If you remove the light cluster the bolt can be seen nearest to the side of the car. It has made for a shorter earth wire from cluster to earth point. I also used male bullet/female terminals to allow easy disconnect when changing a bulb, rather than a wing nut.
Thanks for this mod... I was quoted 120 just to look at the problem!!!

TT-driver · Mar 22, 2014

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.

Stolberg · Mar 23, 2014

Yes my first post, but have been visiting the forum since buying my TT last year. It has saved me cash and put my mind at rest on many occasion

The bolt point I used appears to be one of the fixing points of the bumper. I cleaned the points before fixing but did not apply grease or wax to protect the points. What products would you recommend?

After doing the wiring, I had a look underneath the car and wasn't sure which of the two bolts I could have wired to. I would have thought the chances of water getting at the earth would be greater here?

Fingers crossed the point I have earthed to will have done the job and I won't hear that annoying fault signal again. If it does work... a picture may be a good add on to this how to. You would have to pardon my novice wiring skills though!
Thanks for the feedback.

TT-driver · Mar 24, 2014

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.

mwad · Mar 24, 2014

Guys, what is the usual fault ?

UMZ · Mar 25, 2014

Hi guys got my light sorted from Audi. They replaced the bulb holder the pin connector faulty wiring,a bulb and the earthing issue for £114. All ok and the work looks decent and clean.

Stolberg · Mar 25, 2014

Sporadic tail light failure message on start-up, drive a mile or so, press the break, signal disappears and tail light comes back on again. Amassive pain!

mwad · Mar 26, 2014

Stolberg said:
Sporadic tail light failure message on start-up, drive a mile or so, press the break, signal disappears and tail light comes back on again. Amassive pain!

Damn........

Started getting this on Monday.......

Stolberg · Mar 26, 2014

It is really easy to do (the OEM repair), as long as you have right tools and components.
Took me half an hour to do.
You'll need:
Wire strippers, Wire cutters, Drill, Connector Crimper Tool (I used this rather than soldering iron), Electrical Tape, Thick 1m length of Earth wire depending on whether you are attaching to the bolt under the car or the bumper bracket next to the light fitting, wax or grease to protect earth connections from water.
1-off Male and 1-off female bullet connector - So you can detach earth wire when removing light cluster in future. (wing nut is another option as per top of this feed).
2-off connectors. The connector needs to be big enough to fit over the bolt you are connecting to (on car and bolt fitted to light fitting).
4-off washers and one nut and bolt. The smaller the better. I will try and take a picture in the next day or so to show how mine looks.
Trust me, I am a real novice with this sort of thing, but it is very simple to do.

Philplop · Mar 26, 2014

I did this yesterday. Now I've noticed that my innermost rear indicator bulbs don't work (this may have been the case before the fix, though) and my DIS is saying the rear fog isn't working. Jesus!

Stolberg · Mar 26, 2014

Nightmare. Did you put the connect to underneath the car or next to the light cluster/bumper bracket?

I guess you cleaned up the area you connected to, so that it was metal to metal (no paint or muck in between). And you placed the connector inside the light to the outer most metal wire run as per the pictures above?

[smiley=bigcry.gif]

TT-driver · Mar 26, 2014

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?

Philplop · Mar 26, 2014

I actually fed it through into the boot and used the earth point in the rear corner, which I think is where the rear lights earth. I cleaned the contacts.

It's definitely the right 'wire' on the light, as I traced it myself before checking this how-to.

The new earthing has noticeably improved the light brightness, as I tried with and without the new earthing. Tested the indicators with and without too, with no luck.

I thought that the fog light may be unrelated, just seemed a big coincidence. I just haven't had a chance to check yet.

Thanks for the help and the initial how to.

Rear Light Assembly Ground Fault ~ Non-OEM Repair

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