I also tried swapping the 2 brake light bulbs around, both bulbs light up, but after a while the outer most one fades/goes out.
I think I'll buy 2 new bulbs tomorrow just incase, looking inside the bulbs, the metal bits which hold the filament look rough/brown rusted?
Don't know if the bulbs themselves are causing too much resistance?

 

Similar symptoms to what I had. Pay the £24 for the bulb holder in the first instance and go from there. It's highly likely that the bulb holder will corrode and go at some point, so as well to do the cheap and easy fix first?

 

1) Is it just corrosion/resistance between the connector and the pin I need to worry about, or is it surface corrosion across all the metal stripes in the bulb holder tray (visually everything looks fine).

3) How hard is it to replace the female connector end? Can you just replace the plastic connector, or does the whole wiring need replacing?

4) (forgive my lack of electrical knowledge).
With connecting the another earth, can I just use a piece of wire to connect to the metal stripe in the tray, and connect/splice to the brown wire just before the female connector, by passing the actual point the connector and bulb holder meet.

5) (forgive my lack of electrical knowledge).
If I want to test the resistance on the connector and the battery with a ohm meter, what should I be touching with the 2 needles? The inside of the female connector with the brown earth wire, and then which terminal of the battery??

1) Yes. Only corrosion where two metals meet matters.

3) you'd 'only' need to replace the pin inside the connector, provided the connector isn't molten. Audi does sell them as replacement part. No need for replacing the wire as a whole.

4) Yes, the point of the additional wire is bypassing the overloaded/corroded connector. Only reason why I choose to use a different earth point for me was to leave the original wiring in tact. Normally I'd like to avoid tampering with original wires. Think of water ingress and so on.

5) Measuring the resistance of the whole earth chain would be measuring from the earth strip on the bulb holder all the way to a known good earth point: the battery itself. Alternatively you could measure the remaining voltage on the earth strip in the bulb holder. In an ideal situation the voltage on any earth point in the car is zero. As soon as there is any resistance in the earth connectors a voltage starts building up. Once you get to the positive side of the bulb (which effectively is a resistor too) you'll measure something like 5 volts or so and 12 or more when breaking. Under normal conditions the rear lights are dimmed, only when acting as brake lights they get full voltage.

The connector in the boot itself (under the carpet) can suffer from a bit of resistance too, but not as bad as the connector at the bulb holder.

 

Thanks. How do I actually open the connector to get at the pin part (tried randomly prying with screw driver, couldnt seem to find a seam).

I've sanded point where the pin on the bulb holder, and the inside of the earth pin in the plastic connector make physical contact with each other the best I can, not had any problem today driving to work, but it can go days without issue.
If I could get the plastic connector open I could sand the female pins all over, or would that make no diference as the other sides don't make contact with the bulb holder pin?

Phoned Audi today, ordered a new bulb holder tray, was gonna order the plastic connector as well, but they said its just the plastic housing, no pins inside.

They said they can replace the earth wire for me for £90 for labour, excluding parts...
Think im gonna DIY my own earth wire, to the point you mentioned under the boot.

How does earth work? Can I just connect to any metal part of the car?

 

I haven't opened up the connector yet, so I'm not sure exactly how it's done. But before making any attempt, do take note which colour wire goes where.

Perhaps there is some sort of locking piece of plastic that keeps the pins in place, often has a different colour like pink or so. You'd need to pry that out. Or the pins are held in place by small little hooks that prevent them from popping out at the front of the socket.

example:

The car's body is basically one large conductor. Anything metal that is welded to that body or is screwed into it becomes part of that conductor. But beware: paint acts as an isolator. It's the bare metal that is the conductor.

 

Thanks for you reply on the rear light issue I had.

I went and bought a new bulb holder tray, a new connector and earth wire. But after the scraping the contacts a few more times with a flat screw driver everything has been ok for about a week now.
The earth wire Audi sold to me has 'hooks' connector on both ends, like both ends are ment to be plugged into plastic connectors, but I could easily cut one end and fit some kind of spade terminal, so it can be screwed to some part of the car.

I hope the problems gone, but I read other users having to clean the contacts once every 3-4 months

I figured out how to open the plastic connector, it has a purple clip which locks the wires in place, it just slides out.
But removing the wires are real fiddle, as you mentioned the wires have hooks on them.
I foolishly put the earth wire into a random hole just to see how it went in and it hooked in, took me about 1 hour to get it out.
I tried tiny screw drivers (snapped a few), paper clips, needles, tooth picks, in the end I found that some really fine sharp tweezers and some force works. The hooks on the both sides needs to pressed at the same time, not easy.

There must be some kind of tool you can buy especially for unhooking the wires??

 

There's probably a tool available, but I never looked for it. Usually I dive into the wife's sewing kit and take what ever pin or needle that works for me.

I believe the newer connectors are gold plated so less chances for corrosion. Clear picture how the plug works

 

To conclude this thread.

Scraping the connector on the bulb holder/tray worked for a short while.
But found myself scraping every month or so, till finally it didnt work no more., brake light would fail a second after turning on.

Changed the tray (and all the bulbs for good measure) just after xmas. So far no more problems.

I couldnt get the wires out of the originally connector, they seemed to be glued in.

 

I couldnt get the wires out of the originally connector, they seemed to be glued in.

There's a piece of locking plastic that keeps the pins in place (the purple thing). And there might be small pins that block too. Yes it's not easy to get them out. The seal against moisture may be molten too. That doesn't help either.

 

Finally found a tool for unclipping the metal clips inside the platic part.

Its called a VW terminal tool
http://www.lasertools.co.uk/item.aspx?item=3175

This eBay user who also sells the plastic plugs, seals and terminals, pointed me in the right direction.
He uses the laser brand.
http://www.ebay.co.uk/itm/171630612481

 

Yup that's the one. Nice tool for an evening guessing: what's this tool used for? :lol: