Trailer connections

[alogbe]

I have a 2007 A6 Avant with an Audi towing attachment, and I'm trying to find out why the right-hand indicator on the trailer isn't working.

Using a circuit tester I got a reading of around 11-12 volts on each bulb circuit except the non-working indicator. When I used the tester on the car's attachment socket, I got a voltage reading on each pin. So I'm fairly sure the fault is in the trailer wiring.

However, the voltage reading on the socket was only about 3-4 volts (same on each pin). I'd like to understand why: can anyone explain?

I also noticed that the ground connection (white wire in the standard ISO format) doesn't seem to have any effect; none of the trailer lights work unless the trailer is actually on the towing bracket, which suggests that the ground connection is really through the bracket. Is this normal?

[Alan02]

No, it seems like the white earth lead is either wired to the body of the trailer, or it is nipped or broken and shorted to the trailer. Do a continuity check from the trailer lamp earth to the plug, and again from lamp to trailer chassis. You should get a good reading from lamp to plug.