Good idea, all we need know is some guidance... Knowing my luck I would blow the car up
All Audi earth wires are coloured Brown so not really to hard to identify the wires - unless you are colour-blind maybe!!!
As Mike says create a new one for test purposes from a good earth point like the battery negative terminal.
To check earth wires use a reasonable quality Multimeter and set to Ohms and make some measurements from the test point with respect to the negative earth terminal of the battery, (Infinity means open circuit, small number of Ohms like 1-2 Ohms is a short circuit.)
HOWEVER.... the level sensors do not have a direct earth on them!
Here is the circuit on the sensors:
How to test the G78 sensor.......
With the system fully connected and ignition on,
On G78 I would expect to measure a fixed voltage between G78 "5/+" [19] (positive) and G78 "1/-" [20] (negative) connections - the schematic shows the polarity which I have put in brackets. The headlight range control circuit probably has a diagnostic check to make sure this connection is made as the load is a fixed resistance.
NOTE: J431 is the Headlight range control unit behind the glovebox - it's respective connections are shown in [brackets].
Check this voltage at the control unit end and also the sensor connector end - voltages should be the same at both ends - or near as damn it. If not at the sensor end then there is a broken wire along the way.
The output voltage between "4/S" [15] and "1/-" [20] will vary as per the level of the suspension at the front - maybe you can move the suspension by jacking up and down the front??
Similarly measure the voltage at the control unit end and the sensor end with no change in suspension level - if they are the same voltage then working OK, if not then there is a break in the wiring.
Have ago at this and feed us back your findings.
