A4/S4 Quattro Fuel Gauge Repair
Your car may have some miles on it now. You notice your fuel gauge starting to act goofy? Reading too low or not at all? More than likely it is the sending unit in the tank. A common problem with vehicles over 60-70K miles. The problem is the wire on the sending unit. Over time, it's up and down movement causes it to fray and break in half. Audi would gladly have you buy a new sending unit, but yours can be repaired with some simple wire and a soldering iron.
Pull the carpet back in the trunk and remove the 3 screws in the cover.
Disconnect the electrical connector. Take a hammer and punch and tap the big plastic nut loose. There are little ridges all the way around it making it easy. Audi has a special tool to do the job but it is not required. Just be careful and you won't damage it. Once that is removed, disconnect the 2 hoses. They have 1 time use hose clamps on them. You can reuse them if you were careful about removal, or you can replace them. Be careful when you remove the hose pictured here on the top. It will be pressurized and fuel will spray out. Have a bolt or something ready to plug the line.
Carefully pull up on fuel cover. There are wires and 2 hoses connected to it. Undo the 2, 2 wire connectors and the squeeze/snap connector that has the 2 clear plastic lines running to it. Be careful not to break anything. You do not need to undo the last clear hose that is accordion shaped.
Now comes the fun part. You have to reach down in the tank (careful not to drop anything in!) and undo the sending unit. It can be tricky if you are not sure what you are looking for. Part of the problem is, you can not see what you are supposed to undo. There is a tab on the side of the sending unit housing. Push in on that and then you can lift the whole thing out. Here are some pictorials that should help you in finding it and feeling for it.
You have it removed. It probably looks like the below picture. Frayed and broken wire. I went about fixing it by running a new wire from the connector on the housing side to the metal arm. This part does require some minor soldering skills. I spread the connector on the top side and crimped a new wire down in it, then ran it inside the plastic tube cover. I de-soldered the old wire on the arm, and then re-soldered the new one on. I just used regular stranded wire on mine, but a better solution would be to use wire that is found in speakers. You can use the wire that connects the speaker cone to the speaker connector. That stuff is very flexible. I happened to get an old useless speaker that I was able to cut apart and re-use the wire.
Update: My fix died with standard wire as shown here (the wire in red) I replaced it with speaker wire I got from an old pair of speakers. That stuff is much more flexible, almost exactly like the OEM stuff. It should hold up longer than a year at least.
Also got this from another AW
member. You may need to do this to calibrate it. I did not. "After
the repair, I also confirmed the correct resistance across the float's range
(~50 full and ~200 empty).
Turns out, I just needed to run the float up and down a few times manually with the power on to recalibrate the gauge-sender relation."
That's it! Installation is reverse of removal.