Thanks for your replies. Here's more info that you asked for. The white wire goes to the hold down screws and the black goes to the centre stud of the sender. This was the configuration since I've had the truck and it used to work like this. The white wire is very brittle so maybe it 's broken somewhere. The problem is that the wires are inaccessible under the tank and I can only see the white wire under the cab, which goes into the loom so I can't see where it goes after that. I'll study the previous threads and hopefully I'll find out what the problem is and get back to you.