Take a look at the tank where the sender is located. There will be a pink wire connected to the sender. If you disconnect it and make sure it doesn't touch anything , the gauge should read empty. Now connect the pink wire to a good ground. The gauge should read full. If it does, then your sender is bad or the tank ground is bad. If it doesn't read full when grounded, you have to gain access to the fuel gauge and find the pink wire. Run a ground to the connector on the gauge where the pink wire connects. The gauge should read full. If it does, you have a bad pink wire between the tank and the gauge. If it doesn't read full you may have a bad ground or power at the gauge or the gauge is bad.
What style fuel gauge do you have?
