If you measure the resistance between the sender terminal and ground you should see about 0 ohms when the tank is full, about 80 ohms when it is half full and about 160 ohms when the level makes the warning light come on. A completely empty tank is about 200 ohms.
Your gauge has three terminals for 12V, GND and the sender. You can test the gauge by connecting suitable resistors between the sender terminal and ground.
Your gauge has three terminals for 12V, GND and the sender. You can test the gauge by connecting suitable resistors between the sender terminal and ground.