2003 Toyota Tundra is the first generation of a pickup truck made by Toyota. The first generation was produced from May 1999 to November 2006. And what we are talking about this time is the Toyota Tundra that uses 5VZ-FE engine. To drain the fuel from the fuel tank to the engine combustion chamber needed a tool called a pump or fuel pump. In other words the fuel pump on the engine serves to pump fuel from the fuel tank to the carburetor to the engine.
The diagram below shows when the machine is turned on, current will flow from the terminal ST relay coil ignition switch to the starter relay active starters, then the current will flow into the circuit opening relay coil L1. So that electricity will flow to the fuel pump and can work. When the ECM get a signal from the STA and NE, Tr be active and current flows to the coil L2, then the relay will be activated and the fuel pump operates. While the NE signal is generated (engine running), the ECM keeps Tr ON (circuit opening relay ON) and the fuel pump also keeps operating.
The following schematic illustrates the 2003 Toyota Tundra Fuel Pump Control Circuit Wiring Diagram: