Fuel Pump Relay & Fuel Pump Resistor (LS400)
Fuel pump operating speed is controlled by operating condition of the engine such as: starting, idling, light load or heavy load. When engine is cranking and start signal is received at Engine Control Module (ECM), ECM energizes circuit opening relay and voltage is applied to both sides of fuel pump relay. Fuel pump relay contacts are closed and voltage is applied directly to fuel pump in which fuel pump operates at high speed. After engine starts during idling or light loads, fuel pump relay is energized by FPR terminal of ECM and contacts in fuel pump relay open. Voltage is then supplied to fuel pump by going from circuit opening relay and through fuel pump resistor. Fuel pump now operates at low speed. If a problem exists in fuel pump relay or control circuit, Diagnostic Trouble Code (DTC) P1200 may be stored in ECM. See SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - INTRODUCTION article for retrieving of DTCs. Fuel pump relay is located in fuse/relay box at driver's side of engine compartment, just in front of strut tower. Fuel pump resistor is located below cover at driver's side front corner of engine compartment, near battery. See Fig 1.