F 550 2WD Super Duty V10-6.8L VIN S (2000)
Fuel Pump Relay: Description and Operation
The Fuel Pump (FP) is a PCM output signal that is used to control the electric fuel pump. With the electronic EC power relay contacts closed, vehicle
power (VPWR) is sent to the coil of the fuel pump relay. For electric fuel pump operation, the PCM grounds the FP circuit, which is connected to the
coil of the fuel pump relay. This energizes the coil and closes the contacts of the relay, sending B+ through the FP PWR circuit to the electric fuel pump.
When the ignition key is turned on, the electric fuel pump runs for about one second, but is then turned off by the PCM if engine rotation is not detected.
For applications with two speed fuel pumps, a normally closed low speed fuel pump relay (Figure 48) is wired into the fuel pump ground circuit. With
the low speed fuel pump relay contacts in the normally closed position, there is no extra resistance in the ground circuit for high speed operation. For low
speed fuel pump operation, the PCM will ground the Low Fuel Pump (LFP) circuit, which opens the relay contacts. With the relay contacts open, the fuel
pump ground circuit now passes through a resistor that is wired into the circuit.
