Fuel Pump: Description and Operation

Fuel Pump:

The Fuel Delivery Module (FDM) includes a high pressure fuel pump, venturi jet pump, supply check valve, and a shuttle selector valve located internally to an in-tank reservoir assembly mounted from the fuel tank sender flange. The fuel sending unit is attached to the reservoir body and is a separately serviceable component.

Fuel Delivery Module:

HIGH PRESSURE FUEL PUMP
The in-tank, high pressure pump replaces the need for a two pump fuel delivery system which requires a low pressure pump in the tank combined with a frame mounted high pressure pump. The high pressure fuel pump is capable of supplying 27.7 gallons (105 liters) per hour at a working pressure of 39 psi (270 kPa). The pump has an internal pressure relief valve to provide overpressure protection in the event the fuel flow becomes restricted. Overpressure is restricted to 123 psi (850 kPa) and reduced fuel flow will result. The system pressure is controlled by a pressure regulator on the engine.


SHUTTLE SELECTOR VALVE
The shuttle selector valve opens the return line to the tank when the diaphragm force, resulting from the fuel pressure applied to the shuttle diaphragm, exceeds the opposing spring force. Thus, the return flow is directed to the appropriate tank on vehicles equipped with dual tank.


SUPPLY CHECK VALVE
The supply check valve, which is normally closed, opens when the outlet pressure from the energized pump exceeds the opposing check valve spring force. When the pump is de-energized (i.e., engine is shut-off), the supply check valve closes to maintain pump prime and fuel supply line pressure.


VENTURI JET PUMP
A portion of the high pressure flow from the pump is diverted to operate the venturi jet pump. The venturi pump draws fuel from the tank into the FDM reservoir to augment the engine return flow. This process ensures an adequate fuel supply to the pump during extreme vehicle maneuvers and steep vehicle altitudes with low-tank fill levels.

The inlet of the venturi pump has a nylon filter on it to prevent dirt and other particulate matter from entering the system. Any water accumulation in the fuel tank can pass through the filter without restriction.

Fuel Delivery Module Control:

FUEL DELIVERY MODULE CONTROL
When the ignition switch is turned to the "ON" position, the Electronic Engine Control (EEC) power relay is energized and provides power to both the fuel pump relay and a timing device in the EEC processor. Fuel pump power is provided through the contacts of the fuel pump relay and the inertia switch If the ignition switch is not turned to the "START" position within approximately one second, the timing device in the EEC processor will open the ground circuit (No. 97 T/LG) to the fuel pump relay. Opening the ground circuit de-energizes the fuel pump relay (opening its contacts), which de-energizes the fuel pumps. This circuitry pressurizes the fuel system.

When the ignition switch is turned to the "START" position, the EEC processor energizes the fuel pump relay to provide fuel while cranking. After the engine starts, the ignition switch is returned to the "ON" position, the power to the fuel pump is supplied through the fuel pump relay. The EEC processor senses speed and shuts off the fuel pump by opening the ground circuit (No. 97 T/LG) to the fuel pump relay if the engine stops or engine speed drops below 120 rpm.