Fuel Pump: Description and Operation

FEATURES
The fuel pump module has the following characteristics:
- The assembly rests within an internal retainer on the bottom of the fuel tank.

- The pump is mounted with its inlet at the bottom of the reservoir. This design increases satisfactory pump operation during extreme vehicle maneuvers and steep vehicle attitudes with low fuel levels.

- It is capable of supplying 140 L (29.0 gal) of fuel per hour at a working pressure of 269.0 kPa (39 psi) @ 13.2 volts.

- It has an internal check valve to maintain pressure in the system after the ignition switch is turned to OFF.

- It also has an internal relief valve to provide overpressure protection in the event that the fuel flow becomes restricted (for example by a clogged fuel filter and base or damaged fuel tube). Overpressure is limited to 850 kPa (123 psi), and reduced fuel flow will result if the pressure relief valve is activated.

- The module is protected at its inlet by a nylon pick-up screen. It filters dirt and contaminants which could plug or damage the internal fuel pump components. This screen allows passage of small quantities of water which may accumulate within the fuel tank reservoir.

COMPUTER CONTROLS
The electrical system:
- has a fuel pump control relay controlled by the Powertrain Control Module (PCM), which provides power to the fuel pump module through an Inertia Fuel Shutoff (IFS) switch, which is normally closed.
- provides power to the fuel pump module using PCM power relay signal from the Variable Control Load Module (VLCM) and a high speed fuel pump relay.

OPERATING CONDITIONS
The current path is dependent upon the operating conditions.

When starting or when engine fuel demands are high, the fuel pump module electrical supply will be at system voltage. At other times, voltage will be reduced.
- When the ignition switch is in the OFF position the contacts of both relays are open.
- When the ignition switch is first turned to the RUN position:
- the PCM power relay is energized, closing its contacts.
- power is provided to both the fuel pump relay and to a timing device in the PCM.
- the fuel pump runs through the contacts of the fuel pump relay.

If the ignition switch is not turned to the START position, the timing device in the PCM will open the ground Circuit 57 (after approximately one second). Opening the ground circuit de-energizes the fuel pump relay, opening its contacts, which in turn de-energizes the fuel pump. This circuitry provides for pre-pressurization of the fuel system.

When the ignition switch is turned to the START position, the PCM operates the fuel pump relay to provide fuel while cranking.

After the engine starts and the ignition switch is returned to the RUN position:
- power to the fuel pump is again supplied through the fuel pump relay.
- the PCM senses engine speed and shuts off the fuel pump by opening the ground circuit to the fuel pump relay when the engine is below 120 rpm or the engine stops.