Fuel Pump Control

Fuel Pump Control:

PURPOSE
The fuel pump is controlled by a relay, resistor and a modulator. The relay is used by the ECM to provide power to the fuel pump. The modulator is used by the ECM to control the ground circuit, switching between full ground and partial ground through the resistor.

OPERATION
Positive circuit
The fuel pump relay operates for a few seconds when the ignition is turned ON. The relay only remains engaged if the ECM detects the engine running, or the starter operating. Power is supplied to the relay when the key is ON, the ECM controlling relay ground.

Negative circuit
At low engine speeds, the source current flows directly through the fuel pump to ground and the pressure discharge reate remains constant,
As engine speed increases, the ECM sends a signal out to the modulator to allow the source current to flow through the modulator and resistor to ground.
In this way, the amount of current flowing through the fuel pump varies with engine speed, thus fuel pump discharge is controlled.