Quantity servo potentiometer (Engine management system - EDC)

Quantity servo potentiometer

Mounted at the top of the servo control unit is a rotary potentiometer. As the eccentric shaft of the servo control unit moves, the wiper of the potentiometer is moved across the resistive part of the potentiometer. The voltage output from the potentiometer is proportional to the position of the control spool. From this input the ECM can calculate the quantity of fuel being delivered by the pump and adjust the amount of fuel being delivered by energising the control coil to move the control spool.

Injection timing device

1. Plunger
2. Solenoid
3. Spring
4. Pump feed pressure
5. Pump internal pressure

The injection timing device consists of a spring loaded plunger and a solenoid. The spring loaded plunger acts against a cam plate driven by the FIP shaft. The spring loaded plunger moves under the influence of the pump working pressure.

The injection timing solenoid is responsible for the characteristic ’buzzing’ sound when the ignition is turned on. The solenoid operates at 50 Hz and by varying the operating duration, the advance/retard of the fuel injection point can be adjusted.

With the engine stopped and the solenoid de-energised, the plunger (1) is in the fully retarded position. When the engine is started, the fuel pressure (4) passes through the plunger (1) and acts on the plunger face. Simultaneously, the solenoid (2) is energised by the ECM. Depending on the signals received from the sensors, the solenoid (2) is energised for varying lengths of time. This allows the fuel pressure (5) at the plunger face to relieve to the spring side of the plunger, which is at pump inlet pressure (5), retarding the injection timing. As the plunger moves, a cam plate, connected to the plunger via a pin, moves and adjusts the injection timing. The ECM continually modulates the solenoid, advancing or retarding the injection timing to achieve the optimum timing position.

The ECM receives a feedback signal from the needle lift sensor and will attempt to correct the injection timing accordingly by altering the signal to the solenoid (2). If a change does not occur or differs by a substantial amount, the ECM will assume a fault exists and reduce the amount of fuel injected.