Fuel quantity servo potentiometer (Engine management system - EDC)

Fuel 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 injection pump shaft.The spring loaded plunger moves under the influence of 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 frequency the ECM alters the fuel pressure on the plunger, moving it against spring tension. By moving the plunger against the spring, the height of the plunger acting against the cam plate is changed, retarding the ignition. Injection timing is fully advanced with the solenoid de-energised.

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. 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.