| 1: Control Box (ECU) | 6: Cold Start Injector | 11: Throttle Position Switch (TPS) |
| 2: Injector | 7: Electric Fuel Pump | 12: Trigger Contacts |
| 3: Pressure Sensor (MPS) | 8: Fuel Filter | 13. Air Temp Sensor |
| 4: Coolant Temp Sensor | 9: Fuel Pressure Regulator | |
| 5: Temp Time Switch | 10: Auxilary Air Valve | 1 Bar = 1 atmosphere = 14.7 PSI |
The ECU determines how much fuel is needed and controls how long the injectors are held open during each piston stroke. The ECU will vary this duration in accordance with inputs from the following sensors:
- The main input is the intake Manifold Pressure Sensor (MPS) which measures airflow by monitoring the intake manifold pressure (for the oldies - vacuum) It is connected to the intake manifold via a rubber hose.
- Throttle position and relative movement of the throttle from the Throttle Position Switch (TPS), which is connected to the butterfly shaft.
- Coolant temperature from a Temperature Sensor, which is generally fitted in the cylinder head near the thermostat housing.
- Ambient air temperature from an air temperature sensor, which is generally located in the air cleaner box and,
- a set of dual trigger points, located in the base of the distributor which synchronize the timing of the fuel injectors to the correct piston strokes.
The entry point of fuel into the intake manifold. There is one injector per cylinder and they are electro-magnetically activated. The time they are activated is measured in m/s (milliseconds). The injector is designed to spray fuel into the manifold in an atomized state so that the fuel mixes effectively with the incoming air.
Vacuum in the intake manifold varies greatly for any given engine RPM with variations in throttle opening and load. The Pressure Sensor compares relative air pressure inside and outside the manifold and provides a corresponding voltage to the ECU. The lower the vacuum, the more the mixture is richened.
The coolant sensor when cold, tells the ECU to richen the mixture. As the engine warms up the sensor tells the ECU to progressively return injection duration to normal as it reaches operating temperature.
In later cars, the cold start injector will not inject extra fuel if the coolant temperature is higher than 95oF (35oC) Above this temperature the switch opens and no ground is supplied to the circuit therefore no cold start injection takes place.
Two or three more components (depending on year model) are used to aid cold starting, acting like an automatic choke would on a carburettor. They are the Auxiliary Air Valve, the Cold Start Injector and in later cars, the Thermo Time Switch. If you were starting a cold carburetted motor, you would pump the accelerator pedal a few times while cranking the engine, thereby making the accelerator pump squirt a little raw fuel down the intake manifold. The D-Jet system does this automatically.
The electric Fuel Pump and filter are located at the rear of the car, external to the fuel tank. The pump operates for a few seconds whenever the ignition is switched on. This allows fuel pressure to build up before the engine is cranked. It operates continuously when the motor is cranking or running. The fuel pump supplies fuel several times more than the required pressure to the Fuel Pressure Regulator.
The Fuel Pressure Regulator, maintains the injector supply pressure in the fuel rail at 2 Bar (approx 30psi) and can be adjusted by turning the nut on top of the regulator. It maintains the pressure in the fuel rail by restricting the amount of fuel returned to the tank.
A cold motor needs to idle slightly faster requiring a little additional air and fuel. This system uses an Auxiliary Air Valve, which allows a small amount of air to bypass the throttle butterfly when cold, gradually reducing the amount as the coolant warms up. This is the same as holding the throttle slightly open. The ECU automatically supplies the matching fuel as dictated by the pressure and temperature sensors.
Under the cover is a circuit board with 22 segments, across which three wipers are swept as the throttle butterfly shaft moves. One segment and one wiper make contact and signal the ECU when the throttle is closed. Another segment and wiper make contact to signal wide open throttle. The middle wiper sweeps the remaining 20 segments as the throttle is opened. Each time it "climbs" a segment, it signals the ECU to fire all injectors one additional time. This is, essentially, what D-Jet uses in place of a carburetor's accelerator pump.
The trigger points housed in the lower section of the distributor body are used as the trigger to synchronize the timing of the injector groups. These two contacts are present because the ECU opens the injectors in groups, not all simultaneously. These are the same two contacts, which are used to indicate engine RPM to the ECU, also used to control the opening of the two groups of injectors. Each group of injectors, has two injectors on 4 cylinder engines and three injectors on 6 cylinder engines.
The air temperature sensor is for fine tuning of the mixture. The Air temp Sensor causes the mixture to richen slightly when the intake air is cold. A failure of this sensor will affect performance and economy in cold weather areas.