Mono Injector
General description
Injectors are electrically operated valves which accurately control the quantity of fuel delivered. By adding the fuel to the air sucked in by the engine, a mixture is created with the required fuel/air ratio.
Single-point injection (SPI) uses a single injector at the throttle body (the same location as was used by carburetors). This system features one centrally positioned fuel injection nozzle. It was introduced in the 1940s in large aircraft engines (then called the pressure carburetor) and in the 1980s in the automotive world (called Throttle-body Injection by General Motors, Central Fuel Injection by Ford, PGM-CARB by Honda, and EGI by Mazda). Since the fuel passes through the intake runners (like a carburetor system), it is called a "wet manifold system". The justification for single-point injection was low cost. Many of the carburetor's supporting components such as the air cleaner, intake manifold, and fuel line routing could be reused. This postponed the redesign and tooling costs of these components. Single-point injection was used extensively on American-made passenger cars and light trucks during 1980-1995, and in some European cars in the early and mid-1990s.
Appearance
Fig. 1 Single point injection (throttle body injection)
Principle of operation of the mono injector
In the beginning, because of the lower supply pressure of 0.75 to 1 bar, only a single-stage turbine pump is installed. The system pressure regulator has, as opposed to the Multi-point injection, no vacuum connection, because the system pressure regulation is independent of the vacuum pressure. The injection amount is exclusively dependent on the injection time, because the injection takes place above the throttle flap and not in the vacuum area. Apart from the Lambda sensor, the throttle flap potentiometer is an important sensor for the determining of the injection amount. In addition, the control device still also needs the air or the coolant temperature and the relationship to the crankshaft. The Mono-Jetronic (Bosch) controls the idling through a servo-motor with worm-drive and only relies on a throttle position sensor for judging the engine load. There are no sensors for air flow, or intake manifold vacuum. Mono-Jetronic always had adaptive closed-loop lambda control, and due to the simple engine load sensing, it is heavily dependent on the lambda sensor for correct functioning. The Multec (GM) controls the idling through a bypass.
Fig. 2
Possible damage to the injectors:
- Open circuit or short to positive or to ground in wire(s);
- No or poor plug connection conduction;
- Ground connection is loose or corroded;
- Mechanical fault in component.
CHECK RESISTANCE
- Make sure ignition is off and the engine is not started;
- Disconnect the two-pin injector connector;
- Connect a precise ohmmeter between the terminals of the injector connector. Resistance must be around 1 ohm or less;
- Plug in the injector connector.
TESTING THE OUTPUT SIGNAL WITH OSCILLOSCOPE
Injector Voltage vs Current
1. Channel A:
Plug the 10:1 Attenuator to channel A of the CarScope and connect a BNC test lead to the attenuator. Connect the red test lead to one of the injector wires and the black crocodile lead to the chassis ground.
2. Channel B:
Connect the CA-60 AC/DC current clamp to channel B.
Range ±20A
Clamp switch should be in 1mV/10mA position.
Switch the current clamp on, press the ZERO button before connecting the clamp to the circuit.
It is important to note that only one of the two wires have to be clamped, and not both of them. It doesn’t matter which cable is clipped with the current clamp: the positive or the negative one. This will only affect the polarity of the measured current. But incorrect connection will lead a reading of incorrect polarity. The clamp arrow matches the injector current direction.
Note: the CA-60A probe is supplied with a 4 mm banana plug type connectors so it cannot be plugged directly to a CarScope Pro oscilloscope. A banana plug to BNC adapter must be used to connect the current clamp to the oscilloscope.
Note: When performing a DC current measurement, always push the ZERO button on the clamp until the CarScope displays a zero line.
Important note: Only one of the two wires should be clamped, and not both of them. It doesn’t matter which wire will be clipped with the current clamp: the positive or the negative one. This will only affect the polarity of the measured current.
3. Start the engine, warm it to operating temperature and leave it idling.
Note: The test set-up may distort the recorded signals slightly.
Injector Voltage
1. Channel A:
Plug the 10:1 Attenuator to channel A of the CarScope and connect a BNC test lead to the attenuator. Connect the red test lead to one of the injector wires and the black crocodile lead to the chassis ground.
2. Compare result with the waveform in fig.3
Fig.3