Peak and Hold 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.
Peak/hold injectors are low impedance injectors and are usually used in aftermarket high performance systems. Because they are more expensive and complex than saturated circuit drivers, they are not generally used with domestic production ECUs. When the ECU calls for fuel to be injected, it sends out voltage via the wire clips until a certain current level is reached (the peak part) (varies by injector size, company). For the duration of 1 pulse width, that current is slightly reduced and held (the hold part).
Appearance
Fig.1 Typical peak and hold injector
Principle of operation of the peak and hold injector
These types of injectors and drivers may also be called current sensing or current limiting. Peak and hold injectors are low impedance (0,5-5 ohms) and use a peak and hold driver to activate them. The peak and hold circuit opens the injector with a high current (Peak) pulse and then switches the current down to hold the injector open. The Peak current quickly opens the injector while the lower Hold current rating is used to keep it open for the duration of the ECU command. Because these injectors have larger physical parts and often work against high fuel pressure, they require an extra “kick” from the higher current to keep the opening and closing time of the injector stable at the higher fuel flow rate. The current required to open a (fuel injector) solenoid is several times (usually 4 times) greater than the current necessary to merely hold it open. The current is then automatically reduced to the sufficient holding level for the duration of the input pulse. Advantage of this design is the minimized injector "on" time, resulting in faster response and the total power consumed by the system is dramatically reduced. Disadvantage is the increases coil heat, which can lead to failure over time.
A current flow diagram, with not much detail, should look like this in fig.2:
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 between 2 and 5 ohms;
- 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.
4. Compare result with the waveform in fig. 3.
Fig. 3
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. Channel B:
Plug the 10:1 Attenuator to channel B 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.
3. Channel C:
Plug the 10:1 Attenuator to channel C of the CarScope Pro/LAN/Plus 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.
4. Channel D:
Plug the 10:1 Attenuator to channel D of the CarScope Pro/LAN/Plus 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.
5. Start the engine, warm it to operating temperature and leave it idling
6. Compare result for each injector with the waveform in fig.4
Fig.4