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Wiring Fundamentals: Fuel Injectors

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Fuel Injectors

03.15

00:00 - For your ECU to control the air to fuel ratio of the combustion charge entering the engine, it needs to control the length of time it opens the fuel injectors for during each engine cycle.
00:10 Fuel injectors can be broken down into two main groups, high impedance and low impedance.
00:16 There are other types in use, direct injectors that inject directly into the combustion chamber for example, but their wiring and configuration is a more advanced topic outside the scope of this course.
00:26 High impedance injectors are the most common type you will encounter, and they're also the simplest to wire and configure.
00:32 They have two terminals, one of which is supplied with 12 volts from the power supply system when the engine enable switch is on, or the key barrel is in the on position.
00:41 The other pin is wired to a dedicated injector output channel of your ECU.
00:46 Which pin you choose to use on the injector does not matter, they're not polarity sensitive and will work either way.
00:52 To open the injector, the ECU simply provides a ground connection, allowing current to flow from the power supply system, through the injector, through the ECU, and to ground.
01:02 The amount of current that will flow is limited by the injector's impedance.
01:06 The simple switching strategy is known as saturated drive which is a term you might frequently see in ECU documentation.
01:14 It simply means that the injector output is capable of driving a high impedance injector.
01:19 Low impedance injectors are wired in exactly the same way as high impedance injectors, however they're controlled by the ECU in a more complicated fashion.
01:27 If the ECU simply provided a ground connection for the injector, and allowed the current to flow in the circuit unrestricted, the low impedance of the injector would not limit the current sufficiently.
01:38 Too much current would flow and either the ECU or the injector would be damaged.
01:43 To correctly control a low impedance injector, the ECU will initially allow a large current to pass through the circuit which opens up the injector very quickly.
01:51 Once the injector is open, the ECU will then limit the current to a lower level for the rest of the injection period.
01:57 As less current is required to hold the injector open, compared to opening it initially from a closed position.
02:04 Once the injection period is over, the lower level of current flowing in the circuit means there's less energy in the circuit overall, and when the ground connection is severed, the injector can also close very quickly.
02:15 Your ECU documentation will tell you if any of its injector output channels are capable of driving low impedance injectors.
02:22 You can determine if your injectors are low impedance or high impedance by measuring the resistance across the two terminals with a multimeter.
02:28 For a low impedance injector you'll typically see a reading of 2.5 to three ohms.
02:33 With a high impedance injector giving a reading of around 12 ohms.
02:37 It is possible to run a low impedance injector from a saturated injector drive by adding some resistance to the injector circuit.
02:45 This is called ballasting and is accomplished by adding a 4.7 ohm, 25 watt resistor to the injector circuit, one for each injector.
02:53 This resistor can either be before or after the injector in the circuit, but it's typical to have the power supply system connected to all the resistors, with the other terminal of each resistor then connected to each injector.
03:05 This is frequently seen on OEM vehicles.