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Wiring Fundamentals: Engine Position and Speed

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Engine Position and Speed


00:00 - The engine position and speed sensors, also commonly called trigger sensors, are the most important in the EFI system.
00:07 If the ECU cannot accurately determine the engine position and speed, it cannot fire the fuel injectors to deliver the correct amount of fuel and cannot trigger the ignition coils to provide a spark to initiate the combustion process at the correct time.
00:20 At best this could result in a rough running engine that hesitates.
00:24 At worst, it can result in severe engine damage.
00:28 For this reason they are always the first sensors I consider.
00:31 And they have some particular wiring requirements.
00:34 There are three common types of trigger sensor, optical, hall effect, and variable reluctor.
00:40 The trigger sensor type you are wiring will be particular to the engine you're building a harness for and may require some research to identify its type.
00:48 Optical and hall effect sensors both output a digital switching signal, and for the purposes of wiring them to your ECU, can be considered the same.
00:56 A hall effect sensor will usually be mounted into the engine block or cylinder head and will read a tooth wheel attached to a rotating engine component.
01:04 They will typically have three wires, power supply, power ground, and signal output.
01:09 Optical trigger sensors are usually fitted inside a distributor or in a housing connected to the end of a cam shaft.
01:16 They will have three or more wires.
01:18 Power supply, power ground, and one or more signal output wires.
01:22 As optical sensors almost always spin at the speed of the cam shaft, it's common for them to output both engine speed and engine phase signals.
01:31 Variable reluctor, or VR sensors, look similar to hall effect sensors, and will also be mounted into either the cylinder head or engine block next to a toothed ferris wheel that is attached to a rotating engine component.
01:43 A VR sensor will only have two wires, signal positive, and signal negative.
01:48 It's critical that the polarity of these wires is correct when they're connected to the ECU.
01:54 As if they are around the wrong way, it may still be possible for the engine to run but the ignition timing will be unreliable, possible resulting in engine damage.
02:03 If reliable documentation on the sensor you're wiring is not available, it's still possible to confirm the correct wiring of a VR sensor.
02:10 Many aftermarket ECUs include a simple oscilloscope function in their software and this can be used to determine if the polarity is correct.
02:18 If your ECU has this function, and you can view a trace of the trigger signal, you should see the signal increase in the positive direction as the tooth on the ferris wheel approaches the sensor.
02:28 Then drop very sharply through zero and become negative exactly as the centre of the tooth passes the sensor and the tooth is now moving away.
02:37 If you see the opposite of this, the signal dips negative, then rapidly shoots through zero to a positive value, the positive and negative signal wires need to be reversed.
02:47 If your ECU does not have an oscilloscope function, you can perform a simple test to determine the sensor polarity.
02:53 With a digital multimeter set to measure DC voltage in the 200 millivolt scale, connect the red lead to either one of the terminals on the VR sensor and the black lead to the other.
03:04 Now touch the end of the VR sensor with the shaft of a large screwdriver.
03:09 As the sensor has a magnet within it, it will attract the screwdriver shaft to its centre.
03:14 Now while watching the multimeter, flick the screwdriver away from the sensor as fast as you can.
03:20 If you see the voltage reading on the multimeter dip negative, you have the red lead of the multimeter connected to the positive terminal of the VR sensor and the black lead connected to the negative.
03:30 If you see the voltage jump positive, you have the red lead connected to the negative terminal of the VR sensor, and the black lead connected to the positive.
03:39 As VR sensors are simply a coil of wire around a magnetic core, they have no moving parts or internal circuitry and do not require a power supply.
03:48 For this reason they're very robust.
03:50 It is a slightly more involved process to configure an ECU to read the alternating current output of a VR sensor, and this is covered in both our EFI tuning fundamentals course, and our practical standalone tuning course.
04:03 It's important you have an understanding of the difference between the types of engine speed and position sensors, as it's common for ECUs to have dedicated input pins for these signals, and the pin used might change depending on the type of sensor.
04:16 What will not change though is the use of shielded wire between the sensor and the ECU.
04:22 Strictly speaking it's only completely critical when using a VR sensor.
04:26 The digital signal, hall effect, or optical sensor's output is reasonably immune to electrical noise.
04:32 So shielded wire may not be needed.
04:34 However with the engine speed and position signals being so critical to the system and the consequences of them being incorrect so severe, I always run shielded cable to any trigger sensor.

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