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Variable Cam Control Tuning: Sensor Types and Signals

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Sensor Types and Signals

05.22

00:00 - There are a couple of considerations when it comes to wiring up and configuring the cam position sensors for your engine and getting this wrong can result in a lot of frustration and wasted time.
00:10 Fortunately as we'll see in this module, it's all pretty straightforward, provided we understand what we're looking for and how the sensors operate.
00:18 Broadly there are 2 types of sensor we'll come across for cam position.
00:23 Either a digital sensor that provides a square wave input to the ECU or a magnetic or reluctor sensor which provides a sinusoidal waveform.
00:32 Both work equally well provided you understand a few basics when it come to the wiring and configuration.
00:38 Let's start with the digital sensor which can be broken down into either hall effect or optical sensors.
00:44 While the operating principles of these 2 sensors are different, the configuration wiring and the signal are essentially the same.
00:52 From a wiring perspective, this style of sensor will have 3 wires.
00:57 These sensors will require power from the ECU, usually in the range of 8-12 volts, a sensor ground and then the third wire will be the signal into the ECU which needs to be connected to either a dedicated cam position input or a digital input.
01:13 This is a broad overview and you should of course check with the ECU manufacturer that you're using for specifics relating to your application and engine.
01:23 This style of sensor will output a square wave signal every time a tooth goes past the sensor and this signal will usually vary between ground and a high voltage state.
01:34 The trick here is that in most cases the sensor will switch the signal to ground when the tooth goes past and the voltage will float the rest of the time.
01:43 This means that in order for the ECU to actually see the varying voltage of the signal, we need to enable what's referred to as a pull up resistor within the ECU software.
01:53 We don't need to get too deep into what the pull up resistor is but essentially it just means that instead of the voltage floating, it would be pulled up to 5 volts when the tooth isn't passing the sensor.
02:04 In this way we get a 0-5 volt signal and the ECU can clearly define the teeth.
02:09 Failing to do this will result in a signal that floats at or close to zero volts which the ECU can't make any sense of.
02:17 A magnetic reluctor or variable reluctance sensor on the other hand will only have 2 wires and these will be connected to a sensor 0 volt and a digital input at the ECU.
02:28 The important aspect with this style of sensor is that the polarity of the wiring is critical.
02:34 If you get the polarity wrong the ECU will still see a valid signal, however the shape of the signal will be inverted.
02:41 Here's what a reluctor input signal should look like.
02:45 The voltage starts at 0 volts and then begins to rise as the tooth approaches the sensor.
02:51 As the sensor goes past the tooth, the voltage peaks and then drops back through 0 before becoming negative.
02:57 As the tooth moves away, the voltage then rises slowly back to 0.
03:02 The importance of this is that the ECU will see a valid trigger input at the point where the voltage crosses 0.
03:09 If we now reverse the polarity, we see the opposite shape to the signal.
03:13 The problem with this is that the ECU is still looking for the 0 voltage crossing point to trigger off which now happens as the voltage bleeds back down after peaking.
03:23 This will work however this point won't coincide with where the tooth actually passes the sensor and to make matters worse, since the voltage amplitude of the signal is relative to engine RPM, the trigger point will actually move as the engine RPM varies.
03:39 Particularly when this style of sensor is used for engine RPM, this can result in what's referred to as timing drift where the ignition timing actually drifts from what the ECU is asking for as the RPM changes.
03:51 With a cam position sensor, it's understandably going to make accurate cam control impossible.
03:57 Our last consideration is that your ECU may also incorporate a table that defines the arming threshold for the sensor input.
04:05 This sounds a bit daunting but it's not overly complex.
04:08 This just defines a voltage above which the input must reach before the ECU will become armed and start looking for a valid input.
04:16 The intention here is to help the ECU ignore the background noise that will inevitably exist.
04:22 With a digital input, some ECUs may not require an arming threshold at all but otherwise we'll usually want to fix this at half of the signal amplitude which will usually be 5 volts hence the arming voltage will become 2.5 volts.
04:36 Things are a little trickier with a reluctor sensor since the voltage amplitude increases with RPM meaning that the arming threshold will be a 2D table relative to RPM.
04:46 The general rule of thumb with this table is to set the arming voltage at a 1/3 of the peak voltage at each RPM break point.
04:54 This might be as low as perhaps 0.2-0.5 volts at cranking speed and could increase to 4 through to 5 volts at 7000 RPM.