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EFI Tuning Fundamentals: Auxiliary Outputs

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Auxiliary Outputs


00:00 - While the ECU is there to supply the engine with fuel and spark, this is only scratching the surface of what they're capable of.
00:07 As both ECU and engine technology has advanced the ECU has become responsible for a huge range of tasks.
00:14 Really the only limit to what can be achieved with the ECU is down to the software and your own imagination.
00:20 To achieve these extra tasks, the ECU uses auxiliary outputs.
00:24 There are two general types of auxiliary output, switched or pulse width modulated.
00:30 Let's look at each of these in a little more detail.
00:33 First up, the switched output.
00:35 You could think of this just like a light switch that the ECU can turn on or off.
00:39 Rather than operating a light though, the switched output can be used to operate any device that needs to be turned on or off.
00:46 The possibilities here are just about endless.
00:48 It could include a radiator fan, a shift light, a VTEC solenoid or maybe a driver warning light.
00:54 A switched output can be controlled by the ECU to operate when a certain operating condition exists.
01:00 For example, we may want to turn on a shift light any time the engine RPM exceeds 7,500 RPM.
01:07 When this condition becomes true, the ECU will turn on that output.
01:11 Many of the outputs that we want to control with the ECU are going to draw quite a lot of current though.
01:16 For example, a fuel pump or a radiator fan could easily draw more than ten amps.
01:21 The delicate circuits inside the ECU can't handle this amount of current and would simply burn out if we tried to pass this sort of current through them.
01:30 To get around this, when we want to switch a high current device, we use the ECU to control a relay, and then the relay can be used to switch the device that we want to control.
01:40 When it comes to many of the switched outputs that the ECU controls, you'll also come across the term hysteresis This means that once the output turns on, the input controlling the output needs to change by the hysteresis value before it will switche back off.
01:56 Hysteresis is designed to stop the controlled output cycling on and off repeatedly if you happen to be sitting at the control point.
02:04 Let's take a radiator fan for example.
02:07 It might be common to have the radiator fan switch on at 90°C.
02:11 If we had no hysteresis, then once the engine temperature reached 90° the fan would tend to rapidly turn on and off.
02:19 Instead, what we might want do is add a hysteresis of perhaps 2°C.
02:23 In this case, once the fan switches on at 90°, it will remain active until the temperature drops by the hysteresis amount, in this case 88° before switching back off.
02:35 Next we have pulse width modulated outputs.
02:38 These are often referred to as PWM or DPO, which stands for digital pulsed outputs.
02:45 With some devices, we need finer control than a simple on or off state.
02:49 We may want to able to vary the output between two extremes.
02:53 You could think of a PWM output as light dimmer instead of a light switch.
02:58 With a light switch, we could only turn the light on or off, but the dimmer allows us to vary the amount of light we want.
03:04 PWM outputs use the square wave output that we looked at back in the waveform module.
03:09 The ECU can vary the duty cycle of the output, which you will remember as the amount of the amount of time the output is on compared to the cycle time of the wave.
03:17 Looking back at the light dimmer analogy, if we didn't want much light, we'd use a very low duty cycle where the output isn't in the on state for very much time.
03:26 If we wanted a lot of light, we would increase the duty cycle, which would have the output in the on position for much longer.
03:32 Adjusting the duty cycle gives the ECU very precise control over the output.
03:38 PWM outputs are used for many functions in the ECU.
03:41 This is the sort of output we'd use for idle speed control, variable cam control and boost control, just to name a few.
03:48 While an aftermarket standalone ECU will give the tuner a lot of flexibility in terms of controlling auxiliary outputs and functions, if you're reflashing a factory ECU, then the outputs may be fixed and a lot less flexible as these will be dependent on the factory ECU architecture as well as the software coding.
04:07 Even with this in mind though, software for reflashing many popular models, will allow certain functions to be reassigned and used for different tasks.
04:16 A common example of this is reassigning the purge solenoid output to function as a boost control solenoid.
04:22 This is useful when adding a turbo to a naturally aspirated engine.
04:26 So to recap here, the ECU will also offer a range of auxiliary outputs, and these can be separated into switched and pulse width modulated outputs.
04:35 The type of output we need to use will depend on what we want to control.