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Practical Wiring - Club Level: Step 1: Specifications and Circuit Design

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Step 1: Specifications and Circuit Design


Connection Document, save a local copy to be able to edit.


00:00 - We're going to go through a worked example now of a complete harness build from scratch for a Mazda FD3S RX-7.
00:08 We will use the HPA eight step process to design, specify and build this harness, showing you each step along they way in detail to teach the process that we use for EFI wiring harness construction.
00:20 This process breaks down the large task of EFI wiring harness construction into eight manageable steps that when completed in order with attention to detail, will result in a well designed, correctly specified robust wiring harness that will be reliable for the life of the vehicle.
00:36 The car we're going to be using for this worked example is a 1998 Mazda FD3S RX-7.
00:42 This is a lightly modified street car in need of a completely new wiring harness due to the original item becoming brittle and cracked over many years of heat cycles.
00:50 The first step of our HPA eight step process is the one of specifying the basic materials that we're going to use, and then designing the circuits of the EFI harness to ensure that everything will operate as expected.
01:03 Specifying our materials broadly comes down to two main choices, being what type of wire we're going to use and how the harness will be sheathed.
01:11 As this is a modified street car application we're going to use TXL wire as this is going to save on cost.
01:18 We're going to sheath the harness in DR25 shrink tubing and use raychem SCL for our connector booting and branch points.
01:25 This will give the harness a nice professional appearance once it's complete.
01:29 It will also allow it to remain flexible and withstand the heat cycles that it's going to be subjected to in the engine bay.
01:35 The next thing we do is to list out the components of the EFI system.
01:39 The best starting point for this is the ECU that will be used as its selection may impact the rest of our circuit design.
01:45 Our RX-7 is going to use a Link Fury ECU which will have more than enough sensor input and actuator output channels for this application.
01:54 The inclusion of a wideband oxygen sensor controller within the ECU is also beneficial and will mean that we will need to include the wiring for such in our harness.
02:04 I have a list here of the rest of the components that will make up our EFI system.
02:08 You will either create this list yourself if you're responsible for the system design, or the person that is can supply it to you.
02:15 EFI wiring harness construction should be undertaken in close proximity to the EFI system designer or the tuner if possible, as their input on system components can be invaluable.
02:26 Often if we're working on our own projects we are the designer, harness builder, and possibly the tuner as well.
02:33 Which can be a good position to be in as you will always have access to all the required information.
02:38 Our circuit design begins with the power supply.
02:41 We'll divide our system components into those powered from our main relay, those from our enable relay, and any that require their own relay.
02:48 For our RX-7 we're going to power the ECU and the oil metering pump from the main relay as the ECU documentation specifies that the oil metering pump needs to be powered from the same supply as the ECU.
03:00 The majority of the other system actuators primarily injectors and ignition coils, will be powered from our enable relay, with our fuel pump being the only part of the system needing its own dedicated relay.
03:12 The ECU will also be in control of the original cooling fans fitted to the engine, but it will do so by triggering the original relays which are already located in the engine bay.
03:23 To physically house our power supply relays and fuses, I've chosen to use an integrated fuse and relay box for this application as the space where I'd like to fit it is pretty tight and the smaller relays and fuses these boxes use, is really gonna help out.
03:37 These types of fuse and relay box are a grid pattern of a specific spacing that allow you to plug in a standard iso 280 relay and mini glade fuses.
03:46 In the supplied documentation below you'll see how this fuse and relay box has been laid out.
03:51 An added benefit of these fuse and relay boxes is that all of the wiring approaches them from the same direction.
03:57 This actually makes the harness construction much much easier.
04:01 The relays and fuses essentially become part of the harness and that's gonna keep everything nice and integrated.
04:07 Moving onto our power grounding, we're going to obey our star earthing rules here.
04:12 All of our power grounds are going to be made to an M8 threaded hole which is on the engine block.
04:17 All our power wiring will be run back to this point individually, collected together at that point and crimped into an eight millimetre ring terminal.
04:25 The exception to this is the fuel pump which is going to be grounded to the vehicle chassis at the rear of the vehicle.
04:30 However we will ensure that there is a good single earth strap connecting our engine block to the chassis and the engine bay.
04:37 Our power supply and ground wiring is sized based on the expected current draw of our system components.
04:43 The exception to this is our fuel pump power supply wire which we have actually oversized at 12 gauge.
04:49 This is to cater for our possible future upgrades.
04:52 Our sensor wiring design is fairly straightforward for this application as our Link Fury ECU has more than enough input channels for us to wire our sensors to it directly.
05:02 This means that we don't need to add any additional interface boxes.
05:05 A detail here to note is that the Link Fury ECU has two sensor grounds, one on each of its connectors.
05:12 We will make sure to connect a sensor to the sensor ground on the same connector to which its signal line is connected.
05:18 It is possible that the two sensor ground pins are directly connected inside the ECU, but there are ECUs out there that need you to pair specific sensor input channels with specific sensor grounds, so it's a good habit to get in to.
05:32 Where a sensor needs a supply, this will be given by the ECU from the sensor supply pin on the first connector.
05:38 We don't have any sensors in this application that require and eight volt supply so this pin is going to go unused.
05:44 We're going to use the OEM engine position sensors which are a variable reluctor type.
05:50 These signals will be run with twisted, shielded pair wiring.
05:52 We'll splice to the shielded braids at the ECU end and run these to a specific pin that the Link Fury ECU has for this purpose.
06:00 The actuator circuit design for this application does require some special attention.
06:06 The injector circuit design is a standard saturated drive type with the slightly unusual detail that it is a staged setup.
06:13 It has two different sized injectors per rotor.
06:16 The documentation for our Link Fury ECU clearly specifies which injector drivers the primary and secondary injectors should be connected to however, making this a relatively simple process.
06:28 It's a similar case for the ignition system with the rotary engine having two spark plugs per rotor, a leading and a trailing.
06:34 Once again the ECU documentation specifies to which ignition outputs these need to be connected.
06:40 The rotary engine also includes an oil metering pump which has a metering valve, the position of which is adjusted with a stepper motor.
06:47 This stepper motor has four coils which need to be wired to the ECU.
06:51 Now to drive home the point that it really is important to read your ECU documentation, once again the wiring of these outputs is clearly specified for us in the PC Link documentation.
07:02 While we don't have any CAN capable devices planned in the EFI system at the moment, it's always a good idea to include a couple of CAN access ports in your access port design.
07:11 We're going to put one in the interior and one in the engine bay, both of which also have power supplied via our main relay and a power ground to allow for easy additions later in the vehicle's life.
07:21 Included below this module is the complete connection document, specifying every circuit and connection in the EFI harness.
07:29 I would highly suggest that you look through this document thoroughly, as it will really help cement how the EFI harness design process is undertaken.
07:36 The circuit design process is one that would be different for every single harness that you build, but as you get more experienced, you're going to see the common circuit patterns and be able to simplify the design process even further.
07:49 That is how the design process has been undertaken for the wiring harness on our FD3S RX-7 worked example.