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Practical Wiring - Professional Motorsport: Step 1 - Specifications and Circuit Design

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Step 1 - Specifications and Circuit Design


00:00 - While it's always tempting to jump in the deep end and get stuck into the construction of our harness, proper design and planning is critical and even more so when designing a professional level harness using techniques like concentric twisting.
00:13 With this in mind, our first step is to determine the design specifications for the harness and the circuit design.
00:20 This is a part of the process that many tend to skip over but this will form the basis for every step to come and hence it's really important we spend the time here and make sure that we've covered all of our bases.
00:33 In this step, we'll look at the electronic components that we're using for the truck to control the engine, determine their power requirements and current draw, how they need to be connected and what sub harnesses, if any will be required.
00:47 You'll find a document attached to this module that includes the information we'll cover in this module, which you can print out and use as a reference as we proceed.
00:55 Often this step will be conducted at the same time the electronics package is being selected and this is a sensible workflow since this is the ideal time to ensure that the ECU that you're going to use can actually handle the input and output requirements for your project.
01:12 This process should start by thoroughly documenting every single input, output and connection that's required for the harness.
01:20 I usually start this with a notebook and a pencil while I'm actually viewing the engine that I'll be wiring and I can note down everything that I'll need to wire up.
01:29 This is a very easy place to miss something out so it's important to be thorough and work through the engine sensors and actuators in a systematic way.
01:37 Some familiarity with the engine that you're working with is obviously helpful and for example, a couple of potential traps with the 3UZ-FE that could catch us out include the knock sensor wiring which is hidden down under the upper inlet manifold and terminated in a plug at the rear of the engine which also shares the starter motor solenoid wiring.
01:58 On this note, the factory Toyota knock sensors on this engine are a single wire design which earth through the body of the sensor to the block.
02:07 I've previously had poor results from these sensors and they seem to be very noise prone so they'll be swapped out for a modern Bosch wideband sensor and wired accordingly.
02:17 The other easy bit to miss on this engine is that it uses a set of flaps in the inlet manifold to effectively alter runner length.
02:25 This is controlled by a vacuum reservoir and actuated with a 12 volt solenoid.
02:29 Failing to wire for this will sacrifice torque when it comes time to hit the dyno.
02:34 On the other hand, not all of the factory actuators are going to be required.
02:39 For instance we'll be eliminating the factory EVAP canister and the solenoid that controls this as the new fuel tank will incorporate a vent to atmosphere.
02:47 We don't just want to consider the engine wiring though, as it may make some sense to incorporate other functionality into this harness.
02:55 For example, I'd typically add any of the gearbox wiring into this same harness to get the wires through the firewall.
03:02 I'm talking here about inputs such as a reverse switch, a speed sensor and in the case of the Toyota transfer case, the 4WD switch.
03:11 While there is a provision for a speed sensor in the R151F gearbox that we're using, we're actually sourcing individual wheel speeds from a standalone ABS unit so the gearbox speed sensor is redundant in this instance.
03:26 In addition, we've chosen to incorporate the starter solenoid wiring too.
03:30 We've also made the decision to incorporate the alternator ignition feed and voltage sense wiring into the engine harness.
03:38 This could just as easily be incorporated into the chassis harness since the actual main power feed from the alternator will go directly to the battery however in our case it made sense to add these wires into our engine harness.
03:52 The sense wire will connect to the starter motor power stud under the inlet manifold and the ignition feed will come from our PDM.
03:59 The alternator does also have a light circuit to indicate when the alternator is charging and you need to be careful here as some alternators won't begin charging if they don't have this light circuit wired.
04:11 The older Denso alternators like the one fitted to our 3UZ-FE typically don't require a light circuit but to be certain, we had the alternator bench tested by our auto electrician before committing to our wiring design.
04:24 It's aspects like this that we want to have complete confidence about prior to committing to our harness.
04:31 And if you're wondering how we're going to get on with an alternator warning light now that we've removed that circuit, that's no problem because we can use our MoTeC C127 dash and a set of parameters to bring up a warning light if the alternator is in fact not charging once the engine's running.
04:49 Once we've double checked everything, we can transfer this information down onto our Google sheet which will start fleshing out the requirements for our ECU.
04:57 This begins with simply listing the inputs and outputs required.
05:01 For example, we know that we're going to need 8 injector drives and 8 ignition drives.
05:06 We also need 4 analog temp inputs which will be used for air temperature, coolant temperature, oil temperature and the air conditioning pressure switch.
05:16 I'd recommend going through this sheet once complete and adding up all of the inputs and outputs and listing these together.
05:23 This just makes it very easy to compare to the ECU spec sheet and will let us confirm that we have the required capability with the ECU that we've chosen.
05:32 In our case, we're really close to using all of the inputs and outputs on the M170 ECU, with just 2 half bridge outputs to spare.
05:42 Since we are comfortably able to run everything we want to though, this ECU does in fact end up being an ideal choice for our application.
05:50 While I'm developing this sheet, I also prefer to go one step further and include everything I'll need to run for an input or an output.
05:58 For example, to operate an injector, we'll need an injector drive as well as a 12 volt supply.
06:04 For a fuel pressure sensor for example, we're going to need a 5 volt sensor supply and a sensor 0 volt to go along with the analog voltage input.
06:13 We want to do this as it's essential information when it comes to speccing a suitable bulkhead connector with sufficient positions which we'll look at further into this example.
06:23 Providing a 12 volt power supply to the ECU and the associated components also requires an understanding of the current draw so that we can spec an appropriate conductor size.
06:34 However, in our case, the ECU connector and the bulkhead connector that we've chosen will both use size 22 contacts so we'll be using M22759-32, 22 gauge tefzel wire.
06:49 We can of course double up the conductors in the event that we need to handle more current than the 6 amps or so that we can reliably support using a single 22 gauge wire.
07:00 Let's take a couple of specific examples so that we can see how this is handled.
07:04 First of all, we'll look at the VVT solenoid, and if we use a multimeter to measure the impedance, we'll find it's about 7 ohms.
07:13 Using Ohm's law, we can divide our running battery voltage of 14 volts by the impedance of 7 ohms and of course we get a current of 2 amps.
07:22 This would be the current draw if the solenoid was constantly powered, however that's not actually realistic for this type of output.
07:30 Instead, it'll be pulse width modulated by the ECU at perhaps 100 to 300 Hz and will most often sit at or relatively close to a 50% duty cycle, meaning we can expect the actual current draw to be about half of that we just calculated.
07:47 Likewise, if we consider our injector impedance, we'll find that this is 14 ohms and again dividing our batter voltage of 14 volts by our impedance of 14 ohms, we of course get a result of 1 amp but this would also be at 100% duty cycle.
08:03 This shouldn't be a common operating condition but we should account for it nevertheless.
08:08 Everything so far could be capably handled with a single 22 gauge conductor and in the case of our injectors, we can actually get away with running a single 12 volt feed for each bank and then paralleling the 4 injectors on each bank off this single conductor.
08:24 How about the starter solenoid wiring though that will pass through the bulkhead connector to the PDM32? In this case, it's quite common for the starter solenoid to draw 12 to 15 amps continuously while the engine is cranking.
08:39 We generally don't recommend exceeding about 6 amps with a 22 gauge conductor so clearly this isn't going to work.
08:45 The solution here is to simply run 3 22 gauge conductors which together can handle around 18 amps, plenty for our application.
08:54 These can be run through the firewall connector and these can then be spliced together at the starter solenoid.
09:00 The reason that this is important to work out during this design stage is because it lets us know that we're going to need to allow 3 positions in the connector, rather than just 1 for our starter wiring.
09:13 Additionally, we'll also have some breakout harnesses on the ECU side of the harness.
09:17 The passthrough wiring that we've just discussed will be an example as this needs to go to the C127 dash but we'll also have the ethernet connection for communication with a laptop, power supply interface to the PDM and a CAN bus connector to keep in mind just to name a few.
09:35 We also need to be mindful of the design of our CAN bus to make sure that we're following proper protocols.
09:41 It's recommended for CAN buses longer than 2 metres that we have a 120 ohm terminating resistor at each end of the bus and that each device or node as it's referred to that's attached to the main trunk of the bus is located within 500 mm of the main bus.
09:58 It should go without saying that the CAN bus also requires a twisted pair of wires.
10:03 Laying out the bus in such a way that it meets these criteria can be trickier than it seems with a more complex bus.
10:11 In our case, we have the MoTeC C127 dash, the M170 ECU, a lambda to CAN unit, 2 power distribution modules, a Grayhill CAN keypad and an XLR connector, all connected to the same bus.
10:26 I generally like to consider where the components are physically mounted and decide what's the most sensible way of connecting them with our CAN bus? One tricky part is that the LTC is mounted in the engine bay which can make it tough to meet the 500 mm distance rule from the main bus, particularly if we're only running the main bus up to the interior side of the bulkhead connector.
10:47 There's a couple of ways around this though.
10:50 Either we can make the LTC one end of the bus and add the terminating resistor at the LTC connector in the engine bay, or we could make the LTC a node along the bus and run the CAN bus wiring out through the firewall connector to the LTC and then back through to separate positions on the firewall connector.
11:08 This just requires splicing our CAN high and low together at the LTC connector on the engine harness.
11:15 Of course it does eat up 2 more locations in the bulkhead connector that we need to allow for though.
11:21 In our situation, with real estate on the bulkhead connector at a premium, we decided to make the LTC one end of the bus.
11:27 We can see the full layout of the CAN bus design here which meets the best practice standards and should offer a safe and reliable bus design.
11:36 For the construction of the bus itself, we're using 22 gauge M22759-32 wire that is supplied as a twisted pair, saving us the trouble of doing so.
11:47 This is available in several colour combinations and we're using yellow and green for our application.
11:53 With the basics of our design now understood and documented, we can move onto the next step of our process.

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