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Practical Wiring - Club Level: Harness Routing and Branching

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Harness Routing and Branching


00:00 - The harness routing process is the one of determining how our wiring harness will physically run throughout the vehicle and engine bay and confirming that it will reach all of the required EFI system components.
00:11 The devisions made during this step will drive much of the rest of the design and they are very difficult to change later..
00:18 As such, it is important that the location of the rest of the EFI system components have been finalised by this stage.
00:25 What we aim to achieve in this step is to make a mock up of the harness running through the engine bay and vehicle using another material that is a good analog for the finished harness.
00:35 I like to use cheap nylon rope for this as it tends to form a similar radius throughout a bend as the finished harness is likely to, and its low cost means you don't need to be sparing with its use.
00:46 Once this mock up is constructed, it can be removed from the engine bay and vehicle, the lengths of each branch measured and used to generate the documentation in our next design step.
00:56 One of the biggest considerations when planning how your harness will run throughout the vehicle is where a main harness section will branch out into the smaller sections and individual branches that connect to EFI system components.
01:09 Branch points are one of the most complicated and time consuming parts of the physical harness construction process and we do want to limit their number as much as possible.
01:18 Often this may mean we will branch out to multiple sections that will run next to one another for a short time before deviating off and heading to their individual components.
01:29 Another key consideration to ensure we get a tidy finished result is the orientation of the smaller sections as they exit a branch point.
01:38 We want the orientation of these to be such that they do not need to cross over one another as they head off to the components that they connect to.
01:45 We add this orientation information to our final documentation as it can be helpful to refer to during the construction process.
01:53 If we can somewhat organise the wires in our harness into these sections, before we get to the sheathing and branching part of the process, both of those steps will be much easier and tidier.
02:04 We'll head out to our example FD3S engine bay now and discuss some of the routing decisions that need to be made, and show the process of building the mock up.
02:14 Out next to our RX-7 engine bay here, we can have a look and see how our harness is going to be routed through the engine bay and the vehicle to ensure it attains a really nice fit and it's not going to be strained when it's in operation.
02:26 The key things you want to look for here are to avoid any substantial heat sources in the engine bay.
02:32 So typically these are going to be exhaust components.
02:34 Now with our rotary engine here we are at a slight disadvantage because the intake and exhaust port to the engine are actually on the same side, we do have to run our wiring harness to the inlet manifold of the vehicle, as this is where the fuel injectors are located, so it is going to have to get close to the exhaust manifold also.
02:51 We're going to follow what the factory has done here and we're going to only run our wiring harness on the engine side of the intake manifold, essentially using the manifold as a heat barrier.
03:01 Now this would be a really good place to mention that often the way that the OEM manufacturer has run the wiring harness through the engine bay can be a really good guide as to how we should run our aftermarket EFI wiring harness also.
03:14 OEM manufacturers are not in the habit of wasting money so it's likely they've chosen a good routing path that can reach all of the required sensors and actuators without being unnecessarily long.
03:24 Now as mentioned, we do want to avoid hot components in the engine bay, but we also want to avoid any components of the engine bay that are going to move in relation to the engine.
03:32 So by this I mean any pulleys on the front of the engine that will be spinning quite fast when the vehicle's in operation, but also the throttle linkage that will be moving back and forth.
03:42 Particularly important is to ensure that we do avoid that throttle linkage as if any wiring gets tangled up in it, it could be possible for it to be jammed in an on position which can be a really dangerous situation.
03:52 We also want to make sure that as our wiring harness runs through the engine bay it doesn't have any really sharp bends in it.
03:58 Where it does need to make a bend, we want to keep that radius as gentle as possible and this avoids excess strain being placed on the wiring harness.
04:07 Where it does need to make a bend we're going to want to support the wiring harness usually by cable tying it to an existing component close by.
04:14 Often hard fuel lines in the engine bay or in fact brake lines than run along the fire wall of the engine bay can be really good options for this.
04:21 The last key factor you really want to keep in mind is the orientation of the harness branch points, and by branch points I mean a section where the harness takes a main trunk and branches it out to either sub trunks that would head off to other branch points or in fact harness sections that will head off directly to a sensor or actuator.
04:41 Now it's the orientation of these branch points to each other that is largely going to determine the final fit and finish of the wiring harness in the engine bay.
04:49 And when we talk about a harness section that terminates to a sensor or actuator, we can always leave this a little bit long during our build process and then trim it back for a really good fit and finish when we're doing a test installation throughout the build process.
05:03 However when we're talking about a branch point, once these are built into the harness, their locations are essentially fixed, it's very very difficult to change them when we're part way through the build process.
05:14 So we want to make sure that they're in logical locations and that their location relative to one another is going to make sense.
05:21 Now the way we're going to do this is we're going to use some cheap nylon rope to build a mock up of the wiring harness as we would like it to run through the engine bay and the interior of the vehicle.
05:32 We can then remove this mock up from the vehicle, get it on our bench, measure all its lengths and use this to generate the documentation.
05:39 We can then build our wiring harness on the bench to our documentation.
05:43 So we're got our length of nylon rope here and what we're aiming to achieve is to run it through the engine bay along the engine here as we would like a finished wiring harness to run.
05:54 Now the OEM wiring harness for our FD3S example does actually run along the top of the engine here.
05:59 There's normally a big bank of control solenoids for the twin turbo system but we have managed to remove that because we are converting this vehicle to a single turbo.
06:08 So I know I'm gonna want one of my main harness trunks to run along the top of the engine here.
06:13 There's going to be a branch point at the end which is going to split off here to our oil metering pump and our crank sensor and our knock sensor here.
06:21 Now this branch point I am going to locate back here on the top of our gearbox.
06:26 We did talk about this earlier on in this section as quite often you will locate a branch point further away from the sensors and actuators that the sections exiting that branch point might need to reach and you would think is somewhat logical and the reason for that is we're going to be able to orientate all these wires really nice and tidily and cable tie them together as they run off to the sensors and actuators that they need to reach.
06:50 And by doing this, we're minimising the number of branch points that we actually need to create in our harness and that is going to make both the construction and planning processes much easier.
06:59 Now to do this I'm simply going to get another section of nylon rope here.
07:04 I'm going to run it where it needs to run from our branch point up to our injectors here.
07:11 Just going to pass under this fuel line.
07:13 And then I'm simply going to tape it in place and then we're going to trim this off to the correct length.
07:27 Now if we tape a section of nylon rope onto our branch point here for every harness section that is going to exit off to our sensors and actuators it is going to get very bulky very quickly.
07:36 So instead what I like to do is use a single branch here and approximate where it's going to meet up with our injectors and I'm simply going to mark those locations using further pieces of tape.
07:51 Then when we pull this mock up harness out, we can measure those lengths and use them to generate our documentation.
08:00 So that should give you a good idea of how we go about building our nylon rope mock up harness, particularly the engine bay side here.
08:06 Now you'll notice I've got a nylon rope heading over the side of the engine bay and that's because I know the grommet where our wiring harness is going to pass through the firewall to the interior is down in the corner of the engine bay here.
08:19 So what I'm now going to do is pass this rope through into the interior and then we'll pop inside the vehicle and have a look at what we have to consider where it's going to connect to the ECU.
08:28 So you can see in the interior here it's very much a similar process.
08:31 We're just working so the harness can bend in a nice natural way.
08:36 Ensuring that it can reach the header plugs on the ECU.
08:39 Now when you cut this nylon rope it does fray apart quite quickly so what I like to do is use a bit of electrical tape to mark my position where I need to cut and then when we make that cut it's also going to keep the nylon rope together and stop it fraying apart.
08:53 There are two body harness connectors that we're going to need to interface with in the interior of the RX-7 here.
08:58 The blue one is going to be very close to our ECU header plugs, and there is another one up in behind the heater box here that we're also going to need to make a connection to.
09:07 Now there is another white connector in the interior here and this is where much of the body harness interfaces with the OEM ECU in the vehicle.
09:14 There's not many wires in here that we're going to have to make a connection to if any at all.
09:19 Any that we do need to make a connection to we will splice into before the connector and we will break them out to a Deutsch DTM plug which our aftermarket EFI harness can then interface with.
09:28 Now that we've got our mock up built and out of the engine bay, we can lay it out here on our work bench just to have a quick talk about some of the key things that you're going to be looking for when you're building one of those.
09:40 You'll notice here we've got a section that has no branches coming off it.
09:43 This is going to be pretty typical when you're creating one of these routing layout mock ups.
09:50 This is what I refer to as the central trunk of the harness and it is where most of the wires are going to run through.
09:54 It's also where the harness is going to run through our bulkhead from the interior to the engine bay and I've marked the position where that's going to happen as that it going to be a key reference point we're going to use for measuring the lengths of all the other branches.
10:08 It's also going to be where our sealing grommet will end up.
10:12 On the end of each of these branches I have put a piece of painter's tape and I have written on there in a sharpie what that branch actually connects to.