Combining Non-Shielded Wires Into Shielded Cable (plus a other questions)

I am currently building a harness for a 2014 USDM Subaru BRZ. I have a donor factory engine harness (same part number as the car's factory-installed harness) on the bench and I have all the factory looming and tape removed. I have some questions regarding the factory engine harness that relate to building my club-level harness. I'm calling it club-level, primarily since I won't be using Autosport connectors, but I am comfortable with concentric-twisting and will be using this technique on the harness. I am also using /32 tefzel and SB shielded cable. So overall it's in a middle-ground with the materials and techniques between a professional and a club-level.

1. There are a lot of shielded cables in the factory harness. I'm wondering if all of them are completely necessary because if I run shielded cable in my club-level harness, it's going to be quite a large main trunk. I'm mainly interested to know if the wiring for the direct-injectors, throttle body, direct-injection fuel pump, and the two 02 sensors really require the shielded cables as seen in the factory harness. Are these devices/sensors commonly wired with shielded cable?

2. There are some devices/sensors that are wired in the factory harness with a mix of shielded cables and non-shielded wires. A few examples of this are the throttle body, knock sensors, MAF sensor, and both 02 sensors. Should I do exactly as the factory and keep the non-shielded wires separate, or is it okay to combine the non-shielded wire into the shielded cable? One example: the wiring to each knock sensor is comprised of one unshielded wire and one shielded wire for each knock sensor. I'd like to combine both as a twisted shielded pair for each knock sensor if possible. I'm imagining the factory does this for cost savings and it's probably okay to combine them, but can anyone confirm on this example and any of the other devices I mentioned?

3. Factory wiring size. As I've built my spreadsheet, I am noticing that almost all of the wiring in the factory harness is 22 and 20 gauge (and 18 or 16 gauge for the DI fuel pump) from what I can tell by stripping back the wires on the donor harness. I'm wondering if it's okay to build my harness exclusively with 22 gauge (and 18 or 20 for the DI fuel pump).

Thanks for your time and thank you for producing these courses!

Eric R.

G'day Eric.

1: Yes, most likely all that wiring does need to be shielded. The DI injectors and pump take pretty high power signals to actuate, and this will radiate noise out into the rest of the harness. The shielding here couples this noise away to ground so it doesn't infect other signals. Are the O2 sensors narrowband, or wideband? Both of these sensors output a pretty weak voltage signal that is best to keep shielded. This signal is read directly in the case of a narrowband sensor, or used as the feedback for the charge-pump current loop in a wideband sensor... Either way, yes, I'd keep it shielded if possible. E-throttles are another case of a comparatively high-power device, the wiring of which can radiate noise. Shielded cable is best for sure, but I've run E-throttle wiring many times without shielded cable and not struck any huge issues.

2: You're on the money there, and yes, it most likely will be a cost saving measure. In the knock sensor example you've mentioned, I'd run twin core shielded wiring all the way to them, after confirming that they are an isolated style of sensors (the ground pin on the sensor is isolated from the actual metal body of the sensor, so cannot make a connection to the engine block when it's bolted on).

3: Following the factory wiring sizes is a good way to go, just make sure you're using equal quality (or better) wire. The DI pump (and injectors) can draw quite a bit of current, so I'd certainly stick with the factory sizes there.

pretty good post and thanks for your answer Zac!

I'm in a similar situation.

I wonder if you guys ever tried to start with a core of 3 or 4x twisted pair shielded wire? and if I do so, should I twist the core too?

not sure of the best layout approached...

I need 3x shielded pair and about 26x 20ga wire... so I though about a core of 3 shielded pair, not twist together, a first row of 10 and second of 16. should be right in the ball park?

probably will be using /16 tefzel.

thanks for your time!

Yes you can twist the core but be careful of the direction of twist so that you don't create a quad!

Once you get that core twisted, the rest of the layering should fall into place!

Heres a pic from a current project for you to visualize it! Also note that three pairs will look much nicer than my example of two pairs! Ypu should have no problem getting a smooth second layer. But I often make a 8-12 inch test section as it's really easier to work with and lets you do some experimentation on the best layer design for the application.

Hi guys, I’m on the start of making a Subaru loom also, if going going as far as using tefsel 22 g for the bulk what equivalent is there for shielded wire? Any suggestion where to get it. More than likely will be in USA as Australia has almost nothing and what we have is literally 20 times more expensive. Mainly chasing wire for Cam, crank, knock sensors.

there is many supplier, but on the website of prowire, you have a lot of info, like wire sizing.

you can use single shielded wire from them, but sizing will be way off from your 22ga. maybe you can put them on the same layer.

I just finished doing my 3rd layer, and if I had to do again, I would do more single shielded wire.

I ended up doing a core of one 4x22ga twisted (in a single shield) with a second layer of 3x22ga single shielded and 5x 16 ga /16 wire (size were pretty close) and the rest of the harness in 20ga /16.

thanks for the help guys!

You mean single shielded for each sensor? They’re all lov voltage under 5v as a typical signal wire only. That’s why I was going with 22g. Anything thicker would be over kill and bulky. Shielded wire is bulky enough already.

Subarus use a heap of shielded wires for there sensors so if I get it right I should be able to do a smaller layer just with the shielded gear. I’ll have a look at pro wire and see what they have.

thanksnfor the reply

22ga but shielded, something like OEM was doing. at least use this as a guideline.

for example, I didn't shield my coil igniter wire while OEM was doing it. we'll see if it creates problem.

just in case it is not clear... it's not about the thickness of the wire in relation to what amperage he can handle, but it is to shield signal from interference. under that fat skin is only a typical 20 or 22ga wire.