Project Lexus SC300 Road Racer: Part 11 - Engine Bay Wiring

by Erik Jacobs


When we left Project SC300 last, we had done most of the preparation work in the engine bay getting ready to start pulling the engine harness work. For reference, let’s refer to the engine harness diagram:

The Cadonix Arcadia harness diagram for the engine bay.

The engine bay harness is fairly trivial, but it does have a few important sensor/signal circuits that we need to be mindful of. In terms of trivial power circuits, here is the list:

  • Head lamps
  • Turn signals
  • Accusump solenoid power
  • Horns
  • Alternator
  • Cooling fans
  • ABS solenoid (future)
  • Windshield wiper motor
  • Starter solenoid

While we don’t expect to do much racing at night, we were trying to be thorough. And, in multi-class racing, it can be nice to have your lights on when you are coming up on slower cars. We made sure to have a power circuit to control the low beams, as well as a future circuit if we decide to run high beams (or driving lights if we do an evening/night enduro, or whatever).

Why not have turn signals? And, along the same lines, why not a horn? Plus, the horn button on the steering wheel can easily be reprogrammed to turn on some other circuit, like the headlights for flashing slow cars. Although I’ll probably be the slow car…

The car still has a wet sump (2JZ dry sumps are very, very expensive), and we had been running an Accusump previously. The Accusump has an option for an electronically controlled solenoid valve to effectively turn it on and off. This solenoid needs power. Check.

The 2JZ Haltech harness is designed just for running the engine, but makes no accommodation for the charging circuit. Alternator charging circuits aren’t all that complicated -- just four wires in the case of a 2JZ alternator -- but, without it, your battery would quickly go dead. We are incorporating the alternator wiring here.

The Haltech has a few digital outputs. These can be used for things like cooling fans. In our case, when the coolant reaches a high enough temperature, we will activate a digital output to go to a relay in the front corner of the engine bay. This relay will turn on the cooling fans, which draw quite a bit of current. Along with the cooling fan relay and power distribution submodule that we’ll detail later, we will also have a relay for a future ABS solenoid (if we decide to put in motorsport-grade ABS later).

I don’t intend to let a little rain scare me away, so windshield wipers are a must. The Racepak provides some neat ways to control wipers, too.

Lastly, starting the car requires sending power to the factory Toyota starter solenoid, and there is a circuit for that in this harness as well.

The rest of the circuits in this harness are signal circuits:

  • Knock sensor #1 and #2
  • Oil temperature
  • Brake pressure

If you remember from the previous few articles, we had removed the factory ABS and used several hydraulic tees to maintain the OEM brake line setup and introduce an additional brake pressure sensor. A brake pressure sensor is a great tuning tool when you have data logging at your disposal, as it can tell you a lot about not only how you are applying the brakes, but how you are utilizing them overall.

Keeping track of oil temperature is vital for any respectable car. While it isn’t necessarily used for tuning like water temperature is (coolant temperature can be used as a scaling factor for fueling and other engine management parameters), oil temperature is critical to understanding whether a motor is operating in safe conditions or not. Alerting on it is a necessary extension.

If you read the planning article before this, you remember that the original Haltech harness did not have knock sensor provisions. The new Elite 2500 ECU supports two knock sensors, and as the 2JZ block supports two, we figured we would use them both.


A Bosch flat response knock sensor, and some super secret special parts.

Older engines typically used what are referred to as resonant knock sensors. These were simple one-wire affairs that were pretty limited in their ability to not only detect knock, but also to be able to isolate it from all of the other noisy things that happen inside of an engine.

In a simplification, suffice it to say that a typical older resonant knock sensor (like you would find on the 2JZ) is basically a fancy microphone that is specialized to detect a specific frequency (of a knocking engine). If the engine is generating that specific knocking frequency, the output signal of this “microphone” is largest, and the ECU can detect that as knock. Pretty simple, and works OK.

There is a better option available, and that is something like these multi-wire “flat response” knock sensors from Bosch. These are very typical in modern cars, and are widely used in motorsport. Because of the former, they are a good solution for the latter -- they are cheap and readily available just about anywhere. The benefit of a “flat response” sensor is that it actually can be adjusted at the ECM end for the knock frequency that you want to listen for, more or less. This allows a single sensor to be used across multiple engines, as the ECM is what is calibrated for the knock, and not the sensor. There’s a problem, though. This sensor has a hole in it, and the factory Toyota 2JZ knock sensor screws directly into the engine. What to do?

Page 1 of 10 Next Page
Posted in: Project SC300
Bookmark and Share
Wednesday, July 05, 2017 8:52 AM
It is nice to see someone share some insight into some of the details that should be observed when doing high quality performance wiring.

FWIW, The smartwire allows you to pair 20a outputs into 40a outputs, depending on if you had available outputs you could have paired them instead of having to run external relays. Also, did you use 1 16awg wire for both fan power feeds(30 on the relay) or one for each relay? When you look up the spec, and this can vary from different places but to keep everything MilSpecWire/MoTeC, they show 16awg Tefzel wire only rated for 15amps...
Mike Kojima
Mike Kojimalink
Thursday, July 06, 2017 12:20 AM
I would think that filler wires add weight and complexity only for the sake of making the bundle look neater?
Thursday, July 06, 2017 3:24 PM
^that. I would have thought they would have some specialized foam cord or something to do that purpose if necessary.

Also, area Deutsch connectors really a higher quality connector than the OEM weatherproof? I feel like Toyotas from that era where pretty much at the pinnacle, though I do recognize that the repeated heat cycling on my 1JZ has made some brittle.

Thursday, July 06, 2017 5:21 PM
@ThisGuy 20x2 = 40, which, if these fans are 25A each, would require 3... so, there's that. I am not sure about the supply wire. Now that I think about it it might have been 12AWG. I'll have to re-check when I get back in the shop. But I do believe we used one wire for each fan supply, and I think it was 12. I'll let you know!

@kojima a perfectly round loom is easier to bend and drape. They add a tiny bit of weight, depending on the situation, and not much complexity. It's just kind of "how it's done". I suppose that if you were making a harness where you got one wire into the 2nd layer and then needed 10+ fillers to run 18 feet you might rethink using filler, though :)

@SM_Clay72 There are a bunch of reasons to go Deutsch DT* that were discussed in a few of the previous parts. The main reasons are environmental performance and connection performance (staying connected, not falling out, not falling apart). The "correct" way to have done the alternator would have been to actually remove the alternator, use a Faston .250" tab pressed onto each of the terminals inside the connector cavity on the alternator, and then fill the cavity with non-conductive epoxy. This permanently affixes the wires inside the cavity. In other cases where tabs are not small enough, you can solder a wire onto the terminal and then fill with epoxy. This is called "potting" -- we ended up not doing it anywhere, although I have the materials handy. The flyers that come off the alternator would then get a motorsports grade connector (DT*). I just kinda cheated.

Anyone who has dealt with an older *JZ engine knows all too well how terrible the factory Toyota coil plugs are. They disintegrate and/or the retention clips fail. Potted coils would be the way to go.

Factory connectors are designed for minimally harsh environments and with "just enough" durability and reliability to get the job done without failing. They're not generally designed with sustained extreme temperature and high RPM use in mind. It's all really about insurance at the end of the day, honestly.
Thursday, July 06, 2017 7:25 PM
@thoraxe - Strange I went through my loom specifically to remove 'unnecessary' wires, and was able to drop a few pounds. I know this doesn't sound like much, but I don't know why I would want to add weight, just to improve the looks. How would you even know, most of these bundles are wrapped with split loom anyway right?

And, I would definitely argue that more wires does not make the loom more flexible. That doesn't make any sense. Each wire adds a certain amount of bending resistance, and obviously, more wires would lead to more bending resistance.
Friday, July 07, 2017 9:00 AM

Were your wires made out of iron? lol Some of these unnecessary wires could be connected to things that are required via safety regulations. Fully working lights front and rear, possibly other mechanical functions.

I like this work! keep it up!
Sunday, July 09, 2017 6:32 PM
A few things here:

There are many differences between industrial automotive-grade wire and motorsport/milspec wire, but weight is a big factor. When pruning a factory harness, especially in a modern vehicle (anything built after 1990), there is an awful lot of cruft. If you guys remember back to an earlier part of the series, I took out many pounds just of ECUs and unneeded control boxes. It would be no surprise that if you simply got rid of the wiring for power windows, you would save a few pounds.

When it comes to proper concentric twisting, the reason for the twisting and alternate lays is to aid in the flexibility of the bundle. As we previously discussed, 19 filler wires is probably unnecessary. Not everyone uses filler, but it generally only provides for a better end product, both aesthetically and performance-wise. And, if you don't take my word for it, trust the folks at MilSpecWiring.
Tuesday, July 11, 2017 3:29 PM
filler doesnt equal quality or performance, filler is purely aesthetics and weight. most looms, be in the engine or chassis do not just flap around in the wind, they are attached to something, and in most situations the high flexibility of Handlay/concentrically twisting is not needed or can be achieved without it.

you will most likely never see filler in higher end motorsports. telling someone they need to pay additional money and have a heavier product just in the name of aesthetics will not fly. If you look at build photos from bf1, DCe, Zentec, Sakata,... You will rarely see filler. you will see more straight lay twists, with an outter single twisted layer. or you will see concentric twisting with gaps in the layers. most looms are built to the customers spec and most of the time the customer will not request it.

I have built quite a few harnesses in my day and I did use filler for the asthetics reason but the better I got the less filler was needed. I am not saying using filler is a lack of experience, I am saying once you get over the hype of filler you will find alternative ways to get a quality product.
ekka decker
ekka deckerlink
Saturday, July 29, 2017 6:00 AM
Good tip while making your loom up is to create a test pin set (alligator clip to appropriate pin or socket) for later fault finding so your not jamming multi meter probes into your connectors.

Also your number 1 or A should always be directly under the master key-way of the mil-spec plug if you ever find it hard to line up while populating it.

Any reason for going glue boots over a back shell with strain relief and then just sealing with tef tape , bi seal and a spot tie ?

Post Comment Login or register to post a comment.

MotoIQ Proudly Presents Our Partners:

© 2018 MotoIQ.com