S13 Sr20DET Blacktop to Mk1 CA18ET Electrical/wiring Guide

[George Lambadas]

I'll be documenting my work on wiring a S13 Blacktop SR20DET into my Canadian, early 1986 S12 that came factory with a CA18ET and the needle type cluster. My goal is to have everything on my factory cluster and dash be functional.

Word of warning, I'm not a mechanic. I know a thing or two about electronics, but I'm no expert. This guide is informational, and I'm not responsible if you fry your ECU, chop up your harness in an irreversible way, or never complete your swap after however many dollars spent. With that said, let's get to it!

First, I removed the driver's side fender to remove the original upper engine harness. When doing this, take note of the three connectors bunched together near the hole at the front of the engine bay. This is where a lot of the wiring work is going to take place. My intention is to reuse the S12 connectors when making the adapter harness.

Next, I removed the original lower harness from the passenger's side of the car. There is an 8 pin connector mounted to the frame that goes to the switches on the transmission as well as to the oil pressure sending unit. There is also a 2 pin connector that's full of contact grease (white gunk) which goes to the positive battery terminal.

I like to test things, so here are some tests you can run to make sure you have the right wires identified.

Upper Harness:
First 2 connectors:

6 pin connector:

• Fuel pump: This will be a heavier gauge, either black or white, and is identifiable by the shielding that splits off into its own useless pin. Test with a multimeter and you should see 12v regardless of the key position. Ground this pin and your fuel pump will run. Obviously only do this if you're ready to catch the fuel your car is about to dump out, or if you've run the supply to the return.
• EFI power: Light Green/Black. Test with a multimeter. Should read 12v with the key "On".
• Coolant temp sensor: Purple/White. Ground this wire with the key "On" and watch your coolant temperature gauge rise.
• ECU backup: Black/White. Test with a multimeter when the key is out of the car and you should still see 12v.
• Start signal: Black/Yellow. Test with a multimeter and you should see 12v only when the key is pushed to the "Start" position

8 pin connector (with only 7 wires):

• Sensor power: Green. Test with a multimeter. Should read 12v with the key "On".
• Air regulator power: Black/Red. Test with a multimeter. Should read 12v with the key "On".
• Tach: Blue/White. (this is actually confirmed NOT WORKING. If anyone can give me guidance here, I would appreciate it).

The one wire that's not mentioned so far is the Black/White wire that powers the coil. Either coil wire will do. This is 12V switched power that you can test with a multimeter. It should read 12v with the key "On".

Lower Harness:

8 pin connector (with only 7 wires):

• Oil pressure sending unit: Yellow. Ground this pin with the key "On" and watch your oil pressure gauge rise.
• Reverse switch: Green and Green/Yellow. Connect these two pins with the key "On" and watch your reverse lights go on.
• Battery charge light: White/Red. Ground this wire with the key "On" and watch your charge light go on.
• Alternator field coil: White. I don't know how to test this.

2 pin connector full of contact grease:
Both of these just run to the positive battery terminal.

On the SR20DET upper harness. (numbers in brackets correspond to ECU pinout numbers found here: https://imgur.com/j7NTItb):

Brown 8 pin connector:

• Sensor power: Brown
• Fuel pump relay: Black/Pink
• Neutral (safety) switch: Green/Orange
• Ignition Switch: Red/Black (to be verified)

Grey 8 pin connector (Forgot to take a better picture of this):

• Power from EFI relay: Black/White(to be verified)
• Fused 12v for Air Regulator: Black/Yellow
• Constant 12v (ECU Backup) Power: Red
• Coil Pack Harness switched power: Blue/Orange (Might be Blue/Red. I'm bad at colours)

White dash connector next to ECU:

• Start signal: Orange (34)

ECU connector:

• Fuel pump ground: Black/Pink (104)
• Tach signal: Yellow/Red (3)
• ECU Backup: Red (46)
• ECU Power: Black/White (47 & 38)
• Ignition switch: Black/Red (36)

On the SR20DET lower harness:

• Oil pressure sending unit: Yellow/White
• Reverse switch: Green and Green/White
• Alternator field coil: White.
• Battery charge light: White/Red.
• Starter solenoid signal: Black/Yellow. Thick wire.

With that said, here's how things should be wired.

For the upper harness in the engine bay:

On the S12.	On the S13.	What is it for?
Light Green/Black	Black/White	EFI Power
Black/White	Red	ECU Backup
Green	Brown	Sensor Power
Black/Red	Black/Yellow	Air Regulator Power(IAV)
Black/White Coil Wire	Blue/Orange	Coil Pack Power
Black/White Coil Wire	Red/Black	Ignition Power

NOTE: I would recommend using one coil white each for the coil pack power and the ignition power. While I haven't tested this, I know that the coils will draw progressively more power as RPM increases, so distributing the load will better allow the wire to provide the necessary power. We don't want an unstable power source for the ECU.

Let's talk about the fuel pump.

As we know from testing the wire, it's a ground-controlled circuit meaning that grounding the fuel pump wire causes it to run. The SR20DET Black/Pink wire is grounded by the ECU when the ECU wants the fuel pump to run. I don't know if the ECU can handle grounding the amount of current that the fuel pump draws, so I've wired in a relay, controlled by the ECU and is used to ground the fuel pump wire.

Your relay will have either 4 or 5 wires. Two wires (sometimes smaller than the other 2 or 3) will be the coil wires. We'll collectively call these the control side of the relay. The other two (or three) wires are the switched side of the relay. One of these wires will be treated as the input, and the other(s) as output. In the case of a 5 wire relay, one of the outputs will be normally closed (NC), and the other normally open(NO). The NC wire will have continuity with the input wire when there is no power to the control side of the relay. The NO wire will have continuity to the input wire when there is power to the relay. Test your relay directly on your battery by putting the control wires to either terminal to hear a tick. You'll also be able to use your multimeter to see which wire has continuity to the input when the relay is active. That's your NO wire. You'll want to take note of it.

Now, let's talk about how to actually wire the relay in!

On the relay.	What to connect to	Why?
Control wire 1	Black/White Coil Wire	Power the relay's coil
Control wire 2	Black/Pink on the SR20DET Brown connector	Close the control side circuit and activate the coil
Input wire	Black or White S12 fuel pump wire	Give the relay control over the fuel pump
Normally Open wire	Chassis Ground	Run the fuel pump!
Normally Closed wire (not always present)	Cap it securely	If this wire is accidentally grounded, the fuel pump will run. This is a safety hazard.

In summary, here's what's going on. The chassis provides power to the relay. The ECU completes the control side of the relay circuit, thereby powering the coil. The fuel pump is grounded by the relay when the coil is powered.

For the upper harness in the cabin:

On the S12.	On the S13.	What is it for?
Black/Yellow	Orange	Start Signal
Purple/White	Blue	Water Temp Gauge
Black/Yellow	Orange	Start Signal

For the lower harness:

On the S12.	On the S13.	What is it for?
Green	Green	Half of the reverse switch
Green/Yellow	Green/White	Second half of the reverse switch
Yellow	Yellow/White	Oil pressure sending unit
White	White	Alternator field coil wire. This is the thick wire
White/Red	White/Red	Battery charge light

[kelso840]

I'll be very interested to follow this thread now that I'm not an 18 year old idiot and have a better understanding of wiring.

[George Lambadas]

Update 1:
I've added some pictures of connectors as well as the beginning of how things should actually be wired.
I also added a picture of the ECU pinout that I'm following for the SR.

At this time, my engine cranks when I turn the key, and the reverse switch on the trans turns on the reverse lights. I still haven't finished the fuel or cooling systems so I'm not actually able to test a start yet.

[George Lambadas]

Update 2:
I've added more information on how to wire things together
I also added another wire to identify on the engine harness - power to the coils.

Most importantly, the car actually starts!!! I wasn't able to idle it for more than a few seconds as there was no radiator installed, but I can confirm that this wiring guide is good enough for a smooth idle.
First start: https://www.youtube.com/shorts/ytJbDm1_zys

[George Lambadas]

Update 3:
Alright, this is as close to done as I think I'm going to get.
I still can't get the tach to work and I have no idea why. If someone can point me to exactly where to send the signal, I would be extremely grateful.

This wiring guide will get you very far along on the wiring, but does have limitations.
Obviously, the tach doesn't work. This is the big one.
Less importantly, the oil pressure gauge will move, but not in the way you would expect. The CA18ET oil pressure sender is progressive where the SR20DET's oil pressure "sensor" is a switch. Additionally, the CA reads high (5v) when there is no oil pressure, and grounds with high pressure. The SR oil pressure switch grounds when there is oil pressure and reads 5v when there's no pressure. This means your oil pressure gauge will read only the minimum or maximum value, and it will be the opposite of the actual oil pressure. The silver lining is you'll still know if you have oil pressure or not!