The most time consuming part of the swap. I was somewhat reluctant about doing this swap because I was afraid of the daunting task of the wiring. However, once I got into it it was fairly straightforward, but very time consuming. I had no previous experience doing anything like this before. I do have an electrical engineering background, but nothing automotive. (If you are new to this as well, I suggest reading up on a set of Toyota Technical manuals that are available at Autoshop101.com. Great way to learn about Toyota's systems.)

I took my time with the wiring, basically working on it throughout the mechanical part of the swap. Since it doesn't involve any real physical dirty work, I worked on it when I wanted to take a break from those sort of things. Plus once I got the wiring harness mapped out on paper, I could do all the work inside the AC'd house.

1. The first step is to label each connector as you remove it from the engine.

2. Once you get the harness out of the engine, start taking notes on each connector. Include the orientation of the pinout and the wire colors. Make sure to keep some space between the difference pins, as you will be making more notes here shortly.

3. Next, start taking apart the harness, removing all the tape and wire loom. Make sure to keep the harness organized and untangled, as well as in it's original layout. Once you have the original loom & tape off of a section, loop a piece of electrical tape around the wires. A loop of tape every 12-18 inches will keep everything from getting tangled. Also, make sure to keep the wire loom so that you can reuse it once you get the loom completed.

4. When this is done, begin mapping out each and every wire. This is where most of the time is spent, but it is very important in getting to know the wiring harness. It also makes removing unneeded accessories a breeze. If you did not get a clip or half cut, this is going to make this sorta difficult, as there are quite a few wires that will just terminate at the body harness connectors. Most of these are the instrument cluster, AC, ABS, and ignition switch signals. A couple of them are for the idle-up circuitry as well. If you do have a clip, trace these pins at the body harness connectors if you can't figure out which signals they are by where they terminate on the engine harness. There's no need to torture yourself and map out the entire body harness. Unless you really want to. Though it is nice having the body harness stripped so that you can use it for spare wire and pins.

5. Now, start removing these unneeded circuits. These circuits won't have all their wires on exclusive connectors. So you will need to remove some pins/wires from other needed connectors. To do this, you need something with a very tiny point. I just used a safety pin the whole time. On the late style Toyota connectors, there is usually a lock on them that you need to raise. See the white rectangular area on the connector below? Pry up on the two indented spaces with the safety pin and the lock should raise up. Do NOT attempt to remove the lock entirely from the connector. Not only is it unnecessary, but you could also destroy the lock.

   

   Once it is in the up position you can remove the pins by releasing the tab that is holding the individual pins inside. Look at the front of the connector and you should easily see the tab that you need to push down. While pushing down with the safety pin, pull the wire out from the back of the connector. For the older style Toyota connectors (what is on the MKI MR2) this is the only thing holding the pins inside the connectors. There is no lock.

   

   The circuits I removed from the 20V engine harness were the ABS, headlights, radiator fans, horns, and AC. Now's the time to decide whether or not you will want to use AC.

6. After that is done, set the 20V harness aside and begin work on the MR2 harness. I also removed the body harness from the MR2, which made it easier to trace wires back into the fuse box (the engine fuse box is on the body harness of the MR2, engine harness of the 20V). I also needed to run more wires to the MR2s body harness for the Blacktop, more on that later.

7. No need to label every connector on the MR2. Only the main ones that you will reuse, the trunk connector, the grey connectors in the engine bay, and the body harness connectors that interface to the MR2's interior harness. Also, are you going to use the MR2's engine bay fan? If so, make sure to label those.

8. Once removed, begin stripping the old tape and loom from the MR2 harnesses. Try your best to keep the firewall grommets intact. I ended up slicing them down the middle to remove them from the harness. This worked farily well.

9. If you are going to use the engine bay fan, remove that circuit. It's very straightforward. Now, start mapping out the wires that interface between the body and engine harness. Use the wiring diagrams in the BGB and/or Haynes manual to help. There may be some discrepancies within the documentation so be sure to check the circuits by hand just in case.

10. By now, you should understand what all you need to do to join the 20V engine harness to the MR2 body harness. Here's a diagram that will hopefully help you out, that I put together after the swap (click on it for larger pic). This is the Blacktop wiring diagram drawn in the form of Toyota's MR2 diagrams. The red lines indicate that it is part of the engine harness, and the black is the MR2's body harness. So...wherever black meets red is a signal that you will need to pull to your connector interface.



• The Silvertop harness will be slightly different, at least with the COR. The Alternator Sensing fuse is optional. You could just tie this to B if you wish.
• Notice how I wired the engine bay fan as well. This way it's on whenever the ignition is in the ON position. You could wire it up like the 16V MR2 with the temp sensor and the computer if you wish. Just takes a couple more wires and mounting of the sensor in the engine bay.

However, these are not the only signals needed to complete the interface. There are a few other wires on the body harnesses needed for the ECU. Below is a diagram of all the ECU signals. Those in blue indicate signals contained the above wire diagram. TC1 and TC2 are the two trunck connectors I used to complete the interface. So those in black that go to TC1 and TC2 will need to be in the interface as well.



I used the original trunk connector that was on the MR2 harness and an additional connector I grabbed from the 20V body-engine harness interface. I was able to eliminate both engine bay grey connectors on the original MR2 body harness which makes things a little neater in the engine bay. It may be possible to use only one connector, especially if you are doing a silvertop swap.

New Connector Diagrams

Here are pin description tables of the 2 connectors I used. By no means is this the only way to do this, it just is here to give you an idea of the signals you will need to run:

Update (Feb 14, 2008): I changed the first connector around a bit to keep this with what I'm actually currently using. When I shortened the harnesses in the trunk a year or so ago, I moved the 4 ELS signals to the second connector to reduce the number of contacts in this main connector. It was becoming too hard to detach/reattach the connector. I've also added the 2nd connector diagram as well as the stock 16V connector diagrams.

New 20V Trunk Connector #1:



	Body Harness (Male) Side		Engine Harness (Female) Side
1	G/W	Check Engine Light to Dash	R/Y	Check Engine Light from ECU
2	Y/Blk	Oil Pressure to Driver's seat connector	W	Oil Pressure Sensor
3	R/W	Batt from EFI Fuse	R/W	Batt to ECU
4	NC		NC
5	Blk	Starter Relay	Blk	Terminal 50 on Starter
6	Y/G	Water Temp Gauge to Driver's seat connector	Y/G	Water Temp Sensor
7	Blk	+B EFI Relay	Blk	+B COR, ECU, O2 Sensor, etc
8	Blk/Y	10A Engine Fuse	R/Blu	Alternator IG signal
9	NC		NC
10	Purp/W	Speed Sensor to Driver's seat connector	Purp/W	ECU Speed Sensor input
11	R/Blk	L&R Reverse Lights	R/Blk	Reverse Switch on Tranny
12	Blk/R	Power from Ignition Main Relay	Blk/W	Ignitor/Coil & Injectors
13		Cooling Fan Relay coil side	LG/Blk	CF from ECU
14	NC		NC
15	Blk	IG- to Tachometer to Driver's seat connector	Blk	IG- from Ignitor/Coil & Diagnosis
16	W	Alt 'S' 5A fuse (added to fuse/relay box)	W	Alt 'S' Signal
17	Y	5A Charge 'L' Signal from Alternator	Y	'L' Signal from Alternator
18	Blu	Fuel Pump	Blu/Blk	Circuit Opening Relay & Diagnosis
19	NC		NC
20	Blk/G	Engine Bay Fan	Blk/G	Engine Bay Fan
21	Blu/W	Engine Bay Fan	Blu/W	Engine Bay Fan
22	Blu/Blk	Engine Bay Fan	Blu/Blk	Engine Bay Fan




Optional

Here's just another way at looking at the connector pin descriptions (click to zoom in):



Blue = Body Harness side
Red = Engine Harness side
* = there's 2 ways to do this. I wired the engine bay fan to be on whenever the ignition is on. If you would like to keep the original wiring with the cooling fan computer and the temp sensor, just use the 2 empty spaces for the other 2 wires (Blk/G & Blu/W) in the circuit.

New 20V Trunk Connector #2:

The second connector contains the STA signal, a ground, the idle-up signals, and some optional things I wired in (air/fuel meter and a VVT light in my dash (which i need to fix, cause it's still not wired right in my dash)).



	Body Harness (Male) Side		Engine Harness (Female) Side
1	NC		NC
2	W/Blk		W/Blk
3	Blu/R	Cooling Fan Relay switch side	W/Blu	ELS1 to ECU
4	Blu	Blower Relay	Blu	ELS2 to ECU
5	Blk/W	Starter Relay	Blk/W	ECU STA
6	NC		NC
7	--	A/F Meter Ground	--	O2 Sensor Ground
8	G	Taillight Relay	G	ELS3 to ECU
9	Blk	Defog Switch	Blk	ELS4 to ECU
10	R	Dash	R	ECU VVT
11	NC		NC
12	--	A/F Meter Signal	--	O2 Sensor Signal




Optional



Blue = Body Harness side
Red = Engine Harness side

Old 16V Connectors

I've gotten some emails in the past to document the stock 16V connectors. These vary by year, but here's what I had on my '85:

Stock 16V 4-pin Grey Connector in the engine bay:

Battery + Various Big Fuses Battery, Alt, Starter Black	Battery + Various Big Fuses Battery, Alt, Starter W
Engine Bay Fan Power Fan Relay Engine Bay Fan Blue/Black	Term50 Starter Relay Terminal 50 on Starter Black/W

• The 2 Battery+ were connected together on the other side of the connector and were part of the alternator & starter wiring direct from the battery. This is taken care of by the battery relocation wiring that I did, where I wired directly from the battery to the starter and alternator.
• The other 2 were moved to the trunk connector

Stock 16V 12-pin Grey Connector in engine bay:

Engine Bay Fan NC Engine Bay Fan Blue/W	--	Clutch Start Switch Driver's side kick panel Back to starter relay pin Black/W
AC Clutch AC Clutch Fuse ECU Black/W	--	--
--	--	Starter Relay Starter Relay Back to clutch start switch pin Black
--	CEL Gauge cluster ECU G/W	--

• The AC wiring was removed since I ripped out the air conditioning system.
• Same with this engine bay fan wire, since I didn't use the temp sensor.
• The clutch start switch and starter relay were tied together on the engine harness side. So i just soldered these connections together on the body harness side.
• CEL was moved to trunk connector.

So, to summarize with these 2 connectors: Only 3 of these wires were rerouted to the new trunk connector. The rest were either tied together, used by my new battery wiring, or discarded.

Stock 16V 22-pin Trunk Connector:

Here's the stock 16V trunk connector. The green shows the pins that weren't moved when redoing the connector for the swap. I tried to keep it the same as much as possible.

	Body Harness (Male) Side		Engine Harness (Female) Side
1	NC		NC
2	Y/Blk	Oil Pressure to Driver's seat connector	Y/Blk	Oil Pressure Sensor
3	NC		NC
4	NC		Blk	Injector #3 & 4
5	NC		R	Injector #1 & 2
6	Y/G		Y/G	Temp sensor
7	Blk	EFI Relay +B	Black	COR +B
8	Blk/Y	10A Engine Fuse	Blk/Y	Alternator
9	NC		NC
10	Purp/W	Speed Sensor to Driver's seat connector	Purp/W	ECU Speed Sensor input
11	R/Blk	L&R Taillights	NC
12	Blk/R	Engine main relay&fuses	Blk/R	Ignitor/Coil
13	W/R	7.5A AM2	W/R	ECU
14	NC		NC
15	Blk	IG- to Tachometer to Driver's seat connector	Blk	IG- from Ignitor/Coil & Diagnosis
16	NC		NC
17	Y	5A Charge fuse	Y	Alternator
18	Blu	Fuel Pump	Blu	#1 COR
19	NC		NC
20	Blk/G	Engine Bay Fan	NC
21	W/Blk	L&R Tail Lights	Br	Chassis Ground
22	Blk	Idle up diodes	Black	ECU ISC




Same as resulting connector after swap