Thank you.

Introduction

It is becoming increasingly difficult for ST165 owners to find used replacement engines from the Japanese market, which is typically the holy grail for low mile high end engines such as the 3S-GTE. However, there were much more ST185's built in Japan and finding one in good condition is much easier. This works very much in favour for us ST165 owners, since we can replace our old tired engines with the newer 2nd generation 3S-GTE, which comes standard in Japan with 225 horsepower. Stock ST165 power is rated at 190hp, for the North American market. This swap is becoming increasingly popular as people continue to restore the GT-Four that started it all, the ST165.

In my experience, most people in the past have used the ST165 engine harness and Engine Control Unit ("ECU") to control the ST185 engine. Honestly, I couldn't believe how much this is done. The two engines are very different even though they share the same engine code. For example, they differ in the following ways, among other things: Compression Ratio ("CR"), different Volumetric Efficiency ("VE"), head design, cam design, intake and exhaust manifold design, turbo design, and the list goes on. To me, it just seemed like a very dangerous option to use the ST165 ECU to control the ST185 engine. I do understand why people do it though, it is much easier to just plug in the old harness and call it a day! I am aware of a few people that, with very minimal mods, have been running the ST165 ECU reliably for quite a long time, for what it's worth.

Now there are several ways I believe a person could go about using the ST185 ECU in the ST165 chassis. One way, which I did not explore would be to use the ST165 harness, and rewire the ECU connectors to fit the ST185. However, as you will see, there are many differences in the two harnesses, so this might cause problems during the engine swap. In my view, it is just a better idea to use the harness and ECU for the engine you are using, and convert to the ST165 plugs that you may need down the road.

So, why is it so difficult that most do not bother to do the conversion? Well, there are a few major trouble spots that you need to deal with. Firstly, the two harnesses use different fuse boxes. You can not simply "plug it in". To solve this you have to rewire one of the two harnesses that plug into the fuse box. Next, there is a difference in the way the two engines go about arranging the engine harness. There are many systems in the ST165 engine harness that the ST185 locates in the engine room harness. (Note: the engine room harness wraps around the outside of the engine bay, and supplies power to things like your lights, horn, ABS, and from there goes into the interior on a separate harness, so please note this difference in terminology) Also, the two ECU's are wired differently. The 3rd plug on the ST185 ECU comes from the interior into the ECU, whereas the ST165 has all 3 ECU plugs coming from the main engine harness. The last major issue (although there are other obstacles as well, which I will do my best to explain) is where the engine harness plugs into the interior harness, to feed the dash. These plug(s) are completely different between the two cars, as you will find out below.

Wiring Harness

If you are transplanting a JDM engine into your left hand drive vehicle, you will find that the wiring harness enters the interior on the opposite side of the firewall. In this case, your harness will be much too short to be able to reach the existing hole in the firewall. You have two options. One option is to drill and cut a large hole into your firewall, and attempt to route the harness on the drivers side of the interior to the ECU. Although this will work, I do not like it because it leaves you with an empty hole where the old harness went, and also because the harness will go down by the drivers feet, and in some cases might interfere with your pedals. That said, this is the "easy way". The other option, which I chose to do, is to extend every single wire long enough to reach the existing hole in the firewall. This is extremely time consuming and generally not fun to do, however it results in a much cleaner install and the harness will never be in the way.

To extend the harness there are a few things to keep in mind. First, you should measure out how much extra wire you need. I needed an extra 32" for each wire. Second, make sure you are very good with soldering. This is one place you DO NOT want a bad connection, and will be very difficult to diagnose if there are problems later on. Every connection must be covered with heat shrink tubing, and be sure to extend only one wire at a time. Make sure to use the proper wire, and a good quality flexible wire covering. I suggest that you to not seal the wire covering with electrical tape until you finish adding the appropriate wires for the ECU and circuit opening relay. For the normal wires, make sure to use wire that is at least as thick as the original wire. Thicker is good, but as you go thicker it becomes difficult to maneuver the harness and fit it through the hole in the firewall. Some wires are shielded. They are like this to prevent any interference in their signal. Use shielded wire for these, if not, you might have problems. Lastly, you want to use the ST165 rubber firewall boot over the engine harness, since the ST185 one does not fit your exiting hole. While you are extending the harness, you have the perfect opportunity to switch them. Just make sure every wire comes out of the ST185 boot, and through the ST165 one. This way, you will retain a factory look and fit.

To give you an idea of how much work this involves, take a look at the picture below.

Here is what the harness should look like once you are done:

Fuse Box

The rewiring of the fuse box is the most difficult part of this engine swap in my opinion. When attempting to re-wire the fuse box you will see that there are two harnesses that plug into it. One is the main engine harness, which will be referred to from here on as the engine harness (or engine wire). The other plugs come from the wiring harness that is run around the outside of the engine bay, which shall be referred to from this point on as the engine room harness (or engine room wire), to follow the notation in Toyota's BGB (Big Green Book, the factory repair manual which is in fact a big, green book). Now because there are 2 harnesses that directly plug in, you have the choice of keeping one of the sets of plugs in tact, depending on which fuse box you choose. Personally, I chose to use the ST185 fuse box and retain the engine harness' plugs. I made this choice because I found the information on the engine room plugs to be more complete, and I would be able to avoid re-soldering some very thick ground wires. I would assume it is possible to wire either model of fuse box, however I will only discuss the process that I followed.

A quick skim through the BGB and EWD books will tell you that there is no direct list of pin outs that tells you the exact function of the wires in the fuse box. Instead, most diagrams are drawn by the system and reference is made to the fuse box plugs when necessary. This is probably good for trouble shooting a system, however not especially useful for my purpose. With great help from fellow GT4 owners, I managed to devise a cross-reference spread sheet, matching the wires in both fuse box's by function. This way I could basically take the ST165 wire for a given function, and plug it into the corresponding ST185 plug and pin. Simple? Well, nothing is simple on this car. However with a little effort and a good soldering iron, it can be done.

The diagram is given below. Please note that I strongly encourage you to double check this information with your diagrams and manuals, because it is very easy for me to leave out a small piece of very important information that I might have done "on the fly", since these systems can easily give one a pretty severe headache! So please double check! If you find any errors please let me know so that others do not make a mistake in the future. I'd be glad to make changes where needed.

You can use the following diagrams to properly identify the plugs and pins mentioned above.

ST185

ST165

WHAT TO DO

Basically, you must remove the wires ST165 engine room harness plugs, and put them into the corresponding ST185 plug, as described in the chart above. When reading the cross reference chart, please be aware that the plug names, for example 2B, are not the same between the different generations. The plugs are not in the same position between the two generations, and do not represent the same functions. You will find that the ST165 wires won't fit into the ST185 plugs. You have to cut the existing tips off of the ST165 wires, and solder on the ST185 pin ends. Once you do this the wires will pop into place. One very key thing to remember is that before you remove the wires from the ST165 plugs, use a piece of masking tape and label the plug and pin on EACH wire. It is also a good idea to label the connectors and each pin on the connector itself. If you look carefully in the picture below, you can see how I have labeled every wire.

Once you have plugged in all of the wires into the appropriate plugs, install the top and bottom fuse box covers, and you're done! Then keep your fingers crossed that you didn't make any mistakes! One thing to do before starting the car up, is to watch the fuse box very carefully when you connect the battery, and when you turn the key to different positions. This way you can watch to see if you blow a fuse, or be prepared in case of a fire. Having a fire extinguisher around is useful. Might sound like overkill, but you do not want to ruin your car if there is something wrong with your wiring. That said, the fuse box should clean up nicely like in the picture below.

Third ECU Plug

You will find that the new ST185 harness will not have a plug to go in the 3rd slot on the ECU. This is because, on the ST185, this plug comes from the interior. On the ST165, this plug is part of the main engine harness. Therefore, you have to replicate the ST185 system to be a part of the main harness. I strongly recommend you take a look at the ST185 BGB and examine the systems that are on this plug. The diagram below attempts to show which wires you must get the signals from for each wire. There is one wire on the Japanese ST185 ECU that does not exist in any of my books, and I do not know what it is responsible for. The wire is red with a yellow stripe, and sits between the STP and ELS pins. Regardless, the car will run fine without this wire.

INSERT CONVERSION CHART #2 HERE

Interior / Engine Harness Connectors

The main engine harness has several plugs that go in the area behind the radio. You will find that on the ST165, there is one plug that goes into the interior, which powers your dash instrument panel and gauges. On the ST185, there are two such plugs. Clearly, this poses a problem for the swap since these plugs are not compatible. So, how did I get around this? Well, I went through the ST165 engine harness and found every wire on that plug, and recorded where it comes from and it's function. Then, using my observations, I searched for wires on the ST185 plugs that controlled the same things, or received the same signal. I found that the plugs are not perfect matches for each other, since some wires existed on the ST185 harness that didn't on the ST165 harness, and visa versa. Therefore I was left over with some extra wires, and I also had to run a few new wires that weren't in the ST185 engine harness. I can only assume that there are some different functions between the two cars. This makes sense, since there were quite a few revisions between the two generations. Air bags, for example, are present on the ST185 and not on the ST165.

Note: The Plug Pin numbers in the charts below were arbitrarily assigned by myself.

ST165 Interior Connector

ST185 White Interior Connector

ST185 Grey Interior Connector

WHAT TO DO

In order to connect the ST185 engine harness to the ST165 interior connector, you need to use the ST185 grey plug, so that you don't have to swap over a bunch of pins to fit into the original ST165 plug. The grey plug and the original ST165 plugs are nearly identical, and can be used interchangeably. The following chart shows which wires must be used in the grey ST185 plug. The first column shows the final destination of each wire, which is followed by the wire colour, and the function of the wire in question. The last column shows where you are to get the signal for the wire from.

Circuit Opening Relay

You will also find that the wiring for the Circuit Opening Relay ("COR") differs between the 4th and 5th generation GT4's. The COR is the little black relay box that clips on to the side of the ECU. Once again, the ST165 COR plug comes directly from the engine harness, while the ST185 COR plug comes from the interior, leading up into the engine room harness. Therefore, in order to use the COR in your ST165, with the ST185 harness, you must run your own wires to replicate the ST165 set up. There are 5 wires, which must be wired in according to the diagram below.

Fuel Pump Relay and Fuel Pump Resistor

The fuel pump wiring systems are located on different harnesses between the two gens of gt4's. The 165 includes these systems in the engine harness, whereas the 185 locates these systems in the engine room harness. Therefore, when you swap in your ST185 engine you will not have the fuel pump systems anywhere!

As usual, there are a few ways you can go about solving this problem. I am aware that there is a way to bypass the fuel pump relay to give the pump constant voltage, however since the whole point of doing the swap with the ST185 electronics was to retain the OEM systems and thereby OEM reliability, I would greatly prefer to use the resistor and relay.

Since you have hopefully kept your ST165 engine harness, the easiest way to add in these systems would be to remove them from the old harness and add them to the new one. In order to do this you must be very careful to note where each wire comes from. Once you have removed the resistor and relay mini-harness, which consists of only a few wires, you should be able to simply add them into the new harness. All of the wires go either into the ECU or into the interior connectors that sit beside the ECU. If you look at the conversion charts I have created, you should be able to see where these wires go.

I should also note that I used the ST185 resistor and relay, in case that they were different. I simply bolted them to brackets in the engine bay.

INSERT CHARTS AND/OR DIAGRAMS

Miscellaneous Engine Bay Plugs

There are a number of plugs that you will find throughout the engine bay that will not match up to any of your existing systems. In this section I will do my best to describe what must be done with each plug. In some cases, you will need to add in the wires from scratch, and in others you are able to remove the entire wiring sub-system from your old ST165 harness. There are a few plugs that you do not need to connect, because they deal with things that are not used on the ST165.

I will begin with a picture and/or a description of each plug, and then provide information on the wiring of the plug below.

This is a large, round plug found beside the engine harness hole in the firewall (on the LHD ST165), originating from the engine room harness. The actual plug in the pictures is from an ST165. The plug has 3 thick wires. It has a direct match on the ST185 engine harness. On the RHD ST185 harness, you will find the matching plug in the area where the harness would go in the firewall on the right hand drive car. You can identify it because the wire colours are identical on both models. Basically, you have to extend the wires to reach the original ST165 plug coming from the chassis. You also have to solder on the matching ST165 or ST185 plug so that they will plug into eachother. Be sure to use very thick wires, because this plug contains vital power and ground wires.

This is a uniquely shaped plug found beside the engine harness hole in the firewall (on the LHD ST165), originating from the engine room harness. The plug has 6 wires:

• green/white/stripe: to STP pin on ecu

• black/white: plug on transmission (2 wire plug)

• White/Blue: A/C (blue plug 165)

• White/Red: A/C (blue plug 165)

• black/brown/stripe: interior connector pin 19 on st185 interior connector, after it is rewired

• brown: plug on transmission

                                

The first 2 pictures show a single white wire plug that should be located at or near where the factory battery is mounted. The second 2 pictures show a two-wire plug that is located in the same area, with 2 thick white/red wires. The plugs do not match up, so you must use either both 165 plugs from this location, or both 185 plugs from this location. You connect all 3 wires together. Soldering these fat wires is tough, so be warned! The last picture shows the finished product, with the 2 plugs connected together using the ST165 plug.

   

WILL UPDATE PAGE SOON WITH INFO ON THIS PLUG

   

This plug is located by the strut brace, near the hole in the firewall for the harness. I can't guaruntee this one, because my A/C system does not work. However, I removed the connecting wires from my old ST165 harness for this plug. The other end of the wires from this plug connect to the A/C compressor.

   

WILL UPDATE PAGE SOON WITH INFO ON THIS PLUG

   

WILL UPDATE PAGE SOON WITH INFO ON THIS PLUG

   

This grey plug found on the JDM ST185 harness is not used. You do not directly plug it into anything, because it is for ST185 systems that you do not have on your ST165. I do not know the exact functions of these wires. It is located near in the area of the distributor and fuse box, depending on the length of the wires.

   

This black plug found on the JDM ST185 harness is also not used. Again, you do not directly plug it into anything, because it is for ST185 systems that you do not have on your ST165. I do not know the exact functions of these wires. This plug hides near the distributor, sometimes underneath of it.

Intake Manifold and Intercooler Selection

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You must select an intercooler to use on your new engine. You have quite a few choices, although some are easier than others. Your intercooler choice will also play a factor on which intake manifold you use, because the ST185 and ST165 manifolds are quite different.

Firstly, you can choose to use the factory ST185 air to air intercooler, although it probably isn't the best idea. In order to make use of the intercooler, you would need to mold a hood scoop into your hood, to feed the top mounted intercooler with air. This isn't a very feasible solution for most of us, and most choose a different route. Without the hood scoop, the intercooler would not receive the cold outside air to cool it off, and your intercooler would become extremely hot, your EGT's would rise, your engine would start to detonate and eventually blow up. Probably not the best plan. Hood scoops have been used on the 4th generation Celica in the past. A nice example is shown below, with an ST185 style scoop.

       

The next option, which happens to be the option I chose, is to install an ST185 RC edition intercooler. This is the factory liquid to air intercooler used on the ST185 RC / Carlos Sainz / Group A Rallye Edition Celica's in just about everywhere except North America. It is often regarded as one of the best WTA IC's you can buy. To compare, take a look at the difference of its size over the ST165 intercooler. The core is at least 2x as big!

The install is very straight forward, since the ST165 is already fitted with an intercooler water pump and heat exchanger from the factory. If you use the ST165 intercooler to throttle body rubber connector, the IC is a direct bolt on to your engine. You might want to look for an RC engine hook/IC support bracket as well. You will also find that the ST165 uses smaller hoses than the ST185RC intercooler. This is a fairly easy fix. I found an adaptor in the plumbing section of Home Depot, that is a reducer from 1/2" to 3/8" (I think). With a few clamps and about 8 inches of 1/2" hose, I fitted these adaptors to the system. Here is a picture of the adaptors installed:

INSERT PICTURE OF ADAPTORS

Here are a few pictures of my current set up, with the ST185 RC intercooler installed. It is a very nice touch to finish off the engine swap.

And yes, it fits under the hood, just barely!

The cheapest intercooler option for this swap would be to use your original ST165 intercooler. It is a small water to air unit that works quite well at stock power levels, despite it's small size. The core itself is roughly 2 to 2.5 times as small as the ST185 RC intercooler pictured above, as you can see in the picture. If you are looking to increase the boost in the future, this intercooler is not very effective for power levels over 250hp at the flywheel.

The are a few ways to adapt the setup to accomodate the ST165 intercooler core. The intercooler does not line up to the throttle body and turbo the same way between the two generations of engines. Specifically, the ST185 throttle body is slightly to the left of centre on the intake manifold, whereas the ST165 throttle body is in the centre of the intake manifold. Also, the turbo sits higher and at a different angle on the ST165.

There are two ways to mount the intercooler. The first and best way is to replace the ST185 intake manifold with the ST165 one, for it to line up properly. Luckily the bolt pattern is the same and the manifolds are interchangeable. You should decide this before you put the engine in the car, because it is extremely difficult to swap manifolds with the engine in the car...trust me! If you use the ST165 intake manifold and throttle body (the TB's have different mounts), the intercooler will pop in place nicely. You will need to add a small pipe with a bend in between the IC and the turbo however, because of the way the ST185 turbo sits.

The second way is to use the ST185 intake manifold. If you do this, you must use some kind of adaptor to allow the intercooler to sit on a slight angle. An mr2 TB pipe and a rubber connector would work well, which is similar to what is used in the picture below. However, the intercooler will not sit perfectly in place and you will still need to add in the pipe between the turbo and IC.

Out of the two options, I prefer the first.

Intercooler Pump

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Since the ST185 ECU does not have any inputs for the existing ST165 intercooler pump, this renders the stock intercooler ECU useless. This in turn causes the intercooler pump to be non-functional. However, this is a pretty easy fix. You simply need to provide a 12V constant power and ground to the pump itself, and you're IC pump will be running constantly. Running the pump constant is actually a common modification for GT-Four owners with water to air intercoolers, so it's a nice plus that keeps that intake temp nice and low.

Transmission Cooler

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You will notice that the ST185 transmission uses a radiator for cooling the transmission. The ST165 simply has a small cooling "loop". Utilizing the ST185 cooler would be a nice upgrade to your system, and help to prolong the life of your new tranny. In my case, I did not have the time or money to add custom lines to mount the cooler where I would have liked, so I used zip ties to secure the cooler in place just under neath of where the intercooler pump sits. If you can find room, it is possible to mount the cooler in the front bumper area with the use of custom lines. If you don't want to use the cooler for some reason, you can use the ST165 cooling loop and bolt it to the ST185 tranny, since they both use the same inlet and outlet. Definitely up to your discretion, however I personally will gladly take any opportunity to improve the cooling system in any part of the engine.

Radiator and Rad fan

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Since the turbo sits in a different position on the ST185 engine, you can not easily use the original radiator fan. You must swap over to the ST185 fan, which is quite simple. This is another good reason to have purchased a front clip for your swap, because the fan will be included. Your original ST165 radiator must be used, because the 185 radiator will not sit on the 165 mounts. Swapping over is very simple luckily, however you might have to drill a new hole in the fan for one or two of the mounting bolts. This is just something to be aware of as you proceed with your swap.

Air Conditioner

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Many people ask if their AC will work after this swap. I am unable to answer this question because my AC did not work before the swap, and by no coincidence still does not work. I plan on rewiring the system from scratch by removing all associated wires from another 4th gen from a junkyard, and then having the system recharged. This way I know it will work. I would advise that if you wanted it to work, you should try to use the existing ST185 wiring to control the AC, although the systems are different. If this does not work, I'd say your only choice is to try to wire it in from scratch.

If anyone is able to provide me with additional info on the AC system, I'd be very appreciative of it. You can email me at chris@gtfour.net. Thanks!

Useful Diagrams

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