4EAT Diff-Lock Switch and Handbrake Mod - "Torquemada Lite" - Subaru Impreza GC8 & RS Forum & Community: RS25.com
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4EAT Diff-Lock Switch and Handbrake Mod - "Torquemada Lite"
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Old 01-19-2009, 08:44 PM   #1
Car: '99 OBS
Join Date: Jun 2008
Location: Metro Detroit
Trader Rating: (2)
Posts: 117
Arrow 4EAT Diff-Lock Switch and Handbrake Mod - "Torquemada Lite"


Phase 1 - Install a switch that allows the user to bypass the 4-speed Automatic Transmission's (4EAT) AWD torque transfer logic in order to "lock" the system into a 50/50 torque split.
Phase 2 - Add a switch to momentarily deactivate the "lock" so that the handbrake may be used to lock the rear wheels without affecting the front wheels.
Edit - 3/7/09: Since I initially wrote this DIY, I've learned that the basic AWD design of the 4EAT transmission has been around (and mostly unchanged) since the 1987.5 model year and is in use to this day in the non-VDC cars. It was available across the entire Subaru lineup as well.
That means that this mod applies to basically any Legacy, Impreza, Forester, XT, Justy, Loyale, SVX, and possibly whatever else Subaru crammed the 4EAT into since 1987.5!! Obviously in the much older and much newer cars, testing is of paramount importance so that you assure the proper wire is getting spliced, but the basic structure is the same.
Happy modding!


Hey kids! I've come up with a little mod that allows you to "lock" your transmission into a full time 50/50 torque distribution.
"So what" you say. "I've got a Subaru - it's full-time AWD already - the TV told me so, and the car has a sticker that says AWD!"
Well, ok, but not really - it depends on what transmission you have. If you've got the 5-speed manual (5MT) you've got the closest thing to a full time 50/50 torque split. If you got extra lucky and have one of the 00+ RS's you've got a rear LSD as well, so you're all set - you can stop reading if you want.

If on the other hand, you have the pre 02, 4-speed automatic (4EAT), you're not so lucky. Your torque split can vary from 50/50 to a 90/10 front-bias.
The Transmission Control Module (TCM) on the 4EAT tries to intelligently divide the torque based on conditions, and for most situations it does just fine. However, it's practically worthless for precise, low traction driving. It has a 90/10 (F / R) torque split in most forward gears, calls for shifts at all the wrong times, and the overall gearing is poorly suited to manual shifting. Don't even get me started on the stupid shift gate that prevents hassle free 2-1 downshifting. If you ever rallycross your 4EAT, or go for a brisk drive on snowy, muddy, or gravel roads, you'll quickly learn that the time it takes for the TCM to react is precious time.
Since the default bias is 90/10 you're effectively driving a FWD car. When you lose traction, the car understeers (pushes) to the outside of the corner where bad things like curbs/ditches/mailboxes/guardrails, live. Eventually the TCM reacts and transfers some power to the rear wheels, but by then, you've either gotten past the issue, or it's entirely too late.

Here's where my mod comes in - with the flick of a switch, you can lock your car into a full-time 50/50 split - no more waiting for the TCM to react. AWD all the time!
The feature can be turned on and off at will - no need to stop the car (although if you can, it's probably not a bad idea).
It should be noted that this isn't a new mod by any means, and has been around for years. However, I have yet to find a true "How-To" that lays everything out - so I made one. Not satisfied, I had to come up with a little twist. An extra bonus for us lowly 4EAT drivers is that we can also set the system up to allow the Holy Grail of rally driving - the handbrake turn! Finally - the 4EAT can do something the 5MT cant.
Ok - let's get started.

I should note here that I'll be specifically referring to the 4EAT that came on 95-01 USDM Subaru's - it may even work on older 93/94 models, but I can't verify. As near as I can tell, the basic technology was shared across the lineup, so the Legacy and Forester can use this mod as well.
The only "gotcha" is that the OEM turbo models MAY use a different wire, along with different power reqirements.

The following work was done on a 1999 Impreza Outback Sport with a bone stock transmission. This should work on the later (02+) vehicles with VTD/VDC, but it's not really necessary because from what I've seen and read, the VTD is a huge leap forward, and controls torque very nicely. I obviously haven't tried it myself, so feel free to be a trailblazer.

So here's the general idea behind this project. Under normal circumstances the TCM sends a signal to a solenoid to control the amount of rear traction. When the TCM calls for a 90/10 torque split (front bias), the solenoid is almost fully energized, conversely, when the TCM calls for a 50/50 split, there's almost no current going to the solenoid.

So - full power to the solenoid = FWD, no power to the solenoid = AWD. Got it? Good, that's a critical concept to this whole project.

What we're going to do is to bypass that solenoid to get full-time AWD. Lucky for us this is as easy as making sure the solenoid doesn't see any current. Yes, you could just disconnect it, but
A) that's a fairly crude solution, and ...
B) The TCM won't really like it if the solenoid just disappears.
At the least you'll probably get an annoying light on the dash and at the most, you risk the TCM forcing the transmission into limp mode.
So in addition to bypassing the solenoid, we'll wire in a switch to divert the solenoid's current to a network of resistors that will simulate the solenoid's resistance, thus fooling the TCM into thinking all is fine and dandy. These resistors are commonly referred to as "dummy loads".
The standard disclaimer applies: I am in no way responsible for anything that goes wrong if you try this. You need to use your head - you're working with the electronics of your transmission here, not to mention electricity. If you don't have the proper tools and/or skills to do this you should probably think twice.

Here's what you'll need:

4 Resistors - (2) 10 watt, 50 ohm, (2) 10 watt, 100 ohm.

1 Perfboard - This is for mounting the resistors. My piece was 2.75" x 6" and I only needed half of it.

6" Wire (solid core) - This is what you'll use to wire the resistors together. Two colors would be nice - you'll only need about 6" of each color.

25' Wire (stranded core) - for connecting the switch(es) to the resistor bank.

1 12v LED (optional) In case you want to add an indicator light to tell you when the feature is on.

1 Switch (Single Pole, Dual Throw (SPDT)) - you got it, this is for switching the lock on & off. Take care not to get a switch with a "center off" position - it won't work.

1 Roller switch (SPDT) - for the mythical "handbrake turn" feature.

Heat Sinks (optional) - I had a set, so I tossed 'em on. They're not critical, but I like the extra cooling capacity.

Heat Shrink Tubing (optional, but highly recommended) - there's lots of metal in your car, cover all of your splices and solder joints to keep them from shorting to the chassis.

Soldering Iron/solder
Wire Cutters
Wire Strippers


Sharpie marker
Hot Glue Gun (clear silicone glue sticks preferred)

Common Sense (mandatory)

Next - Building the dummy load...

Last edited by James555; 09-18-2017 at 02:24 AM..
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Old 01-19-2009, 08:45 PM   #2
Car: '99 OBS
Join Date: Jun 2008
Location: Metro Detroit
Trader Rating: (2)
Posts: 117

Step 1 - Build the dummy load:

A quick look at the wiring diagrams and diagnostic sections for the 4EAT shows this page:

Side Note: I've included pictures of all the relevant info you'll need, but you really should get a copy of the Factory Service Manual (FSM) for yourself - they're free, AND there's a download link at the top of this forum.
Official "Factory Service Manual" Thread

If you have the FSM, the relevant information can be found here:
DIAGNOSTICS SECTION/Automatic Transmission and Differential/Diagnostics/Transmission Control Module (TCM) I-O Signal.pdf

Over on the far right we see that the solenoid presents between 10-17 ohms of resistance.

Consequently, this is the number that the TCM expects to see when the solenoid is functioning properly. Above or below that number by too much and the TCM will throw a fault code. This means that we need to install a resistor that behaves in a similar way. The easiest way to do this is to go buy a 10-25 watt, 10-17 ohm resistor. Unfortunately you won't find one of those down at your local Radio Shack - that's just too specific - you're going to need to build a custom setup. Now, there are a bazillion ways to wire up resistors to get the results you need. I'm not going to go into gory details about ohms law, and all the options unless asked to do so. Suffice it to say that if you buy what I listed above, and build it as I instruct, you'll end up with a dead-sexy 10watt, 17ohm resistor.

Let's build it -
First I like to check each of the resistors to make sure they are working properly. Set your multimeter to check impedance (ohm symbol - looks like a horseshoe) and test each resistor by touching the probes to the leads of the resistors (polarity doesn't matter here). Ideally you'll see a number close to the rating of the resistor - in our case 50 and 100 ohms.
If everything checks out, grab your perfboard, and mount your resistors by pushing the leads through the holes and gently bending them to hold everything in place. Now would be a good time to add some little pieces of heat shrink tubing to the exposed leads on the resistors (see later pics). Mock everything up and make sure you're happy before moving on.

You'll notice I have my resistors mounted in a set of heat sinks. Again, you don't have to do this - I'm a bit obsessive and tend to overbuild things, plus I had them laying around.

At this point I like to mark the locations of the resistor leads, and in my case the heat sinks. After you've marked everything - disassemble or at least pull everything away from the board an inch or two so you can get the glue gun in there.

Now take your hot glue gun and put a bead where each resistor body is going to rest on the perfboard and quickly push the resistor back in place. This will firmly attach the resistor to the perfboard so it won't vibrate out over time. Repeat for the rest of your resistors and whatever else you may have.

Now it's time to solder up the electrical connections on the back side. Now's a good time for a quick electronics tidbit: We're wiring these resistors in what's known as a "parallel circuit" as opposed to a "series circuit". I'll skip the mumbo jumbo, and math about why, and just say that I like the idea of having multiple resistors to share the load. These things get pretty warm during use, and more surface area to dissipate heat is always a good thing.

What I did was to cut and strip three separate pieces of wire for each set of leads - you can just run one continuous uninsulated wire as well - just make sure you insulate it against shorting out somehow - even electrical tape is better than nothing. Regardless of you wire choice run a wire up one side, and down the other. Now nolder each of the resistor leads to the wire and you're done.
Do NOT bridge the leads of any single resistor - just connect all the leads on each side. When you're done soldering, snip the excess resistor leads - you don't need anything past the actual solder joint.

Now it's time to check the overall impedance of your finely crafted dummy load. Just like before, put the probes of your multimeter on the leads of the first resistor. Only this time, since the resistors are all part of the same circuit, you should get a cumulative reading of all the resistors. If you've done it right, and your resistors were within their specs, the number should be very close to 17ohms.

If so, your dummy load is complete - congratulations. Have a bite to eat and a soda to celebrate, because now it's time to head out to the car - which only means lots of contortions under the dashboard.

Next - how to run wire in a car....upside down!

Last edited by defcon5; 01-19-2009 at 09:22 PM..
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Old 01-19-2009, 08:45 PM   #3
Car: '99 OBS
Join Date: Jun 2008
Location: Metro Detroit
Trader Rating: (2)
Posts: 117

Car wiring:

Now it's time to track down the wire that carries the TCM signal to the solenoid. Back to the FSM:

We're looking for the AWD transfer solenoid which is called the "Duty Solenoid C" in my manual. As you can see, it can be found at terminal 15 of connector B54

Looking at the layout of the B54 connector, we see the physical location of pin 15

So, where exactly is this connector located in the car? Well, it's on the TCM. Which you'll find mounted just to the left of the steering column. Crawl into the driver's side footwell and look up, you'll see it. Here are some pics to give you an idea of what you're looking for. Note the orientation of the brake pedal to get your bearings

One quick note: I highly recommend one of these -

It's much easier to do a job like this in tight quarters when you can use both hands and not have to dick around with a work light.

Ok - so the connector we're interested in is the white one (located closest to the firewall). We've seen the location of the wire on the connector diagram of B54, so we should be able to just cut it and be on our way right? Be my guest - but I like to be dead sure when I cut wires, so some diagnostics are in order. A look back at our diagram shows some tests we can do to verify we've got the right wire. This is an important step because it appears that turbo models use a different wire.

According to the tests procedures - we need to install the FWD fuse and turn the ignition to "on". The FWD fuse is located in the engine compartment fuseblock.

Remove the lid and locate the FWD fuse position

Unless your car is drastically different, the fuse probably goes here:

Spares are kept in the lid. Install the one of your choice - I used the 10 amp with no issues.

On to the connector - I've removed the connector for clarity, but this whole job can be done without ever pulling the plug. In fact the plug has to be in place to do the tests.

With the plug in place, and the FWD fuse installed, turn the ignition to "on" but don't start the car. Jam one of your multimeter probes in the back side of the connector (in the appropriate terminal of course) and the other to a good ground; usually some bare metal of the car chassis. My meter has pretty long leads, so I ran the negative probe out to the factory ground points in the engine compartment.
When done properly, you should get 8.5 volts or more. If your meter is like mine, the probes are too fat to get into the connector, so you may have to jam something metal in there to get a decent connection, and then touch your meter probe to that to get the reading - I chose a small pick.

Now, remove the FWD fuse, put the gear selector in 1st, fully open the throttle, and repeat the test. This time, you should get less than .5 volts. If this checks out, you've got the right wire. If not - use the wiring diagrams and diagnostic sections of your FSM (you did dowload one right?) to troubleshoot and track down the right wire.
If you get totally stuck, post here and i'll try to help as best I can.
Now it's time to cut the wire. Dig a few inches of wire out of the harness and snip it.

Now we can extend the wire out to the switch. I located mine back near the handbrake, but you can put yours wherever you like. Just splice two lengths of wire to extend each side of the white/green wire to your switch location. Solder and heatshrink the connections.

Now route your extended wires to your switch location. If you're planning to put your dummy load in the footwell, You should route your new wires, AND an additional piece of wire of equal length - this will be the wire that feeds the dummy load from our switch (more on that later). If you'd rather put the dummy load elsewhere, you're on your own. Not because I don't like ya, just cuz I didn't do it that way.

Alright, we've got our wires pulled to our new switch location, so it's time to wire up the switch. Let's talk a little about the particular type of switch we're using. I've spec'd Single Pole, Dual Throw (SPDT On-On) switches because they allow you to switch between two different circuits - which is exactly what we want to do here. When the switch is in one position it's like nothing ever happened. The TCM signal flows, unaltered, through your new wires, through the switch, and on to the solenoid - all you've done is made the trip a little longer. But, when we flip the switch to it's second position, the signal will now flow through the new wires, out to the switch, but now the switch will redirect the signal to the dummy load that you built. Very simple, yes?

Looking at the switch, you'll see three posts; they should be labeled as COM, 1, and 2. On my particular switch, the center pin was COM, and 1 & 2 were the outer pins. In order to understand what the switch is doing, you can visualize the pins as "Ins and Outs". Think of the COM pin as your "In" and the 1 and 2 pins as your "Outs". The signal will always enter through the COM pin, but where it exits is decided by which "out" you have selected with the switch.
The first step is to set up your "In" to the COM pin. Since this is the signal that feeds the switch, you need to make sure that you connect the wire that's coming FROM the TCM to the COM port, otherwise, you won't have a common signal and the switch will only work in one position - not good.
One quick common-sense note before we whip out the soldering iron again....unless you're getting all crazy, and wiring up a quick release connector before your switch, you should have your wires pulled through your switch mounting hole before you wire it up. Trust me.
Ok - let's melt some solder!

Solder/heatshrink your TCM signal wire to the COM pin. Then solder the wire that goes to the solenoid onto pin 1. Now you have a complete circuit....that does absolutely nothing special.

Now we'll wire the dummy load to pin 2 - you know what to do.

You may have noticed that my COM wiring changed between those two pictures. I have a very simple explanation; I screwed up the first time. The picture with all three wires is my final configuration (the big fat wire is my COM wire). Multicolored wires would come in handy to avoid screwups like that, and I'll probably rewire everything when the weather warms up. Since I didn't know if this would even work, I was just scavenging wires from old, blown DC wall transformers.

Wiring the 1 and 2 pins is up to your personal preference. I chose pin 1 as my "off" position - meaning that the car behaves normally when the switch is in this position. Conversely, pin 2 is my "on" position and is wired to the dummy load, thus activating the 50/50 torque split. How you do it is entirely up to you.

All that's left is to wire up and mount the dummy load:
Solder the wire from pin 2 to one side of the dummy. From the other side you'll run a wire to ground.

I chose to go through the firewall and hook into the OEM grounding points located on the driver's side strut tower. No pics cuz I'm lazy. I'm sure you can figure it out.

Not knowing how much heat the resistors would generate, I chose to temporarily mount the dummy load directly to the TCM. I assumed that since this feature will mostly see use during the winter months, I'll already have the blower on in the footwell, and that should move enough air to keep things the resistors cool. I used a small piece of rubber between the dummy and the TCM to provide both vibration damping, and electrical insulation.

Well, that's about it for Phase 1. I'm still working on installing an LED to indicate when the AWD is locked, and I'll write that up when I get it installed.
But for now, I suggest a little testing before you fire everything back up. Namely, test the switch to make sure it's doing what you want.
With the key on, and the switch in the "normal" position, there should be no voltage present across the dummy load. Flip the switch, put the car in 1st and open the throttle 100%, and you should get around 8.5 volts. If it checks out, you're good to go. Clean up, and go for a test drive.
On the test drive, take the car to a patch of dry pavement where you have enough room to slowly drive a complete circle. With the switch in the normal position, you should be able to drive the full 365 degrees with no problem. Now flip the switch to activate the lock, and try the same circle - you should feel a binding within the first 45-90 degrees. When you feel it bind - STOP and deactivate the lock. You should hear a "thunk" and feel the tension release. If so, pat yourself on the back - you've got full-time AWD at your control. Now - go play in some slippery stuff.
Days like this are when the system really shines:

I need to give credit where credit is due. Big thanks to MountainBiker, Zhe Wiz, SeaComms, and Kevin over on www.subaruforester.org. This thread was instrumental in giving me the idea to put this mod together.

Next installment - the handbrake mod. (insert evil laugh)

Last edited by defcon5; 01-20-2009 at 01:29 PM.. Reason: added credits
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Old 01-19-2009, 08:46 PM   #4
Car: '99 OBS
Join Date: Jun 2008
Location: Metro Detroit
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Posts: 117

Handbrake switch...coming soon
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Old 01-19-2009, 08:46 PM   #5
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i dont have a 4eat, but this is really interesting!

best of luck, keep us updated!!
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Old 01-19-2009, 08:47 PM   #6
Car: '99 OBS
Join Date: Jun 2008
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extra space
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Old 01-19-2009, 08:49 PM   #7
Car: '99 OBS
Join Date: Jun 2008
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Originally Posted by tnice17 View Post
i dont have a 4eat, but this is really interesting!

best of luck, keep us updated!!
Thanks man - this is my first RS25 "how to", and I hope it helps some people out. Since it's my first crack at this, feel free to tell me what works and what doesn't.
It won't help anyone if it doesn't make sense right?
I'm still going through and proofing it, making sure pics are correct etc, so don't be shy...

I've had this mod installed for about 2 weeks, and have had to slog through countless inches of snow since then (very good timing). It works VERY well. The resistors get warm, but never seem dangerously hot. I really don't leave it on for any longer than I need it though. I think that's the real bonus of this mod - switch on, switch off, no harm done.

I had the opportunity to run in a snow rallycross this weekend, and all I can say is WOW! The car was so much more drivable. The feeling of control that my old 5MT RS had was back, and I was actually able to place two spots higher than I expected. In the process, I beat two cars with snow tires mounted while I only had my all-seasons on.

The best test was the drive home. The event was located over two hours from home, and it had been bitter cold (single digits) and snowing continuously all day (we got over 8 inches at my house).
At one point the freeway was so iced over that cars were sliding around everywhere. Traffic eventually slowed to stop & go conditions which had it's own brand of chaos.
If I left the switch "off" and let the car make the decisions, it would spin the front tires so badly before locking the rear, that I would routinely be in danger of sliding sideways into the next lane if I didn't let off. I flipped the switch to lock everything up, and presto! No spinning tires, just a nice easy drive home.

Let me know what y'all think!

Last edited by defcon5; 01-19-2009 at 10:25 PM..
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Old 01-19-2009, 08:51 PM   #8
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I am interested.
I hope that after I die, people will say of me: 'That guy sure owed me a lot of money!'
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Old 01-19-2009, 10:33 PM   #9
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Holy Crap I have to try this!

Why should you feel binding on asphalt? Would this be safe to use for autocross or is it for offroad only? How long has this mod been done to your car. Is there any chance you could pre fab the resistor board for me and include the wiring and switch so all I would have to do is solder the wires to the car.?

Last edited by DunDunSkeert; 01-19-2009 at 10:36 PM..
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Old 01-19-2009, 11:40 PM   #10
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this depresses me.... I got the 4eat and i dont have AWD?WTF? And i sure aint mechanically talented to pull this off. damn. Nice write-up by the way
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