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4EAT Diff-Lock Switch and Handbrake Mod - "Torquemada Lite"

412K views 357 replies 117 participants last post by  ForesterLove1993 
#1 · (Edited by Moderator)
Synopsis:

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!

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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 can't. 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 requirements.

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.


Tools:
Soldering Iron/solder
Wire Cutters
Wire Strippers


MultiMeter


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


Common Sense (mandatory)

Next - Building the dummy load...
 
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#2 · (Edited by Moderator)
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. http://www.rs25.com/forums/showthread.php?t=73249

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
/page3

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!
 
#14 ·
Step 1 - Build the dummy load:
...
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.

...
One observation - If you had put one 50Ω and one 100Ω resistor side-by-side, you would have more even power dissipation. As it is, the two 50Ω resistors are dissipating twice as much power as the two 100Ω resistors. Have you noticed that heatsink is warmer?

Take Care,
David Baldock
 
#3 · (Edited)
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)
 
#294 · (Edited)
@defcon

First off, I love this idea/writeup, going to get on it ASAP. Just one question. In the end of your write up you state the following:

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.
edit: of course with the switch normal, the dummy load isnt in any circuit and of course there would be no voltage, however....

According to prior information and the info found in the manual, shouldnt the dummy load see only around 0.5V across it in this situation? If the TCM is being fooled into seeing the dummy load as the solenoid, wouldnt this first gear, full throttle situation yield a low voltage result? Please correct me if im wrong. In addition, if you were to insert the FWD fuse, wouldnt the dummy load then see ~8.5V?
 
#7 · (Edited)
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.:D

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!
 
#9 · (Edited)
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.?
 
#11 ·
You'll feel binding on dry pavement because the Multi Plate Transfer Clutch (MPT) is physically locked. The 4EAT doesn't have a "true" geared center differential like the 5MT, so when you activate the lockup, the transmission has no way to make up for the difference in rotational speed of the front & rear axles as you take a tight turn.
Subaru is obviously comfortable enough with the strength of their drivetrain components to allow the "fail" position of the solenoid to fully engage the AWD. If you look up "torque bind" on most Subaru sites, you'll see that it's most commonly traced to a DutyC Solenoid failure - which is exactly what we're mimicking with this mod.
Under normal driving circumstances on dry pavement, you're very unlikely to break anything unless you try really, really hard, but the wear & tear will certainly be accelerated.

As for autocross, I'd be very, very careful - especially if you're running in one of the "super sticky tire" classes. Crappy all-season tires will just skid and dissipate the load, but a nice set of R compounds will probably just transfer it on to your axles, cv joints, and the transmission itself. this mod is probably best left to situations where the wheels can slip just a little .
I also don't know how the handling would be at the limits with a locked differential on tarmac. I'd assume you'd pick up a pretty nasty push in the tighter stuff, but that's just me - and I'm a Rallycross guy.

If there's enough interest in this project, I could apply for vendor status, and sell full kits, but for now, I don't want to run afoul of the RS25 rules... I guess I could source the parts and build it - then just charge you my costs plus shipping, but I'm not sure if "non-profit" financial transactions are allowed. Maybe I'll go ask about that, or one of the mods could chime in.
 
#12 ·
Naw man, you've got AWD - just not all the time. Your car has to sense a bit of slip before it reacts and transfers power to the rear. In other words it's a "reactive" system. That's not a huge deal - lots of other car makers have similar systems. Done right, it's a very capable, and intelligent way to control traction.
How long that reaction takes can be influenced by many factors, including overall transmission health and age. My transmission can take up to half a second or more to react, your's may react instantly. If so, you may not need my mod - test it out and see.
My mod is fairly crude and takes away any intelligence that the system has, but since it's only for occasional/competition use, brute strength is just fine.


Glad you like the writeup;)
 
#15 · (Edited)
Yeah. Me + keyboard = epic reading for everyone else.
Thanks for clearing up the "non profit" question.

One observation - If you had put one 50Ω and one 100Ω resistor side-by-side, you would have more even power dissipation.
Dave - can you please expand on this? Are you saying that by pairing a 50 ohm with a 100 ohm rather than both 50s and both 100s together, the resistor would run cooler overall? I know the same impedance can be obtained using fewer resistors, and wiring them in series, but I figured the greater surface area would aid in heat dissipation - Am I mistaken?

EDIT: I just re-read your post Dave, and now I see what you mean. I read "even more power dissipation", instead of "more even power dissipation". Got it...and yes, you're absolutely correct. I'll monitor each side the next time I use it heavily and report back. If it needs to be rewired, I'll note that in the DIY - Thanks.
 
#18 · (Edited by Moderator)
David - we must've been typing at the same time - I was just editing my above reply when I saw your new comment.

Great stuff man. You're absolutely right, and I feel dumb for not realizing it. I even did all the calculations to make sure my resistors were up to the task. You'd think that once I saw the resulting power figures for each of the resistors, it would have occurred to me to mix 'em up. Guess I was too busy taking pictures :lol:.

Thanks for the expert opinions and hardcore formulas - it'll give the thread some much-needed "electronics street cred"


defcon--I assume the idea for your handbrake turn mod is to duplicate inserting the FWD fuse when the handbrake is pulled, thus not torturing the center diff when you lock up the rear wheels?

Looking forward to the write up on that as well. Great stuff so far
You're very clever....thanks for the kind words
 
#22 ·
Thanks!
Yes, you can do that. But at that point, all you would do is revert to the stock configuration - which is still a 90/10 torque split at 0% throttle on the MPT cars, and subject to lockup when the throttle is opened. On my car with it's ailing transfer clutch, it's no big deal, but in a newer car with tighter tolerances, and/or the VTD system, you risk unnecessary abuse to the AWD transfer components.

What needs to be done is to introduce a full voltage signal to the solenoid when the handbrake is pulled. This is a little more complicated since the native signal is pulse width modulated (PWM), so I'm investigating construction of a PWM controller.
The development of a PWM contoller opens up the option to have variable torque split in the MPT cars, and some degree of variable diff lockup in the VTD cars (unconfirmed until I get hold of some VTD documentation.

There are other PWM sources to pull from in the car, so the controller may be unessesary, but I'm always looking for odd challenges. Not to mention I was sad to see the Torquemada project die, and would love to see something similar come to fruition.

In the short term, I've come up with a working version of the handbrake mod using discreet/analog components, and will write up the how-to as soon as work calms down. Development of a microcontroller version will continue as well....
stay tuned.....
 
#23 ·
The only need for a µC would be to pull off the fully-variable trick, right? I mean, 100% PWM is just DC (battery voltage to the solenoid). I'm guessing that's what your current handbrake mod accomplishes. I think that would be good enough for a lot of people, though the manual control knob would be hella cool, no argument there.

I also think it would be fun to make an analog gauge for the PWM output to the solenoid. That way we can watch it bounce around as we drive, and better understand what the car does normally, as well as be able to easily tell if the manual torque control system is working.
 
#25 ·
I love the idea of an in-car gauge to monitor rear wheel torque split! Great idea.:clap:

Yes, the microcontroller would only be necessary to get more control than on/off for the transfer clutch. As you stated, the low tech on/off switch with a handbrake interrupt would satisfy 80 - 90% of the drivers out there, so the variable controller option would only be a personal project for now, unless I saw a definite need for it.;)

I am looking into the possibility of making kits for this, but I want to have a few more miles on it before it gets put in other people's cars.

If you think you might be interested in something like this - feel free to speak up and reply here. The more people I know are out there, the more serious I'll get about it.
 
#26 ·
I did this mod about a year ago to my 90 legacy ss. It was pretty sweet. I didn't think of the e-brake interrupt tho at that time. Now I don't have a 4eat car thank the lord!! Good to see someone helping people get a little enjoyment out of having a slush box. Fine work!!
 
#28 · (Edited by Moderator)
That is correct. With the exception of the STI , all the manual transmissions will attempt to lock the front wheels shortly after the rears since the center diff's main job is to keep all the wheels rotating at the same speed.

It doesn't know the difference between acceleration and deceleration, so it just does it's job.
Knowing that, you can see that a handbrake turn won't work very well, and is definitely not good for the parts involved.

I've never driven an STI, so, STI guys feel free to correct me if I'm wrong, but I believe the DCCD center differential in the STI is electromagnetically controlled and applying the handbrake will trigger it to "open" momentarily.

The standard 5mt has no such feature, so the front and rear wheels are mechanically bound by the viscous center diff. The lowly 4EAT isn't intended to have the feature either, but the wonderful thing about electronics is that they can be fooled.

Viva la Slushbox!! (at least until it breaks)
 
#31 ·
Speedfreek0o: I'd love to do that , but trust me, you don't want my hand scribbled crap. Since I'm not an electronics guy by trade, I have no access to any kind of schematic layout program and I have yet to find an inexpensive (free) one.

If anyone has any leads on a good application for that, I'll gladly post the schematic. Until then, you'll have to wait until I hack one together in Photshop (graphics guy by trade)
 
#30 ·
Thank you both, defcon5 and dbaldock9 for the write-ups, pictures and information. Seeing it and having a decent write-up in one place instead of sifting through threads all across the internet will make this project much easier to complete.
 
#33 · (Edited by Moderator)
This is what I use:

http://www.cadsoftusa.com/freeware.htm

I'm a computer engineering student and have created many schematics & board layouts using Eagle. If you don't have time for the learning curve, just send me your notes and I'll transcribe them for you.
Nice find - that looks perfect, and it has a Mac version. I'll check it out, and see how the learning curve goes. If it's too much for me, I'll take you up on your offer.

Quick update: I just got back from the Sno*Drift Pro Rally in northern Michigan, and I have to say - the system works GREAT. We usually stay away from the official spectator areas, so we end up on some "seasonal" roads which basically means they never get plowed and you enter at your own risk. Maybe trucks will have beat things down and you'll be fine, maybe not. With about 3 feet of snow on the ground, this can be a risky undertaking.

Through two solid days and over 250 miles on snowy fire roads and two tracks, the car never let me down once. Even after a little driver error had us buried up to the driver's side axles - with a little pushing, she popped right out, and kept on going. This is quite possibly the most effective mod I've ever done - especially if you factor in the low cost.

Not to mention, it just makes the car flat-out fun to drive.
 
#34 ·
Very cool mod and nice of you to post it for us. Good job on the pics as well. One thing to add about resistor heat dissipation would be to put some heatsink paste between the resistors and sink.(unless you already did) Or use the resistors that have the sink made around them ($$$)
 
#35 · (Edited by Moderator)
defcon,
Not sure if I've met you before, but you live in Detroit and I'm assuming you're on xceedspeed correct? I drive the white 05 RX-8 at the current moment, but because of finanical woes my dad is buying my car (the first day he saw it and drove was the day he told me he would be the first to buy it with out hesitation) and I'm going back to my paid off 96 Impreza L coupe with the wonderful, glorious auto tranny and hella cheap insurance. Why I'm telling you this I have no idea, but I do know you live here, I live here and well I suck at any sort of electronics esp hooking wires up (started a small engine fire wiring fogs a different way... different story for different time, lol). So a few questions to you:

1. Can I meet up with you once I get my Subie back (it's in Colorado at the moment) and give you money for parts + a 6 pack, haha?

2. How long did it take from start to finish to wire everything together?

3. Have you ever heard of the RX (ok dumb question)? Subaru made it from 86-89 and it had basically this exact mod done except they put it in with there manual. The RX had a 5 speed, high/low full time 4wd, and a center locking diff control for 50/50 torque split. Not sure if that helps what so ever.

4. If memory serves me right there was a guy on nasioc or clubwrx... I think nasioc, that was building manual DCCD controllers for the WRX owners that did the STi trans swap. Not sure if that would help any, but I can attempt to locate my conversations with him about it. Maybe it might work for controlling the torque split that you talked about doing later down the road.

5. As of right now the only "torque split / whatever you want to call it" mod I have on my 4EAT Impreza is a toggle switch for the "on the fly" FWD. Easy thing I know. How would this work though with your mod? Would I have to basically take it out/uninstall it? Or would I be able to still use it? I guess what I'm thinking is: normal 90/10, your mod AWD 50/50, and "my mod" FWD 100/0. What do you think? It could also be used as maybe a fail safe so you don't accidently hit the switch.

6. You would be able to use this on wet roads as well correct?

7. How well do you think this would work with the other trans mods you can do? Ie. auto trans cooler, torque converter, valve body (I think that's the other mod)?

8. Live anywhere near Mount Clemens? Every other Monday there is a group of Subie owners (past present, and future) that meet up at Madisons for dinner. We're the ESSO group on IWSTI. You should come up. I would be curious to see this in person.

9. Lastly, I've been wanting something like this for ages! Glad someone actually did it. Madd props to you mang! Thanx... 🍌


Jonny Walker
 
#37 · (Edited by Moderator)
Hey guys thanks for the interest.

2slo4u: I'm trying to track down some thermal paste, but my electronics retailers around here are decidedly... umm, lame. The resistors don't really get too hot, but the paste would add another layer of protection. Good call.

The Korean: you've got a PM heading your way. :) Fair warning - I have expensive tastes in 6-packs....

L8APX - I can't take full-on orders right now since I'm not an official vendor, but what I'm doing right now is building "kits" for the cost of parts and shipping. This way I can get the word out, AND get a few field testers at the same time (clever monkey, no?).

The good news is that this usually only costs about $30 - $35 depending on where I get my parts. Anyone interested in this should be aware that this is NOT a commercially produced product - it is simply homebrew/DIY at it's finest. That's why it only costs 30 bucks. What you'll receive will require cutting and splicing one wire (2 splices) and finding a suitable ground. That's it! (well, you gotta mount the switch too, but that's up to you)

Mods - if this is getting too close to the limit of my Non-Vendor status - please let me know. I intend on keeping this on the "up and up", and for now, it doesn't look like I have enough interest to go to a full Vendor account. As soon as it looks feasible, I'll sign up ASAP.
 
#38 ·
Thanks for the write up. But some of us are limited to doing our own work such as changing brakes or oil changes, we're not that electronically inclined to take on such project...so definitely set up a kit for us feeble minded trolls :stafiseiz

And assuming this works across all makes with 4EAT before '02 right? I have a '98 Legacy. Thanks a bunch :)
 
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