Subaru Impreza GC8 & RS Forum & Community banner

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!

---------------------------------------------------------

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...
 
See less See more
4
#204 ·
Well for the first time eVer (installed in both my last car and my current car) I am haVing problems with this... no idea why it's not reading the signals, but I ended up just disconnecting (and reconnecting the stock AWD system) until I can figure out why...:( I haVe a feeling it's one of my connections somewhere... just need to find the silly thing.
 
#205 ·
hey Defcon do you think this would be a useful modd for my 1997 Subaru SVX LSi. with the 4EAT stock gearing is I belive 3.454 or 3.545. either one. the admin of the forum iam at (www.svxworldforums.com says that the 4EAT locks up into a full 50/50 if the lever is down into 1st gear. well thats for my SVX at least.

i would like to do this modd considering that I have done some upgrades to the TCU myself with the help of the quick change modd kit that our forum sells.

thanks
 
#209 · (Edited)
hey i just read through every page of this and now realize what its really for no my rs is manual, so i cant do this correct? also is my rs always in 50/50 or could i still make the led bar graph or maybe a gauge telling me where power goes through acceleration? and has anyone figured out the mythical mod or where you can do e break turns?

also this is from koolaid13 im just on my brother in laws computer
 
#213 ·
this was a very good thread and if i a had a 01 or older i wouldnt hesitate to try the diff lock mod but i have a 03 2.5 TS and i can see they are a little diffrent i have a wiring diaram and it doesnt say duty solenoid C there is a AWD solenoid ( blue/orange ) are these pretty much the same thing ? there is also a lock-up solenoid (white/green ) is this for the awd system or the lock-up torque converter? and what resistor setup will work ? there doesnt seem to be anything in the thread specific to my car were they the same for 02 + ? i hope somone can help me out a bit thanks .
 
#218 ·
ok so,
If your like me, your car is not a standard awd automatic. and by that i mean you have no tcm.

well i called a local Subaru tech and he said it may be part of the ecu. and i think hes right.

i looked in my fsm and i found the duty c wire to be not is connector b84 on the tcm. but on terminal 11-4,8.



now can any one comferm that this connector is on the ecu?
 
#224 ·
So... I read this entire thing in hopes of doing the handbrake mod... Very thorough on the 50/50 mod, but there was all this talk about a mythical handbrake switch that was never explained.

I want to wire a momentary switch into my handbrake release button, and I want this button to put me in FWD mode when it is pressed. Regardless of whether I am in AWD or 50/50 mode. Plus I want a toggle elsewhere to deactivate this handbrake switch, for when I'm using the handbrake to park.

Seems simple enough right? This thread has "handbrake mod" in it's name, but the handbrake mod has not been explained in it.

Even the first page had a roller switch in with the required parts.. which was never used..

So, some people said the handbrake mod could basically insert the FWD fuse. That's great... except when the 50/50 switch is turned on, the entire duty C solenoid is completely cut off from the world... so it doesn't care about the FWD fuse at all.

Unless you made the handbrake button give the solenoid full power.. seems to be 8.5 volts...

Or when you press the handbrake button it could switch you out of 50/50 and into AWD mode AND insert the FWD fuse.. all at the same time.. If you used a relay...

So how do I do the handbrake mod?


Does this diagram look okay? Do you think there is a better way to do it?




Please give me some feedback, I'm no expert, and this could be a crappy roundabout way of doing this.
 

Attachments

#225 ·
Meh, I understand the concept behind this mod but I can't get my resistor bank to register more than 9ohms. No cold solder joints :unamused:, maybe this summer I'll throw some more time in to messing with it.... or just 5spd swap.

Good luck to all you modders out there, remember to check your voltages against that which is described in this thread so you don't fry something or make your TCU shit the bed!
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top