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...
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...