4wd switch for 4EAT trans

Discussion in 'Modifications And Maintenance' started by joebush44, Oct 26, 2014.

  1. joebush44
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    joebush44 Well-Known Member

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    I had been contemplating performing this mod for some time now to my auto '00 Impreza OBS. For any of you Subaru 4EAT owners, I'm sure you agree the 90/10 front wheel bias, frankly, sucks. The way the system works, if the TCM senses front wheel slip, it will then distribute 50/50 front/rear power and then revert back to 90/10 once traction has returned. The downfall with this system however, is that is is extremely slow to react and can be completely unpredictable. This makes winter hooning a bit of a challenge.

    Which brings us to the mod. There is quite a bit of info out there on this and I would suggest researching extensively before taking the plunge. There are some risks involved.

    The "right" way (and complicated way) to do the mod is to build a "dummy load" to fool the TCM into thinking there are no issues (no dummy lights on the dash) while still achieving the 50/50 split (link). (Ignore the handbrake integration in that thread).

    The easy way is to just cut the duty solenoid C wire and wire in a toggle switch. When the signal is interrupted to the solenoid, the center diff locks the clutch packs and the car is then a 50/50 split, just like it would if the TCM was sensing front wheel slip. Flip the switch back, and it's back to it's normal 90/10 front wheel bias. The only downfall with the "easy" way, is that you will have a constantly flashing AT OIL TEMP light, which is quite annoying...but is a small price to pay, in my opinion. Once you restart the car with the switch not interrupting the signal, the light will be off just like normal.

    I haven't had a chance to really put this to the test yet - I can't wait for the snow! I tried to test on gravel, but my poor 2.2 struggles to spin the tires, even in "normal" 90/10 mode :(, so I couldn't truly put it to the test. I may be searching for a mud pit if it doesn't start snowing soon. I can confirm that with the 50/50 engaged, you do get some torque bind when making tight turns on pavement so it does work!

    Final product:
    [​IMG]

    I may try to incorporate a push-button 4x4 button, or something of the like, in the future. For now, a cheap switch from Advanced Auto will have to suffice.

    Anybody else have any experience with this mod?
     
  2. EricS
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    EricS Nooberator

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    Interesting! Seems to me that a much easier way to make a dummy load would be to just grab a second solenoid from a junked Subaru and/or a friendly board member here, connector and all (probably wouldn't even have to be from the same application, just a similar continuous-duty unit with roughly equal coil resistance) , then you could just wire it up instead of having to solder up a dummy load yourself??
     
  3. joebush44
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    joebush44 Well-Known Member

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    I won't even pretend to know how that dummy load works...way over my head, but the idea is to bypass the solenoid that controls the diff lock altogether, while fooling the TCM into thinking, "there's nothing funny going on here, the front wheels must be losing traction". I'm not sure putting a second solenoid in the mix would warrant the same result. But again, I don't know how that dummy load is "fooling" the TCM. I'm definitely no electronics genius.
     
  4. joebush44
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    joebush44 Well-Known Member

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    I once owned a '91 Loyale automatic that had a 4x4 push-button on the gear selector. FWD until you engaged the 4x4 button...I'd be curious to know more about how that system worked too
     
  5. Fatboy Walrus
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    Fatboy Walrus Well-Known Member

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    I read about this mod on another forum once. Was really neat but I remember hearing that while turning it gets a bit notchy.. Can't remember too much though, it has been a while. Is the ratio really 90/10 in the auto subarus?? Then what about the manuals? I've heard lots of debate about that and never got a real answer.
     
  6. joebush44
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    joebush44 Well-Known Member

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    Yep, autos are 90/10 front wheel bias until slip is detected, manual 50/50 all time. The problem that myself and many others have, with the 90/10, is that the transition to 50/50 is delayed and unpredictable in most cases. You don't want to leave it locked on dry pavement with this mod, that's where you get the notchy/wheel hop/torque bind. This is more intended for wintery conditions/low traction situations.
     
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  7. Nhibbs
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    Nhibbs Well-Known Member

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    5mt are all 50/50 split with a viscous center diff so no clutch pack.
    The sti is different depending on year either 35/65 or 41/59 with varying amounts of lockup from the DCCD controller.
     
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  8. Fatboy Walrus
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    Fatboy Walrus Well-Known Member

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    Interesting. I would like to read up more about the subaru transmissions. One thing I am not too over familiar with.
     
  9. joebush44
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    joebush44 Well-Known Member

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    This is basically the next-best-thing other than swapping in a 5mt :eek:
     
  10. EricS
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    EricS Nooberator

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    The dummy load is emulating the resistance that the solenoid coil normally presents to the TCM. So the dummy load is a fake solenoid coil - just seems easier to me to use an actual solenoid for that purpose, instead of messing around building an emulated resistive load...
     
  11. Nhibbs
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    Nhibbs Well-Known Member

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    Unless you know the exact resistance needed, then just stick a resistor in the connector and tape it up.
     
  12. joebush44
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    joebush44 Well-Known Member

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    Yeah, I got that much out of it. Like I said, I don't really know enough about it to say why or why not that idea would work. Sounds plausible, but there has to be a reason why he went through all of that work. Nothing is ever THAT easy! :)
     
  13. fobiawrx
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    fobiawrx Fabiola

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    we do this on our drag autos. we go threw the ecu (stand alone) and trigger when we see 5 psi of boost. that way we get to the staging line fine with out binding up.
     
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  14. Todd
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    Todd The Originator Staff Member

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    That was a vacuum diaphragm that threw a lever that engaged a gear. It was a "hard lock" 4WD.

    Not quite true. The 4EAT control unit does preemptively add rear depending on wheel speed and throttle application. If you snap the throttle it would add in more rear than if you squeezed the throttle.


    I did both modifications to my Legacy GT wagon in the past. It was fun. I could stab the parking brake to start a slide/drift then the 50/50 split would keep it going.
     
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  15. joebush44
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    joebush44 Well-Known Member

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    Interesting about the Loyale setup. It did work well and I'd really prefer that over how the 4EAT is setup. Only downfall was that the Loyale was a 3 speed auto...nothing like running 4k doing 65 in top gear!

    I was not aware the 4EAT "throttled" rear wheel bias based on wheel speed/throttle position. I think that may contribute to its unpredictability. Thanks for the info!
     
  16. Chux
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    Chux Well-Known Member

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    Are you sure about that, Todd? I know the PT4WD 5MTs were like that, but I was under the impression that the 3ATs functioned much like the AWD 4EATs, with a clutch pack, but only programmed with "Off" and "Locked" as options. I've never owned or worked on one....so I'm not sure.





    I also want to say something about the torque split numbers. The "90/10" thing always bothers me....the 4EAT is a Front wheel drive transmission, with the addition of a rear output as an afterthought. There is a mechanical linkage between the torque converter and the front diff, so the front wheels will always spin the same speed as the output shaft of the transmission (which is why the speedometer is on the output shaft....). Then the transfer clutches bleed a little bit of power to the rear diff. This clutch pack is able to vary the power from 0% to 100% locked (As you've seen).


    A guy on Ultimate Subaru reverse engineered the programming on several 4EAT TCUs, and then modified the code (his primary goal was paddle shifters....pretty cool stuff!). He also found, buried in the code, the AWD programming. There's a value that represents the tolerance in speed different between the front and rear output on the transmission. He was able to change that value to improve the response of the AWD, up to and including full lock.

    He also found that the very early 4EATs (XT6s and EA82ts, '87.5-'90) had a 2-bit TCU, and the EJs had a 4-bit TCU. So even with the same values for the AWD system, just changing to the new-gen computer, drastically improved the AWD (I still want to do that to my XT6....).
     
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  17. EricS
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    EricS Nooberator

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    Yeah, but it needs too see a 10-15ohm resistance, so it's got to dissapate about 10w - you can't throw a normal 1/4 or 1/2 watt resistor in the connector :) The dummy load that the thread OP built looks like this:

    [​IMG]
     
  18. EricS
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    EricS Nooberator

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    My guess is that the solenoid in question is not continuous-duty rated, which would make sense in its normal application.

    But for Nhibbs or other electrically-inclined folks, I don't know why you wouldn't be able to use a potentiometer to feed the line the voltage it's looking for, instead of using a big dummy load? Unless the TCU is measuring that line's current using an internal shunt resistor and not just looking at the raw line voltage...
     
  19. euro
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    euro Well-Known Member

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    subscribed, I was just reading about this but most threads ended in complaining with no justification for their whining. besides the binding during dry turns, is there any serious side effects to doing this to a stock 4EAT? (premature wear, ABS issues, etc.)
     
  20. joebush44
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    joebush44 Well-Known Member

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    Premature wear to the center diff clutch packs has been a topic of concern from my research and the only concern that I am aware of. Only other thing is (the way I did it) you get the AT OIL TEMP dummy light flashing constantly when the car is in 4wd mode. I haven't had any real time to test this mod. One might think that "locking" the CD would produce LESS wear to the clutches as compared to allowing them to "slip" under normal operation...
     
  21. Todd
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    Todd The Originator Staff Member

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    Whoops, I think I had the wrong trans in my head.
     
  22. euro
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    euro Well-Known Member

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    you'd think so unless it somehow was causing more aggressive wear during binding from the clutches not wanting to slide on mild turning
     
  23. Chux
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    Chux Well-Known Member

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    IMO, this should be treated like true 4WD in an EA-series Subaru, or most trucks. It should not be used on dry pavement. When you go around any sort of corner, the front and rear wheels need to spin at a different speed....so something has to give. That needs to be the tires against the ground (snow, ice, gravel, etc.). Without the Duty C solenoid allowing the pack to release a bit, it's going to *theoretically* cause more wear on components. How much? Who knows. It's very dependent on so many factors....


    Also, the first write-up I ever saw about this mod mentions that the flashing AT Temp light only happens when the vehicle is turned on with the switch in the "locked" position....(Post #30)
    http://www.ultimatesubaru.org/forum...s-whoah-4eat-automatic-transmission-not-5spd/

    Now, that was on a Phase I transmission....
     
  24. joebush44
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    joebush44 Well-Known Member

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    I read that too, but I think something changed in the newer cars, '98 maybe?

    And I agree that this should not be used on dry pavement/normal driving conditions. There's really no need anyway. Snow/ice driving is what I'm anxious to test it on.
     
  25. joebush44
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    joebush44 Well-Known Member

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    If all goes as planned, I may not even get a chance to use the mod this winter...I might have a 5 speed swap in my very near future
     
    Last edited: Nov 16, 2014
  26. spudd
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    spudd Well-Known Member

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    OK, I really think I need to do this
     
  27. joebush44
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    joebush44 Well-Known Member

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    We finally got some measurable snow down here in SE MN this weekend. I had a chance to do some Subaruing and put this mod to the test...it kicks ass! No matter what the throttle position is, I've got front and rear power. Although, I've noticed it seems to be right wheel bias for some reason in the rear. Still, it takes the guess work out of trying to determine when the rear wheels are suddenly going to kick in, which makes it much easier to hoon and performs a much better from a dig. I wish I had done this last winter! I did have to put a couple pieces of black electrical tape over the flashing AT TEMP light before it gave me a seizure.

    Also, the 5mt swap that I mentioned previously has been put on the back burner. I can't justify the time/cost/inevitable pain in the ass that it will be, at this time, so will have to live with the 4eat for now - this mod definitely makes it more tolerable.

    If anyone is interested, I can post a brief writeup of how I did this. The hardest part is cleaning the dirt off the wires so you can see what color they are.