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Centre diff lock on AWD auto


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175 replies to this topic

#1 Fatz

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Posted 15 December 2005 - 12:00 AM

Hey all, i have a auto AWD series 3 Gen 2 TT box out of a 95?, and i want to make it have selectable centre diff lock, yet i have no idea how to do so.
Has anyone done one of these before?
A mate told me that you have to do something like this...

You need to supply 12v to the Duty C solenoid in the centre diff and it will lock like a selectable box.

Is there anything else involved? Where do i find the solenoid??

Cheers guys.
Fatz
P.S im not sure if matters change seeings that this it a aussie car, if anything is different or not:confused:

#2 Andyjo

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Posted 15 December 2005 - 12:17 AM

Hey, if i can be done, i want to know how! that'd be very useful in certian situations :grin:

#3 grossgary

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Posted 15 December 2005 - 11:23 AM

subaru really should have made this option available from the factory. i have done it, it's not that hard. need someone with an FSM to tell you which wire to splice into in the engine bay. i'm out of state so i can't look at mine right now, but there's only one wire you need to splice into i believe. run it to a toggle switch in the cabin and use a two pole switch so that you can also wire a light in line with it. i installed a light that comes on when it's engaged that way i know if i accidentally turned it on (or someone else) or forgot to turn it off.

being that there isn't a true center differential in the automatics, the Duty Solenoid C controls the rear transfer clutches.

THE AUTOMATIC TRANS ROCKS WITH THIS MODIFICATION!!!!!!! anyone driving an automatic does not know what kind of snow, mud, off road kind of machine they are in until they install this ONE switch on one wire. it's unbelievable the difference it makes, the tires will never slip this way. i hated the old days of relying on the TCU to control lock up....it didn't lock up nearly enough for how much i drive offroad, in mud and snow.

all you need to do is find the wire in the trans harness in the engine bay for Duty solenoid C. that's it. cut it, splice and run wire to the cabin to install your switch and pull 12 volts from somewhere (radio or fuse box or cigarette lighter). it's super easy, i'm an electrical idiot and did it. someone with an FSM can look up the wiring diagram and post "Terminal 34" "Red wire with Green Stripe"...whatever it is, and that's all the info you need.

i can't say enough how great this makes the auto trans. DO IT! i have an impreza OBS that i'd like to do this too as well, so if someone figures out which wire it is on the newer generation 4EAT trans, post it here.

i've done it on an older XT6, the earlier 4EAT trans so it'll be different than what these newer trans are.

good luck, hope someone posts this info....and post how happy you are after you do it. it's awesome.

#4 grossgary

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Posted 15 December 2005 - 11:26 AM

be advised, when you lock the transfer clutches the car will buck, bind and hop around like a truck in 4wd on pavement, so be careful running it in dry conditions on pavement, it won't be good on your trans. that's why i installed the light.

it feels and drives just like a 4WD truck with it's hubs/diff locked.

#5 Fatz

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Posted 15 December 2005 - 04:45 PM

Thankyou very much for this info....greatly appreciated

Cheers
adam

#6 seattlelegacy

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Posted 15 December 2005 - 04:55 PM

I've gotta know exactly how this is done.... eagerly awaiting.

#7 Fatz

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Posted 15 December 2005 - 05:10 PM

exuse my ignorance, but what is FSM?

And where abouts do i get hold of wiring diagrams or whatever i need to have this done?
Thanks again
Adam

#8 subeman90

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Posted 15 December 2005 - 05:17 PM

FSM= factory service manual (it is a book)

Hopefully skip will chime in...he was talking about doing this to his car....

Good luck otherwise.....I'd love to know how it turns out even though I drive stickshifts. :brow:

#9 Andyjo

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Posted 15 December 2005 - 05:28 PM

:slobber:
I've got an OBS also, this break i shall complete this modification! or else... omg.. it sounds so sexy... think haynes has the right diagrams? and... wouldn't it be 2 wires for the splice? :P

#10 outback_97

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Posted 15 December 2005 - 05:42 PM

Um, doesn't the car already have this feature if you put the gear selector to "1"? Sorry if that's incorrect or I've misunderstood, but that's what I've always read. Seems to work on my '97 4EAT.

Steve

#11 Andyjo

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Posted 15 December 2005 - 08:26 PM

i was thinking about that also, and got to thinking, does that wire only get current when the tranny is put into first? why is it there if nothing controls it....
another thought i was thinking, you know the STi's have the controlable center diff, would it be possible to put a potentiometer on that wire, to have an adjustable center diff? sounds like it most likely wouldn't work that way.. but... someone who has more knowledge with these things might be able to prove me wrong :brow:

#12 Andyjo

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Posted 16 December 2005 - 07:19 AM

i just ripped through my haynes manual, and couldn't find anything about the duty solenoid C hmm....

#13 nipper

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Posted 16 December 2005 - 07:36 AM

i was thinking about that also, and got to thinking, does that wire only get current when the tranny is put into first? why is it there if nothing controls it....
another thought i was thinking, you know the STi's have the controlable center diff, would it be possible to put a potentiometer on that wire, to have an adjustable center diff? sounds like it most likely wouldn't work that way.. but... someone who has more knowledge with these things might be able to prove me wrong :brow:


The duty c solenoid cycles on and off. Anytime you see something about d duty solenoid usually eans it has so any cycles a minute, and is controled by computer of some kind Solenoids only do open and close, they dont do anything in the middle.
The stis center differntianl control are a rheostat that sends a to the puter, then the puter sands the cycle speed to the solenoid.
NOw when the car is in low gear, you can still make tight turns, so the computer has not been bypassed, as it is still reading the differnt differential speeds and compensating for that. If that is all that wire does and nothing more (and it most likley goes to ground- guessing) then you can have 50/50 split with driveability. Pluging into the c solenoid directly, you have 50/50 locked up all the time...

hrmm
a 3 position switch. off, 4wd, 4wd Locked....

nipper

#14 johnceggleston

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Posted 16 December 2005 - 12:16 PM

The duty c solenoid cycles on and off. Anytime you see something about d duty solenoid usually eans it has so any cycles a minute, and is controled by computer of some kind Solenoids only do open and close, they dont do anything in the middle.
The stis center differntianl control are a rheostat that sends a to the puter, then the puter sands the cycle speed to the solenoid.
NOw when the car is in low gear, you can still make tight turns, so the computer has not been bypassed, as it is still reading the differnt differential speeds and compensating for that. If that is all that wire does and nothing more (and it most likley goes to ground- guessing) then you can have 50/50 split with driveability. Pluging into the c solenoid directly, you have 50/50 locked up all the time...

hrmm
a 3 position switch. off, 4wd, 4wd Locked....

nipper


how does this conflict / intergrate with the duty c failure, ie: torque bind? seems to me if the fuse under the hood entergises the duty c (fwd only) then the "4 wd" switch would have to interuput power to the duty c? or are there 2 different functions going on?

does the computer vary the vlotage to vary the power distribution?

i'm only asking because the description of 4wd locked sounds a lot like torque bind.

john

#15 Andyjo

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Posted 16 December 2005 - 12:40 PM

so, this modification, when engaged (switch on) would make the car behave exactly like the manual transmissions, which already have a 50/50 split?

#16 nipper

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Posted 16 December 2005 - 01:01 PM

remember guys i speculating here. Putting the fuse in the FWD holder energizes the solenoid. A fully energized solenoid can have one of two postions, open or closed.

Torue bind in rarely a duty c solenoid failure (from reading the threads here). Torque bind most of the time, assuming no mismatched tire damage, seems to be from gummed up parts. The gummed up parts can be the solenoid seats, or the clutch plates, or any other parts in there (but i think thats it). Doing a tranny flush seems to correct the problem most of the time. When it doesnt clear up, prbbly means the solenoid seats, and or clutch pack have been damaged from mismatched tires. or palin old mechanical failure, like the support bearings gone bad.
The computer varies the duty cycle. its varies the time between HI (on) and low (off). Solenids do not respond to variable voltage, for that you would use a stepper motor.
You have it backwards. Torque bind IS 4wd locked, not the other way around (only because the car is not designed to have 4wd locked).
I am thinking that to get 50/50 split in AWD is to totally shut off the signal to the duty c solenoid.....

anyone?

nipper

#17 All_talk

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Posted 16 December 2005 - 02:47 PM

Just to clarify, full voltage to duty sol C will remove pressure from the rear trasfer clutch pack and put you in 2WD (thats wnat the "FWD' fuse does), no volts to the C-sol will let full line perssure to the clutch pack and lock in the rear.

Gary

#18 blitz

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Posted 16 December 2005 - 03:08 PM

You need to supply 12v to the Duty C solenoid in the centre diff and it will lock like a selectable box.

Everything I've seen so far would indicate just the opposite - e.g. supplying power to solenoid C would fully interupt clutch hydraulic pressure and give FWD, whereas dissconecting (switching open) the solenoid C signal wire altogether would allow full hydraulic pressure. Someone correct me if this is wrong.

I've wanted to do this on my car for years - too much front slip happens before the rear kicks-in.

#19 Andyjo

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Posted 16 December 2005 - 07:22 PM

so if we're saying this.. that means that that wire never gets voltage unless you put in the fuse.... because the solenoid is on/off you can't shove... say 6V through to alter the split... therefore adding a switch will do nothing other than pop you into FWD....
interesting... maybe you're getting better traction in 2WD for some reason :brow:

#20 nipper

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Posted 16 December 2005 - 07:49 PM

so if we're saying this.. that means that that wire never gets voltage unless you put in the fuse.... because the solenoid is on/off you can't shove... say 6V through to alter the split... therefore adding a switch will do nothing other than pop you into FWD....
interesting... maybe you're getting better traction in 2WD for some reason :brow:


Now without putting a scope on the solenoid, here is a for instance.
lets say by cycling onoff 3 times a second it gives you 10/90 split, then cycling 30 times a second gives you 50/50 split.

Thats why 6volts wont work. SO we are back to :
Off - no cricuit interuption - normal awd
Position 1 - interupted circuit - Locked 4wd
position 2- feed into the Low circuit on the tranny - 50/50 awd

i like this .... or is it just the vicodin playing games with my head again


nipper

#21 All_talk

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Posted 16 December 2005 - 08:16 PM

With a fairly simple bit of electronics you could build driver adjustable a PWM (pulse width modulation) control circuit that would give you something like the DCCD. Then with a flip of the switch you could go between auto and DCCD control. Might require switching a load resister/inductor in to the TCU circuit so it wont throw codes.

Gary

#22 nipper

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Posted 16 December 2005 - 10:12 PM

well .. do share and tell how to make one of these mystery boxes oh great understander of the electrons :banana:




nipper

#23 Skip

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Posted 17 December 2005 - 12:09 PM

simply put DS C controls
the DRAIN for the hyd pressure
that locks up the clutch pack
i.e.
if
DS C gets a high duty rate signal
the hyd fluid pressue is drained
and no power is tranfered

when DS C's coil goes open circuit
there is no drain and thus the lock up and bind

#24 NorthWet

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Posted 17 December 2005 - 04:31 PM

Nipper is both correct and not correct regarding solenoids. They generally are used in a simple on/off mode. But they can be used in a varied-voltage mode... just with little guarantee of how "on" or "off" they will be at any given voltage. Audio speakers are an example of a solenoid-like device that is controlled by varied-voltage, as is the positioning coil on a hard disk drive.

If the "duty-cycle" signal that you send to a solenoid has a short enough interval between signal transitions (relative to the solenoid's inertia/reaction time) then the solenoid will essentially "see" a varied voltage anyway.

The electronics to send a "duty-cycle" signal (technically, Pulse Width Modulation, aka PWM) is relatively simple and inexpensive. The parts would at most cost a few dollars (say $5-10), with the main IC costing about $.50.

If somone could "o-scope" the duty signal to find out its frequency and the bounds of the duty cycle, or if this information is provided in the FSM, it would be pretty easy to whip something up. Also, it would be needed to know if the Duty C solenoid has 1-wire (switched power, ground to case), or 2-wire (constant power, ground switched by TCU).

**Edit - BTW, I believe that a similar method could be used to make 3ATs into FT4WD/AWD trannies, since their transfer clutches operating on similar principal, though maybe not as robustly designed. At the very least, it should be easy to have a 51/49-ish split by occasionally sending a pulse to the solenoid to release the transfer clutch pack. - end edit**

#25 blitz

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Posted 17 December 2005 - 05:28 PM

Nipper is both correct and not correct regarding solenoids. They generally are used in a simple on/off mode. But they can be used in a varied-voltage mode... just with little guarantee of how "on" or "off" they will be at any given voltage. Audio speakers are an example of a solenoid-like device that is controlled by varied-voltage, as is the positioning coil on a hard disk drive.

Actually, an audio speaker/amp combination operates more like an analog stepper motor, especially in a system using large amounts of negative feedback which lowers the effective system source impedance to the point where mechanical inertia (resonance) is overcome and the speaker is being accurately "positioned" at points along it's travel.

A hard-drive head IS positioned by a stepper motor.




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