How the AWD system of 4sp Autos work

[Gannon]

This may help others understand how the AWD system of the 4EAT transmissions works that is found in all 4speed automatic transmissions fitted to Libertys, Outbacks, Impreza's and Foresters. Turbo and VTD versions are a little different as these have a planetary centre diff (kinda like a 5sp manual centre but with a variable clutch pack in place of the viscous coupling) The control side is very similar though

Ive been reading the 4EAT section of my FSM and although it described how the AWD system worked, it didnt actually stipulate under what conditions torque was sent to the rear wheels. So i decided to investigate.

I bought a Kit from Jaycar to build a Duty Cycle Meter

Once constructed, i connected it up and went for a drive.

Think of the 4EAT AWD system somewhat like the L series part time 4wd. The front wheels are always connected to the engine, but in the auto AWD, the power to the rear wheels can be varied between 5% and 95% through a solenoid which controls oil pressure to a multi plate clutch. Oil pressure is varied by varying a PWM signal from the TCU

Here my results taken on bitumen road
At idle in park = 5%
At idle in D = 40%
Cruise = 30-40%
Medium acceleration = 40-60%
Heavy acceleration from a standstill = Peaks at 95% and reduces to 60% as speed increases
I didnt notice any difference between D, 3, 2 or 1, or even Power or Hold

Now on gravel its a little different.
The TCU is very sensitive and very smart.
Under medium acceleration, it would shoot up to 95% instantly, even though i couldnt notice any wheelspin. It would then vary between 30-70% depending on the traction available.

I tried coasting in D with no throttle and slamming on the handbrake, the wheels skidded a bit but the display didnt change much from 30%
I then tried coasting, but with light throttle and slamming on the handbrake, the wheels skidded only a little and the display instantly jumped to nearly 80%

I will be doing more testing over the next few days so if you have anything you would like me to try, or any questions, just ask

Here is a short video of me driving up to an intersection, stopping, then quickly accelerating to 100Km/h (apologies for the sideways video, the display is housed in half a nokia phone charger and is mounted on the ashtray)
http://www.youtube.com/watch?v=x4k9KIH55fo

[buttwrx]

G'day suparoo, i have done a bit of reading about this as well. i just finished wiring up a center diff lock switch. this sends the tcu signal to a false load, tricking the computer to lock the center diff 50/50.

the reason i've done this is so when i come to take my outback on the beach, i can flick the switch when stopped on sand and can take off with drive to all four wheels instead of waiting for the front to sense slip before sending drive to the rear.

you can find my post here:
http://offroadsubarus.com/showthread.php?t=1348

other useful reading:
http://www.rs25.com/forums/f8/t99075-4e ... -lite.html

http://www.subaruforester.org/vbulletin ... 4eat-3091/

Enjoy

[JG12gauge]

Thats interesting. Mines a standard 4wd not the AWD so its more simple

[Kralle]

do you think in the l-series 4wd 4EAT transmission is the same principle?
that would explain why my rear wheels are spinning only halfe when my front wheels are spinning one times, it should be clear that every wheel is upon the ground.

[Gannon]

Yeah buttwrx, reading about the awd lock is what got me started thinking about it. I was thinking about adding a switch, but im impressed with the stock system and will leave it till i have a need to.


Yep the L series 4EAT is almost exactly the same as the 89-98 Liberty/Legacy/Impreza 4EAT. The only big difference is the bellhousing and the later models have more clutches.

[Kralle]

do you think this automatic rear diff will work althought i changed my automatic transmission to a manual tranny?

[Gannon]

do you think this automatic rear diff will work althought i changed my automatic transmission to a manual tranny?

I think your a tad confused.

My description above has nothing to do with the rear differential. I was talking about the transfer clutch in the rear of the automatic transmission. It is what sends power to the rear differential.

The rear differential that normally comes with a automatic transmission is usually a different ratio. I just read your other thread and it appears that you have a 3.7:1 rear diff that came with the auto.

Chances are you are gonna need a 3.9:1 to match your manual transmission, but thats not always the case.

What car did you get your manual transmission from?

[Kralle]

ah okay thank you, now i understands

i have the transmission from a L-series MPFI, its a 5gear.

[Tone]

I like this thread!

I am trying to sort out why my AWD isn't working (see this thread showthread.php?t=18137)

I think my multimeter will test duty cycle. So I would like to give it a go.

Suparoo, I would love to know where you connected the meter? Would I be right in guessing that there is a connection to the solenoid somewhere on the rear housing of the AT?