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View Full Version : 4eat mod explained "Duty Solenoid C"



blackparis
01-23-2008, 10:30 PM
Thought I would share this info, as I have done it once on my RS, with great results... I searched and din't see anything specifically on this topic...

I dont remember the exact details of Numbers and etc. but I will have to do it again anyways to the new Wagon...

Theory behind "Duty Solenoid C".. It controls the amount of Auto trans fluid that goes to a clutch pack the engages the rear wheels, in "Limp-home mode" failure of the circuit or "Duty Solenoid C" power is taken away from the circuit and a 50/50 trq split is achieved... Which causes binding of the drivetrain and throws on a AWD light on the dash....

Alot of 4eat guys have figured this much out, and have installed switches to make the car cut poer to "Duty Solenoid C" Fin but cause binding, and in racing it is better than 90/10 split, but isn't the BEST trq split fo tackling the corners...


SO... Knowing what I know about electronics I worked a solution to a better torqsplit MINUS the binding and AWD light...

Basically, I measured the resistance of "Duty Solenoid C", measure the voltage going to "Duty Solenoid C" and did some calculations on how less power to "Duty Solenoid C" I would need, to still be mostly Front wheel drive and not allow for binding..

Basically to the rear wheels:

12v = 0% power
0v = 100% power

and its not linear, its exponential... so:

1v = ~4%
2v = ~10%

BUT!

6v = 1/2 of 12v = so the solenoid is 50% open = roughly 50% power to the rear ... Basically now 66%front and 33% rear, ALOT better then 91%/9%

After watching the signal from the TCU I found that it never sends less than ~ 2v when spinning all 4 tyres.... And @ 40-60mph It gives the solenoid a full 12volts, making the car entirely FWD .... Also, during engine braking the Solenoid is being fed the full 12v, FWD...

If, you are looking for the 50% Get the same value resistor as the resitance of "Duty Solenoid C" As far as the resistor goes, It has to be wired in Series to "Duty Solenoid C", Either on the ground side or between the solenoid and TCU...


Throwing some numbers @ it...

Lets say Duty Solenoid C resistance is 400ohms

Normally it takes 12volts and however many amps to pass through the solenoid to open it fully @ 400ohms...

If you add another 400 ohm resistor in series to the solenoid, Now the 12volts has to travel through 800ohms... 6volts of power is wasted in the new resister, the other 6 makes it to the Solenoid, and only opens it half way...





I'll update this again with pics and links to parts when I go ahead and do it myself for the second time...

DOOK
01-23-2008, 10:38 PM
no clue what you just said, but sounds cool. Wouldn't a more even torque split also be detrimental to gas mileage?

blackparis
01-23-2008, 10:40 PM
no clue what you just said, but sounds cool. Wouldn't a more even torque split also be detrimental to gas mileage?

Well the 4eats switch to ENTIRELY Front wheel drive @ about 40mph, so yes gas milage does drop a few...

Sarra
01-24-2008, 12:02 AM
Why not lower the resistance, so you've got a 40/60 front/rear split? That should give you a better cornering setup.

Or is the resistance built into the wiring?

Very interesting. You should setup a potentiometer and experiment. :razz:

Hocrest
01-24-2008, 10:00 AM
I may be wrong, but I have always read and been told that Sol C is controlled by varying the pulse wave, not by varying the the voltage?

Mr. Sociable
01-24-2008, 11:08 AM
I'm confused. What Legacy models with a 4eat had a 90/10 tq split? I know my car has a 40/60 or 45/55 split (though I can't remember which atm).

Maybe you could look into a newer Leg's drivetrain for a way to achieve more than a 50/50?

shazapple
01-24-2008, 11:14 AM
All the auto's except for the special Outback, i cant remember the trim though

2.5GT
01-24-2008, 03:51 PM
hm...interesting read. update this when you successfully experiment with it :grin:

Sarra
01-24-2008, 04:59 PM
I'm confused. What Legacy models with a 4eat had a 90/10 tq split? I know my car has a 40/60 or 45/55 split (though I can't remember which atm).

Maybe you could look into a newer Leg's drivetrain for a way to achieve more than a 50/50?

Every 4EAT has a 91/9 front rear torque split unless you're in 3, 2, 1, or the TCU detects slip, then it tries for a 50/50 split.

DOOK
01-24-2008, 05:40 PM
what torque split does my 5MT have?

Wiscon_Mark
01-24-2008, 05:59 PM
what torque split does my 5MT have?

50/50

The 5MT is a completely different setup for torque split though, it's not electronically controlled.

2.5GT
01-24-2008, 08:34 PM
ok, so 0v= 100% ...then why not disconnect the power to the solenoid?

dont mind me..im just a noob :lol:

blackparis
01-24-2008, 10:52 PM
The 4eats, that I am talking about DONT have a torsion Center diff like the WRXs 4eats have... Torsions' have a 45/55 split... the 5EATs also have this setup... Never heard of a older Legacy 4eat with this setup, but it wouldnt suprise me... (My moms' BE used to get really sideways really easy)

The older 4eats, were basicly just a FWD trans... With a clutchpack that locks up to allow power flow to the rear.. SO.. You cant go better than 50/50

I did this mod, cause when I was auto-xing I was coming out of corners holding it in 1 and the front tyres were lighting up.. after this mod that stopped...

I have always thought of giving a potentiometer a try, Would be cool to "Dial" in you lock up rate ;)

Wiscon_Mark
01-24-2008, 11:27 PM
Yeah, the BE's had a slightly better torque split, 65/35 IIRC.


ok, so 0v= 100% ...then why not disconnect the power to the solenoid?

dont mind me..im just a noob :lol:

Because torque bind is a side effect?

He doesn't want to lock things up, just slide power back a little.

Your electronics seem sound, I'm just not so sure it's great for the differential.

But hey, it's your car, if you're comfortable doing this, it could be a very good DIY mod.

blackparis
01-24-2008, 11:44 PM
Yeah, the BE's had a slightly better torque split, 65/35 IIRC.

[quote="2.5GT":2o8rqcto]ok, so 0v= 100% ...then why not disconnect the power to the solenoid?

dont mind me..im just a noob :lol:

Because torque bind is a side effect?

He doesn't want to lock things up, just slide power back a little.[/quote:2o8rqcto]

that was my purpose of posting this mod..

Most people start looking to "duty solenoid C" and end up just unplugging it, or making a switch the disconnects it when they need to... and Disconecting it throws a AWD light, and also creates TRQ Bind, which does not allow the car to turn in properly/predictably, 1 wheel is going to slip upon turn-in and throughout the whole corner..... Thats why manuals have a viscous type clutch inside the diff, to alow for some slippage front to rear...

and 66/33 is by far better than 91/9

2.5GT
01-25-2008, 12:44 AM
how would it feels in the car with 50/50? slower i assume right?

blackparis
01-25-2008, 08:12 PM
how would it feels in the car with 50/50? slower i assume right?

When you turn one of the wheels is going to "hop", in a straightline there will be no difference, but it will be harder to spin the front tyres...

03LegacyWgn
07-12-2008, 10:14 PM
any updates on this? im very interested

xXGTBspecXx
07-13-2008, 12:53 AM
+1 i wanna do this to my BK. i knew somthing wasnt right when i neutral dropped my 97 legacy brighton, the front wheels had more power. my 98 gt has this same problem and i hate fwd side effects(torque steer,understeer,etc)

03LegacyWgn
07-13-2008, 02:23 AM
i know with my 4EAT locked in 1St, 2nd, And 3rd its a 50-50 split just when driving in 3rd i dont think its a good idea for some reason plus i think my tranny early shifts 1st, off the light with light acceleration it qucik shifts first into second on me and it pisses me off cause if i do have to give it gas when its in second it bogs the shit out of her till it gets higher up in the RPM's cause i lost my bottom end TQ from the Short ram intake i installed.

Any ideas on the quick shift? TCU? tranny on the way out?

legacy4ever
08-01-2008, 09:41 AM
I have the controller for the duty C solenoid, with a dial to change the duty ratio and thus the torque split. An electronic guru made that for me to cure the loss of AWD. As I fixed the AWD, I can get rid of that module. I have it in BC legacy, it should work in all 4eat phase 1 trannys.

makipsee
10-25-2008, 10:21 PM
i have a 95 BK and i lost power 2 the rear wheels. when its raining out and i punch it i go no where fast would this solenoid have anything 2 do with it?

Bluto
10-29-2008, 10:18 PM
i know with my 4EAT locked in 1St, 2nd, And 3rd its a 50-50 split just when driving in 3rd i dont think its a good idea for some reason plus i think my tranny early shifts 1st, off the light with light acceleration it qucik shifts first into second on me and it pisses me off cause if i do have to give it gas when its in second it bogs the shit out of her till it gets higher up in the RPM's cause i lost my bottom end TQ from the Short ram intake i installed.

Any ideas on the quick shift? TCU? tranny on the way out?

I HAVE THE EXACT SAME PROBLEM! :evil: Im into 2nd by two car lengths when leaving a stop. Other trans have a throttle wire or governor that controlls 1-2. A shift kit would fix also. If this trans has a governor or throttle valve- Id bet it was varnished up & sticking. Not much fun driving a car that instantly shifts into 2nd-cant be good for the trans either. :cry:

1-3-2-4
10-30-2008, 10:51 AM
All the auto's except for the special Outback, i cant remember the trim though

The one with VDC? That's the only one I know of that did not do a 90/10 It was something more balanced.. cant think of the number offhand.

2.5GT
02-17-2009, 05:02 PM
Any updates? I know it's been awhile...and the op probably have forgotten about this thread.

levis76
02-25-2009, 01:02 AM
I am very interested in this. Lets keep it alive. Anyone else have any insight on this? Would doing this reduce the trans life? Can the clutch pack that drives the rear end handle being loaded a bit more on a regular basis, say a 70/30 split?

2.5GT
02-25-2009, 12:39 PM
I found out a guy over at RS25.com who made a DIY for the fulltime "AWD"
Go check it out and plus, he's making a kit for sale too.

http://www.rs25.com/forums/showthread.php?t=99075

levis76
02-26-2009, 03:46 PM
Great link man, thank you! Not only is he looking into a variable style control, he is also local!

decke48
02-26-2009, 04:03 PM
just to tell you guys this is a snow only mod. since you loose the varible torque split the TCU does automaticly for corners. so using this on dry road will cause premature wear to the center diff, and problems caused with wheel bind

2.5GT
02-26-2009, 06:46 PM
snow/dirty/slippery roads :lol:

HeresMyMind
03-01-2009, 01:15 PM
http://www.rs25.com/forums/showthread.php?t=99075

read this u can make it 50 50

-B

levis76
03-01-2009, 04:49 PM
Yeah, after some reading and thought, the 4EAT is not capable of splitting the torque up other than the 90/10 or 50/50 for any period of time. I don't even think a 70/30 split is possible. The power to the rear wheels is engaged via an electronic controlled clutch pack in the transfer, much like the drive gears in the tranny. That clutch pack isn't designed to let just a little bit of power through, it's either engaged or it isn't, so no variable control. Even if it is possible, driving around with a 70/30 split would burn that now constantly slipping clutch to hell pretty quick. You need a whole different setup for that, which is what you get with the MT.