Part-time AWD transmission????

[thatswhatshesaid]

The twin-stick T-case setup I'm doing in my Jeep got me thinking...

 

Since it's possible to put in a fuse on our autos to get FWD, why not turn that into a relay of some sort...with a switch, maybe for part-time FWD/AWD? Now I know they say not to run them in FWD for long periods of time, but is it really harmful?

 

On that note, if the rear wheels are electronically deactivated by the fuse, can we deactivate the fronts and make it FWD, RWD or AWD at the flip of a switch or two? Or does that go against the principles of the center differential? I really don't know how it works, so if someone could enlighten me, that would be great!

[Setright]

You cannot switch to RWD without a serious bit of fettling on the driveline.

 

FWD or AWD.

 

Your car divides torque 90/10 favouring the front wheels in full traction driving conditions. The transmission will automatically send more torque rearwards when it detects oncoming slip/spin at the front.

 

The fuse causes a permanent 100/0 split, by activating a solenoid to disengage the rear wheels.

The long term "fear" is burning out that solenoid.

 

 

Hope that helps

[thatswhatshesaid]

So is burning up the solenoid a legitimate worry? Is it really vulnerable and failure-prone? Has anyone ACTUALLY screwed it up, or is it pretty much a wives-tale?

 

And what would need to be done to the center diff to make it RWD-functional and AWD-functional? I know you can weld it and make it FT-RWD, but how about a part-time deal? And how about the switch for the part-time FWD operation? Is that possible?

[johnceggleston]

So is burning up the solenoid a legitimate worry? Is it really vulnerable and failure-prone? Has anyone ACTUALLY screwed it up, or is it pretty much a wives-tale?

 

And what would need to be done to the center diff to make it RWD-functional and AWD-functional? I know you can weld it and make it FT-RWD, but how about a part-time deal? And how about the switch for the part-time FWD operation? Is that possible?

with my limited knowledge and in my humble opinion, there is nothing you can do to make the 4eat a rwd only except remove the front axles, or install disconnects on them. the front output shafts on the trans are a direct connection the gearing of the trans. in theroy, you can remove the front axles and still drive the car but i'm not 100% sure of that. and why would you want to except in an emergency.

 

on the other hand, the rear output shaft is controlled by the TCU through a tranfer clutch and a solenoid the controls the fluid foir the clutch. typically the solenoid cycles on and off at a variable rate (controlled by the TCU) and this controls the pressure on the transfer clutch and therefore the output to the rear wheels.

 

there is no doubt that some of the 'duty c' solenoids have gone 250k miles without problems, but i have one in a 95 leg with 130k that is bad. a bad duty c causes torque bind. one of the associated problems with duty c caused torque bind is that it wears out the clutch plates in the transfer clutch much more quickly.

 

so is it "pretty much a wives-tale?", no it is real. how long can you run with the 'duty c torque bind' before yopu damage the rest of the clutch? unknown!! but it is not recommended. as for the part time FWD switch, you already have one, it's called the TCU. when the rear wheels are not needed for power the TCU only sends 10% of the power to the rear. FWD does not save any gas because all of the rear drive train is still in place and turning. you would have to remove the rear differential to gain any real advantage.

 

this has been discussed here at length.

[nipper]

All subarus are mechanically front wheel drive cars. You can not disconnect the FWD and choose RWD at will without HUGE and expensive (and maybe impossible) modifications.

 

Duty C life is usually starts getting iffy over 150,000 miles, and clutch packs the same. It really depends upon how often the car has run on a flat, and how long the tranny fluid went between changes.

 

Putting the FWD fuse in the car after getting TB is basically a "i cant make it worse" approach. Cinse odds are you have to replace the clutch pack, the Duty C solenoid is right there, you replace that too when the tranny comes apart. Its rare to replace the failed duty C and not need to replace the clutch pack, since most people keep driving with TB after the flashing tranny light appears, and ruin thier clutches.

 

Torque bind is not just an internal clutch issue, but a huge saftey issue. When TB gets really bad, its hard to turn the steering wheel. WIthout the slip in the driveline to safley manuver the car, on a rain slick road, it is possible to have the car not go where you point it, if a wheel looses traction.

 

Also automatics do not have a center differnential, they have a electroniclly controlled clutch pack.

 

nipper

[idosubaru]

nah, there's no reason too except maybe it's cool that you can, and cool to control the transmission.

the follow up question is why bother? why would you want FWD? it does not help gas mileage (do a search and read those that have asked and those, like me, that have done it multiple times on manual and auto trans) and find out why it doesn't work.

 

they can be run in RWD, i know someone doing it, but that's a very, very bad idea and best done by a transmission that it already kaput (like the one i know of).

[CNY_Dave]

All subarus are mechanically front wheel drive cars. You can not disconnect the FWD and choose RWD at will without HUGE and expensive (and maybe impossible) modifications.

 

Duty C life is usually starts getting iffy over 150,000 miles, and clutch packs the same. It really depends upon how often the car has run on a flat, and how long the tranny fluid went between changes.

 

Putting the FWD fuse in the car after getting TB is basically a "i cant make it worse" approach. Cinse odds are you have to replace the clutch pack, the Duty C solenoid is right there, you replace that too when the tranny comes apart. Its rare to replace the failed duty C and not need to replace the clutch pack, since most people keep driving with TB after the flashing tranny light appears, and ruin thier clutches.

 

Torque bind is not just an internal clutch issue, but a huge saftey issue. When TB gets really bad, its hard to turn the steering wheel. WIthout the slip in the driveline to safley manuver the car, on a rain slick road, it is possible to have the car not go where you point it, if a wheel looses traction.

 

Also automatics do not have a center differnential, they have a electroniclly controlled clutch pack.

 

nipper

 

 

In other words, TB causes it to become old-school four-wheel-drive.

 

Snow cannot stop my '92 ranger, but if you want to turn when its slick, you better ease off the gas!

 

Not really dangerous if you expect it, but if you apply traditional subie or FWD driving techniques in that situation, you will indeed find yourself out of control.

 

Dave

[Sushi]

The tranfer case on your Jeep has got me thinking... I would guess this is already the case in some vehicles, but could you make/remake a t-case that has a center diff in it to make a Jeep or other 4x4 truck full time 4x4?

[nipper]

Transfer cases come in many forms, and yes some are already AWD or full time 4wd.

 

nipper

[thatswhatshesaid]

Thanks, guys.

 

I am not really looking for better MPG or anything (if I did, I wouldn't be driving an Outback), I just thought it would be cool to have control over the transmission's output ratio of front/rear.

 

So, I guess since that isn't going to happen, I'll just focus on trying to create some sort of STi-like DCCD and get my hands on a 6-speed!

[idosubaru]

the coolest thing you can do is run a Duty C switch allowing you to "lock" your rear transfer clutches and essentially run true 4WD, nearly identical to locking the center differential. a few of us have done it, have significant threads posted on how to do and it makes an automatic an excellent vehicle for snow or off road purposes.

[thatswhatshesaid]

Awesome. Thanks, grossgary.

 

I'm looking into it now.

[nipper]

But remember to shut it disengage it when your not off road, otherwise you can destroy your clutchpack.

 

 

nipper

[johnceggleston]

the coolest thing you can do is run a Duty C switch allowing you to "lock" your rear transfer clutches and essentially run true 4WD, nearly identical to locking the center differential. a few of us have done it, have significant threads posted on how to do and it makes an automatic an excellent vehicle for snow or off road purposes.

 

how hard / practical would it be to use a cruise control switch or similar relay as a switch for the duty c? that way, when the car is turned off the so is the duty c switch.

[nipper]

how hard / practical would it be to use a cruise control switch or similar relay as a switch for the duty c? that way, when the car is turned off the so is the duty c switch.

 

Ah yes the holy grail we have been looking for. A circuit that will reset the system when X speed is reached (usually 20-25 mph).

 

 

nipper

[johnceggleston]

Ah yes the holy grail we have been looking for. A circuit that will reset the system when X speed is reached (usually 20-25 mph).

 

 

nipper

 

i think..... i'm feeling very smart now.!!!!

 

go ahead, ask me a hard one!!!

[thatswhatshesaid]

Hmm...I can't seem to find any write-ups on the Duty C switch. Can someone point me in the right direction or give me a quick rundown?

[nipper]

i think..... i'm feeling very smart now.!!!!

 

go ahead, ask me a hard one!!!

 

OK here we go.

 

In theory if you can figure out the solenoid cycling time for 50/50 torque split, then you can make a controller that simulates that command from the ECU. Then in theory its just another step to make it variable so that you wont damage the clutch pack, but still will have the same 50/50 split that you get with full throttle.

 

and then i can tell grossgary :-p

 

 

heheheh

 

nipper

[CNY_Dave]

OK here we go.

 

In theory if you can figure out the solenoid cycling time for 50/50 torque split, then you can make a controller that simulates that command from the ECU. Then in theory its just another step to make it variable so that you wont damage the clutch pack, but still will have the same 50/50 split that you get with full throttle.

 

and then i can tell grossgary :-p

 

 

heheheh

 

nipper

 

 

OK, now I'm confusdeded-

 

I thought 50/50 torque split was with the clutch pack fully engaged which is what occurs when no signal is sent down the magic wire to the duty C solenoid (soln. fully closed, dumping no pressure)?

 

100/0 being solenoid C fully engaged (dumping all pressure possible), clutch pack fully disengaged (slipping, spinning free), rear driveshaft essentially disconnected from the engine.

 

 

 

To follow up on something else you said, there is a specific action when you're at full throttle?

 

 

Dave

[johnceggleston]

OK here we go.

 

In theory if you can figure out the solenoid cycling time for 50/50 torque split, then you can make a controller that simulates that command from the ECU. Then in theory its just another step to make it variable so that you wont damage the clutch pack, but still will have the same 50/50 split that you get with full throttle.

 

and then i can tell grossgary :-p

 

 

heheheh

 

nipper

 

ok, i'm not feeling smart any more.

 

enough with the hard questions!!

 

i was just thinking that a switch that turns off with the car would mean you could only leave it on as long as the car was on.

 

kind of like the switch subaru does NOT have on the heated mirrors. off when the car is off, it can't be left on. saab has them.!!!

[nipper]

ok here we go

 

The duty C cycles on and off (hence the term duty). Another factor in this is a internal spool valve in the tranny that is hydraulically controlled.

 

A subaru (auto for this discussion) varies the torque split from 10/90 to 50/50.

 

You get TCU controlled 50/50 at full throttle, revers and Low.

 

At TCU 50/50 the solenoid is still pulsed, to release the torque bind so you can steer the car.

 

To activate FWD the TCU tells the Duty C to stay energized, and dumps all the pressure to the clutch pack. This is done with the fuse. To manually activate 50/50 full lock, you are disconnecting the solenoid from the TCU. This will give you 4wd like a 1950's willy jeep. This will be good on slick surfaces, but on blacktop will tear the clutchpack apart. There is some slip designed into the clutchpack, but not enough to handle full lock.

 

The holy grail would be to go to full 50/50 split on demand just like the TCU does. This would be the best of both worlds.

 

 

nipper

[CNY_Dave]

ok, i'm not feeling smart any more.

 

enough with the hard questions!!

 

i was just thinking that a switch that turns off with the car would mean you could only leave it on as long as the car was on.

 

kind of like the switch subaru does NOT have on the heated mirrors. off when the car is off, it can't be left on. saab has them.!!!

 

 

Oh, that is really easy. You want a locking relay, made from an ordinary relay.

 

 

You take a normal relay that has double-pole double-throw contacts, connect the transmission magic wire through the normally-closed (NC) contacts on one set of the contacts.

 

You connect a pushbutton to activate the coil and close the relay, such that when you push the button the relay closes, when you let go the relay opens.

 

Now, take +12V from a source switched with the key to activate that pushbutton, and also wire that +12V through the normally-open (NO) contacts, and connect this switched-through-the-relay 12V to the coil.

 

Voila, when you push the button, the relay closes, breaking the magic-wire circuit, but also keeping the relay coil energized. When you turn the key off, the coil will de-energize, and the relay will open.

 

Just add a normally-closed pushbutton in the feed from the +12V to the NO contacts to turn off the relay without turning off the ignition.

 

 

Dave

[johnceggleston]

Oh, that is really easy. You want a locking relay, made from an ordinary relay.

 

 

You take a normal relay that has double-pole double-throw contacts, connect the transmission magic wire through the normally-closed (NC) contacts on one set of the contacts.

 

You connect a pushbutton to activate the coil and close the relay, such that when you push the button the relay closes, when you let go the relay opens.

 

Now, take +12V from a source switched with the key to activate that pushbutton, and also wire that +12V through the normally-open (NO) contacts, and connect this switched-through-the-relay 12V to the coil.

 

Voila, when you push the button, the relay closes, breaking the magic-wire circuit, but also keeping the relay coil energized. When you turn the key off, the coil will de-energize, and the relay will open.

 

Just add a normally-closed pushbutton in the feed from the +12V to the NO contacts to turn off the relay without turning off the ignition.

 

 

Dave

you are smart aren't you???????

 

this relay/ switch would help prevent the damage from leaving it on long term. however, it does not address damage, if any, due to high speed or dry pavement.

 

my other idea, which no ones has perfected, was to control / vary the output from the speed sensor on the rear trans output shaft to the rear wheels. the computer uses this to determine the split front and rear. if the rear wheels are turning slower than the front they get more power. as far as no damage goes, this may be easier to accomplish. but it probably would not give you a 4WD locked condition. the transfer clutch should still allow for slipping to avoid damage. it was designed to do this.

[CNY_Dave]

ok here we go

 

The duty C cycles on and off (hence the term duty). Another factor in this is a internal spool valve in the tranny that is hydraulically controlled.

 

A subaru (auto for this discussion) varies the torque split from 10/90 to 50/50.

 

You get TCU controlled 50/50 at full throttle, revers and Low.

 

At TCU 50/50 the solenoid is still pulsed, to release the torque bind so you can steer the car.

 

To activate FWD the TCU tells the Duty C to stay energized, and dumps all the pressure to the clutch pack. This is done with the fuse. To manually activate 50/50 full lock, you are disconnecting the solenoid from the TCU. This will give you 4wd like a 1950's willy jeep. This will be good on slick surfaces, but on blacktop will tear the clutchpack apart. There is some slip designed into the clutchpack, but not enough to handle full lock.

 

The holy grail would be to go to full 50/50 split on demand just like the TCU does. This would be the best of both worlds.

 

 

nipper

 

 

OK, so when you're saying 50/50 'like the tcu does it', its 50/50, solenoid generally off, but with a (likely) occaisional blip to release any variance.

 

I've heard its likely pulse-width modulation, it could also be pulses of a fixed size but with a varying amount of time between the pulses (pulse-position modulation).

 

Interesting to note the only reason there's any variance (assuming all 4 tires exactly the same circumference) is because the rear tires don't take the same path as the front. They tend to take a shorter path, and so tend to push the front. If they took a longer path, the front would tend to drag the rears, and torque bind would likely be much less violent.

 

 

 

Dave

[CNY_Dave]

you are smart aren't you???????

 

this relay/ switch would help prevent the damage from leaving it on long term. however, it does not address damage, if any, due to high speed or dry pavement.

 

my other idea, which no ones has perfected, was to control / vary the output from the speed sensor on the rear trans output shaft to the rear wheels. the computer uses this to determine the split front and rear. if the rear wheels are turning slower than the front they get more power. as far as no damage goes, this may be easier to accomplish. but it probably would not give you a 4WD locked condition. the transfer clutch should still allow for slipping to avoid damage. it was designed to do this.

 

 

If I understand the system correctly, you could make the computer think the rear is slipping relative to the front, or to put it more correctly, that the front was slipping so more power needs to be diverted to the rear. Under those cases, the rear would indeed be going slower than the front.

 

You can never feed the rear with more power (torque) than the front, though.

 

 

Does anyone know what the rear or front speed sensor is, and where its located?

 

 

Dave