Will This Destroy My Tranny?

[crazyhorse001]

97 Legacy Wagon LSI ej25 4eat 120xxxmi

 

My wagon shows no signs of the dreaded Torque Bind, but I happen to have one mismatch tire on the car. I ran the FWD fuse, but decided to install a toggle switch on the dash. Will using this switch cause any damage? (yes there is a fuse inline with it [10a])

My thinking is, that a fuse is actually a bridge. A switch is a bridge. Therefore adding a switch makes an easily removable bridge. With the light damage to the front of my wagon, the hood can be a pain to open.

My intent is to run in FWD until cashflow allows me to buy 4 tires. Then use the switch to "show off" the wheelspin that an EJ25 is capable of in FWD:burnout:

[nipper]

Actually you are only doing the same thing the fuse is. You installed the switch in place of the fuse. As long as you are only energizing the FWD circuit by using the switch, your not hurting anything.

The FWD fuse acts as a switch, sending the signal to the TCU to fully open the Duty C solenoid and not activate the AWD.

What you did was maybe overkill, but fine. As long as the switch bridges the circuit between the two FWD fuse wires.

 

nipper

[crazyhorse001]

Thanks Nipper, just what I wanted to hear.

I MUCH prefer to "overengineer" things. I like reliability, that's why I love Subies

[jamal]

There could be a risk of frying solenoid C by running it at 100% for an extended period of time.

[The Dude]

There could be a risk of frying solenoid C by running it at 100% for an extended period of time.

 

Actually, the C soleniod is operating at 90% full power whenever the car is moving. When the car is shut off, the center diff is 100% locked up. As more power is shifted to the rear wheels the amount of current going to the C soleniod DECREASES.

[nipper]

Actually, the C soleniod is operating at 90% full power whenever the car is moving. When the car is shut off, the center diff is 100% locked up. As more power is shifted to the rear wheels the amount of current going to the C soleniod DECREASES.

 

Almost but not quite. The Duty C solenoid (note the word duty) is an on - off cycling solenoid. It is not designed to have full voltage/current all the time. The cycling of the solenoid allows for the differntial action on turns, and changes the the balance from 90/10 to 50/50 as is needed.`

 

When you use the FWD fuse the solenoid is energized whenever the car is turned on. The car is not locked up when the car is off, as the car needs hydraulic pressure to apply the clutch pack. When you shut the car off, the pressure in the clutch pack goes to zero. When the car is running and in park the solenoid is off, then the car is at full application of the clutch pack. Since the car is not moving, there is no torque split. It's not unusual to feel a very slight lurch in an AWD car as you back out of a parking space, as the computer figures things out.

 

nipper

 

(edit) PS. From what we have seen here, if the solenoid is good you wont hurt it, if it is weak it will burn out, but for the most part it survives as long as its months and not years....

[ccrinc]

I have had the same switch on my '92 Legacy Turbo Touring Wagon LE for at least 4 years. 99% of the time I run it in FWD. It can literally be flipped "on the fly" (highway speeds) when you need it.

 

My transmission is perfect, thank you. You're doing no damage by having it and using it. Certainly, MUCH less damage than running mismatched tires would be!

 

Emily

http://www.ccrengines.com

[The Dude]

........

When you use the FWD fuse the solenoid is energized whenever the car is turned on. The car is not locked up when the car is off, as the car needs hydraulic pressure to apply the clutch pack........

nipper

 

Ok, the clutch pack is in the tail piece of the of the AT. If the clutch pack was COMPLETELY disengaged when the car is off, you might think that towing a Subaru with the front wheels lifted would be OK. The clutch pack is functionally "behind" the rest of the AT. So, it it's open, towing on the rear wheels should not effect the AT. We know that's NOT the case.

[johnceggleston]

........

When you use the FWD fuse the solenoid is energized whenever the car is turned on. The car is not locked up when the car is off, as the car needs hydraulic pressure to apply the clutch pack........

nipper

 

Ok, the clutch pack is in the tail piece of the of the AT. If the clutch pack was COMPLETELY disengaged when the car is off, you might think that towing a Subaru with the front wheels lifted would be OK. The clutch pack is functionally "behind" the rest of the AT. So, it it's open, towing on the rear wheels should not effect the AT. We know that's NOT the case.

 

true, but we also know that with an awd in park and the rear wheels off the ground, you can spin them freely.

 

the towing thing, with rear wheels down, is a lubrication problem not a gearing hard connection problem. a short tow is possible, but speed and distance will kill your trans. that's why they say it can never be done. and that's a good rule, NEVER TOW AN AWD A/T WITH WHEELS ON THE GROUND. unless you disconnect the wheels from the trans.

[The Dude]

 

true, but we also know that with an awd in park and the rear wheels off the ground, you can spin them freely.

 

the towing thing, with rear wheels down, is a lubrication problem not a gearing hard connection problem. a short tow is possible, but speed and distance will kill your trans. that's why they say it can never be done. and that's a good rule, NEVER TOW AN AWD A/T WITH WHEELS ON THE GROUND. unless you disconnect the wheels from the trans.

 

If have another question on the Suby AT, if anyone would care to answer it. Most Suby AT models normally have 90%/10% front/back drive distribution. How does the car "know" that it has achieved a 90%/10% split% Are there any sensors involved? Is this a live, continuous feed back adjustment using the TCU, or is the AT just mechanically set for a nominal 90%/10% split?

[johnceggleston]

 

If have another question on the Suby AT, if anyone would care to answer it. Most Suby AT models normally have 90%/10% front/back drive distribution. How does the car "know" that it has achieved a 90%/10% split% Are there any sensors involved? Is this a live, continuous feed back adjustment using the TCU, or is the AT just mechanically set for a nominal 90%/10% split?

 

nipper will correct me if i stray.

 

the drive split is determined by the on/off cycle of the duty c soleniod. it's usually 90/10 front/ rear.

 

there are 2 speed sensors, one for the front diff and one for the output shaft from the trans to the rear diff. when the TCU detects a speed differential between the 2 , it changes the front/ rear drive split, (it varys) to a maximum of 50/50, (maybe 60/40). the TCU accomplishes this by changing the rate of the on/off cycle of the duty c solenoid.

 

by placing the fuse in the FWD holder under the hood, you change the duty c from an on/off cycle to an always on. this creates front wheel drive only or 100/0 split. but it's more like just fwd. 90/10 may not sound like much, but if you put in the fuse you can feel the differeence when you drive the car, even if the front and rear are turning the same speed. very noticeable.

 

if you place your trans selector in 1ST, 2ND or REV. you will command the TCU to provide a 50/50 spilt (maybe 60/40).

 

hope this helps.

[nipper]

The TC uses the input of the

 

Throttle position sensor*

Map/bap sensor (i forget what subaru calls their combo sensor)

Inhibitor Switch (neutral saftey switch assembly)*

Front speed sensor*

Rear speed sensor*

ABS*

Torque feedback from the ECU*

Engine RPM*

Tranny Temp Sensor

 

*direct effect on AWD

 

Newer models have more , including another speed sensor on the torqu converter, the ABS system itself (individual wheels). The speed sensors are the ears of the system in older subarus. Without these the AWD wont work.

 

There are some very complicated mathmatics involved in figuring out how much torque to apply and when. Thats what puters are for.

 

Personally i like the older system, where there is no softening or wait-and-see thnking from the TCU. I have rented two 2005/2006 outbacks, and there seems to be some annoying delay (split second but noticable) between front spinng and rear engaging. My 97 and my 98 never had that.

 

nipper

 

(i hope i said that right )

[jamal]

Duty C controls the pressure to the hydraulic center clutch pack. When you're rolling along and the solenoid is operating at 90%, it's sending fluid to the clutches which force them apart and allow the front and rear to spin at different speeds. There is still a small amount of resistance, which is where that 90/10 comes from. Once enough of a speed difference is detected between the speed sensors, or the TCU determines it necessary based on throttle input etc, the solenoid will drop pressure to the clutch, which allows the plates to come together and send more power rearward.

 

The power split that is listed, ie 90/10, 50/50 for 5mt, and 41/59 for dccd, only applies when all four wheels are turning the same speed and a limited slip device is not in operation. Once a wheel slip occurs, the split depends on the amount of center lockup and the available traction at the front and rear.

[nipper]

 

nipper will correct me if i stray.

 

the drive split is determined by the on/off cycle of the duty c soleniod. it's usually 90/10 front/ rear.

 

there are 2 speed sensors, one for the front diff and one for the output shaft from the trans to the rear diff. when the TCU detects a speed differential between the 2 , it changes the front/ rear drive split, (it varys) to a maximum of 50/50, (maybe 60/40). the TCU accomplishes this by changing the rate of the on/off cycle of the duty c solenoid.

 

by placing the fuse in the FWD holder under the hood, you change the duty c from an on/off cycle to an always on. this creates front wheel drive only or 100/0 split. but it's more like just fwd. 90/10 may not sound like much, but if you put in the fuse you can feel the differeence when you drive the car, even if the front and rear are turning the same speed. very noticeable.

 

if you place your trans selector in 1ST, 2ND or REV. you will command the TCU to provide a 50/50 spilt (maybe 60/40).

 

hope this helps.

 

Your completely and totally .........

 

 

 

 

 

 

 

 

 

 

correct

 

 

nipper

[The Dude]

Thank you for your eloquent explanations on the 90/10 power split of the Suby AT. Actually, you have answered yet another question of mine.

I have always wondered why a major car manufacturer would seek to purchase Subaru's AWD technology. You might imagine that a major automotive player would prefer to delevop that technology in house. Obviously, putting together the right sensors and more importanly, the proper algorithms, could be quite a task.

[jamal]

On top of that the old clutch pack system has nothing on VTD or DCCD

[nipper]

Thank you for your eloquent explanations on the 90/10 power split of the Suby AT. Actually, you have answered yet another question of mine.

I have always wondered why a major car manufacturer would seek to purchase Subaru's AWD technology. You might imagine that a major automotive player would prefer to delevop that technology in house. Obviously, putting together the right sensors and more importanly, the proper algorithms, could be quite a task.

 

Thats why ford licensed prius technology for their cars. Its not the parts that are tough to make, its the competer software that gets tough to make from scratch.

 

Always cheaper to license a mature technology then build one from scratch. Gm did that from Jeep for years.

 

nipper

[ron917]

On top of that the old clutch pack system has nothing on VTD or DCCD

 

Do you know of any web site or article that explains how those technologies work? The Endwrench article I found basically parrots the marketing stuff. I know there's a planetary center diff (Torsen? like in a 4Runner?) and something that can electronically control the torque split. I kinda understand how Torsen center diff works (magic!), but I don't know how the torque distribution can be varied.

 

I'd really love to get a new Outback XT or H6 with VTD, but I'll have to wait until the used ones get cheap enough.

[nipper]

Do you know of any web site or article that explains how those technologies work? The Endwrench article I found basically parrots the marketing stuff. I know there's a planetary center diff (Torsen? like in a 4Runner?) and something that can electronically control the torque split. I kinda understand how Torsen center diff works (magic!), but I don't know how the torque distribution can be varied.

 

I'd really love to get a new Outback XT or H6 with VTD, but I'll have to wait until the used ones get cheap enough.

 

Start here

 

http://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm