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Old 01-04-2005, 04:12 PM   #1
RightyTighty
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Default Has anyone done the center differential lock mod on early 4eat?

I have a 1991 legacy with 4eat. I've read about people splicing a relay switch from the TCU in order to get center differential lock manually. How is this done? I know all that is required is cutting one wire and splicing a switch into it right? Doesn't the selenoid for the center diff activate when electrical flow is stopped?

Also, would I be damaging the center diff if I left it on when going through really deep snow?


What about swapping a newer 4eat that has 50/50 automatic lock when in 2nd or first gear? When did the 4eat get that feature and will it swap into my car?

thanks!
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Old 01-04-2005, 06:47 PM   #2
Legacy777
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Your 91 legacy has the function to force 50/50 torque split in 1st gear for sure. 2nd gear I'm not 100% positive. I think if the manual button is depressed it will force 50/50 torque split. However with the manual button depressed, the transmission won't shift down into 1st gear when the gear selector is in the "2" position.

The older 4EAT's do not have a center diff. They utilize a MPT (multi-plate transfer) clutch system to transfer power to the rear wheels.

To force 50/50 split, you need to splice into the duty-c solenoid line coming from the TCU. When this circuit is "open" the solenoid is not getting any power and full line pressure is transferred to the rear wheels and gives you 50/50 torque split.

This can be done....however I really don't think it's super necessary unless you're just screwing around. In which case, just leave the gear selector in 1st.
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Old 01-04-2005, 07:12 PM   #3
RightyTighty
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Quote:
Originally Posted by Legacy777
Your 91 legacy has the function to force 50/50 torque split in 1st gear for sure. 2nd gear I'm not 100% positive. I think if the manual button is depressed it will force 50/50 torque split. However with the manual button depressed, the transmission won't shift down into 1st gear when the gear selector is in the "2" position.

The older 4EAT's do not have a center diff. They utilize a MPT (multi-plate transfer) clutch system to transfer power to the rear wheels.

To force 50/50 split, you need to splice into the duty-c solenoid line coming from the TCU. When this circuit is "open" the solenoid is not getting any power and full line pressure is transferred to the rear wheels and gives you 50/50 torque split.

This can be done....however I really don't think it's super necessary unless you're just screwing around. In which case, just leave the gear selector in 1st.
Ummm, are you sure about that? I don't think the 4EAT tranny in the 90-94 models has the 50/50 split like the newer ones in first and second gear. Mine is actually a 90 tranny put into a 91 legacy. I've put the gear selector in 1st and driven around on dry ground before and didn't notice any lockup in the tranny like I do when driving the toyota in 4x4mode.

How can the tranny have an open 90f/10r power split without a center differential? What you described is how the older subaru models go from FWD to 4WD, with the clutch packs. I've never heard of a legacy with that type of system.

The manual button is good for traction control, but doesn't do anything for locking the center diff like the newer ones. I wonder when they started making the 50/50 lock in 1st and second gears?


edit: I just read through the owner's manual and it mentions NOTHING of holding a 50/50 split in 1st. I think the 50/50 thing happened after the 90-94 series 4eat. That's why I'm needing to do the mod. 90-10 power split sucks serious ass for extreme conditions.

Last edited by RightyTighty; 01-04-2005 at 07:38 PM.
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Old 01-04-2005, 10:55 PM   #4
Legacy777
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Don't believe me? Read these pages from the factory manuals. The 4EAT's most definitely do NOT have a center diff.

http://www.main.experiencetherave.co...er_system1.jpg
http://www.main.experiencetherave.co...er_system2.jpg
http://www.main.experiencetherave.co...er_system3.jpg

All Subaru 4EAT transmissions except the newer VTD 4EAT transmissions do NOT have a center differential and transfer power via the rear extension housing, and MPT system.

You will not get that "lockup" feeling you do on a 4x4 pickup or similar AWD system if you just put the lever in the 1 position. The reason for this is the TCU is taking a lot of inputs in, and at lower speeds is not going to give you the same type of power distribution/feel as if you just disconected the line going to the duty c solenoid. The computer is keeping the car from acting like that 4x4 pickup.

If your playing around in snow or dirt and have the gear selector in 1st, the TCU will adjust the power split closer or near to the 50/50 split.

There is a decent amount of threads regarding the AT's operation on www.bbs.legacycentral.org

There's a lot that is not put in the owner's manaul, that is hinted at in service manuals or found out by people with scan tools, etc.
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Old 01-04-2005, 11:24 PM   #5
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Screw that....has anyone successfully put an 4EAT center diff into a 5spd? ....and made it possible to lock?
hmmmm....tasty!

jay Storm
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Old 01-04-2005, 11:38 PM   #6
RightyTighty
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Woa! I'm super confused now. How exactly does this system differe from the 4wd systems in the older cars that have user push button? It seems to me like the pages you linked to were describing those systems, but I'm not smart and couldn't understand most of that stuff.

How does the 4EAT split torque at 90/10 and then change it to 50/50 without a differential?

Would doing the MOD give me the 4x4 type control I'm looking for? The curent system seems like it changes itself too much when I'm constantly driving a long distance in nasty snow and can be really unpredictable. Would the MOD give me more of an STI/4x4 type lock? Does the 4eat not actually lock, but only change ratios in an open setting? I think solid lock is way better for rough riding.

thanks for the help
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Old 01-04-2005, 11:38 PM   #7
RightyTighty
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Default

Quote:
Originally Posted by Storm
Screw that....has anyone successfully put an 4EAT center diff into a 5spd? ....and made it possible to lock?
hmmmm....tasty!

jay Storm
it's called the sti tranny
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Old 01-05-2005, 12:25 AM   #8
Storm
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96L Most Over-
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Quote:
Originally Posted by RightyTighty
it's called the sti tranny
Now that would be the EASY way. I just want to know if anyone has attempted something like this.

Jay Storm
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Old 01-05-2005, 09:28 AM   #9
Legacy777
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1990 Legacy & 97 OBS
AWD 6MT EJ22T AWIC Swap

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Quote:
Originally Posted by Storm
Screw that....has anyone successfully put an 4EAT center diff into a 5spd? ....and made it possible to lock?
hmmmm....tasty!

jay Storm
Are you talking about the center diff in the VTD 4EAT's? If so, I highly doubt it. It's a completely different setup and will not swap to a 5MT. If you don't mean the VTD 4EAT's, again, they DO NOT have a center diff!
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Old 01-05-2005, 09:38 AM   #10
Legacy777
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Member#: 4800
Join Date: Mar 2001
Chapter/Region: TXIC
Location: Houston, Tx
Vehicle:
1990 Legacy & 97 OBS
AWD 6MT EJ22T AWIC Swap

Default

Quote:
Originally Posted by RightyTighty
Woa! I'm super confused now. How exactly does this system differe from the 4wd systems in the older cars that have user push button? It seems to me like the pages you linked to were describing those systems, but I'm not smart and couldn't understand most of that stuff.

How does the 4EAT split torque at 90/10 and then change it to 50/50 without a differential?

Would doing the MOD give me the 4x4 type control I'm looking for? The curent system seems like it changes itself too much when I'm constantly driving a long distance in nasty snow and can be really unpredictable. Would the MOD give me more of an STI/4x4 type lock? Does the 4eat not actually lock, but only change ratios in an open setting? I think solid lock is way better for rough riding.

thanks for the help
You need to understand how an automatic transmission works. Once you understand how an automatic transmission works, you'll probably understand how the MPT AWD system works.

Yes doing this mod would give you a more predictable 4x4 type locking system. I use the term locking very loosely because there is no physical locking in the automatic transmission.

What you need to understand is that on most 4x4 setups they have a transmission, MT or AT that sends power to a transfer case. This transfer case then splits the power to the front and rear drive shafts. When you move the gear selector from 2Hi to 4Hi or 4Lo. You are doing that via the gearing in the transfer case, not the transmission.

The newer subaru's do NOT use a transfer case, and is a completely different AWD system.
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