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Old 05-10-2000, 08:44 AM   #1
efoo
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Join Date: Jul 1999
Post center diff identification?

Does anyone have pictures of the center diff & viscous coupler that I can look at?

I think I've got one in my hands here, but I want to make sure. For some reason, it really looks like a rear diff - at least, I don't see an exit for the front driveshafts. Or is the whole thing integrated in the front so there's only 3 "holes" - 2 for the two front axles, and one going to the back? But then where does the engine plug into?

Please excuse my ignorance - this is as far as I've ever gotten before.

-Edwin
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Old 05-10-2000, 01:00 PM   #2
Joel Gat, 1.8L
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Hello,

A center diff sits at the very back of the transmission. It has an output for the driveshaft and an input for the back end of the tranny. That's about it - 2 holes. If you see three in/out-puts, then you have a front or rear diff (1 input and two outputs). The front and rear look very similar (maybe identical, but I haven't seen what the AWD front diff looks like).

JYD
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Old 05-10-2000, 03:40 PM   #3
dch
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Very Yellow

Question

Wouldn't a center diff require an input for power from the engine, and one output for the front wheel drive shaft and one output for the rear wheel drive shaft? Cuz like with one output for both the front and rear wheels, it's not much of a differential is it (it'd be more like a U-joint!)?

Disclaimer: I've never seen one so I don't know what the hell I'm talking about
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Old 05-10-2000, 04:52 PM   #4
paultg
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Post

Hey Edwin,
I just looked at my original Subaru Impreza Brochure, and the little AWD diagram.
It is not a great picture, but I think you have a rear differential. From the looks of the diagram, the front diff. is integrated into the transmission (so no input from engine to center diff), the center diff has two inputs (one from the trans, and one to the rear), and the rear differential (where we jacked your car up from this weekend)has three input/outputs. One for the center driveshaft, and one on each side from the rear driveshafts (like a "T"). I am not sure how accurate this diagram is. If I get a chance, I'll crawl under my car tomorrow.
Paul G.
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Old 05-10-2000, 05:02 PM   #5
Joel Gat, 1.8L
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Hello,

Since the front diff is inside the tranny, the front diff gets driven off gears located on the inside side of the shaft that goes into the center diff. Basically, the engine turns the gearset and the gearset turns both the front diff and the center diff. The center diff then turns the rear diff... Make sense?

TTTTTxxxxxxxxDDDxxCCCCxxxxxxxxxxxRRRR
TTTTTDDDDDDDDDDDCCCCSSSSSSSSRRRR
TTTTTxxxxxxxxDDDxxCCCCxxxxxxxxxxxRRRR
FFFFFxxxxxxxxddd
FFFFFddddddddddd
FFFFFxxxxxxxxddd

Where x is blank space used to make drawing clear, T is tranny, D is the main shaft in the tranny and a gear made to turn 'd', d is another gear and shaft leading to the input of F, the front diff. C is the center diff, S is the driveshaft to the rear, and R is the rear diff.

Joel

[edit for drawing clarity]

[This message has been edited by Joel Gat, 1.8L (edited May 10, 2000).]

[This message has been edited by Joel Gat, 1.8L (edited May 10, 2000).]
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Old 05-10-2000, 07:42 PM   #6
Trey
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Post

The main shaft has "DRIVE" gears located on it which run corresponding "DRIVEN" gears located on a "DRIVEN SHAFT". The power from the engine goes through the main shaft to rotate the DRIVE gears which in turn rotate their DRIVEN gears. The DRIVEN SHAFT, which the DRIVEN gears are attached to, then drives the center viscous differential. Now here's the confusing part, inside the DRIVEN SHAFT is the front pinion shaft which runs the front wheels. The kicker is that the DRIVEN SHAFT rotates independantly of the pinion shaft which actually sits inside of it. If you open a transmission, it looks as if the DRIVE gears run the DRIVEN gears which would obviously run power directly to the front wheels but actually this isn't the case. Anyway, the DRIVEN shaft turns the center diff and the center diff has the front pinion shaft (inside the DRIVEN shaft) on one side and then the rear shaft to go out to the drive (prop) shaft which turns the rear wheels.

I know I'm not doing a very good job of explaining it but basically you have a shaft inside a shaft on the lower portion of the transmission. So your drawing Joel is partially right but you need to move the diff down and connect it to the driven shaft. If it wasn't like this, then we could simply pull the center diff and run FWD which would making dyno'ng the cars easy. Unfortunately this isn't the case.

And yes, I've spent many hours studying that transmission trying to figure out the easiest way to dyno the car and ended up finding that the easiest way was in fact a hard way I originally hoped to avoid. C'est la vie!

So to answer the original question, it's easy, if it has many bolts around the outer perimeter that looks like a ring(crown) gear should bolt to it, it's a rear diff. If it's cylinder shaped with two weird looking splined inputs on one side and one splined input on the other, and no were to bolt a big ring gear, it's a center.

Good luck,
Trey
trey@cobbtuning.com
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Old 05-11-2000, 01:04 PM   #7
efoo
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Well, here's some pics to settle this once and for all:

<img src=http://hesed.mit.edu/~efoo/pix/car/diff/diff-isoview.jpg>

<img src=http://hesed.mit.edu/~efoo/pix/car/diff/diff-front.jpg>

<img src=http://hesed.mit.edu/~efoo/pix/car/diff/diff-side.jpg>

<img src=http://hesed.mit.edu/~efoo/pix/car/diff/diff-rear.jpg>

So, is this a rear or center diff?

-Edwin
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Old 05-11-2000, 01:12 PM   #8
ColinL
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Post

Rear! You can see where the half-shafts and axles attach.
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Old 05-11-2000, 01:17 PM   #9
rao
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Post

Definately the rear differential and not part of the tranny.
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Old 05-11-2000, 01:21 PM   #10
efoo
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argh. That's what I thought.

I asked the yard specifically for the center diff too. I know it's part of the tranny, but isn't it separable if you take the tranny apart? Or should I have just asked for the whole tranny?

-Edwin
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Old 05-11-2000, 04:41 PM   #11
Trey
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Post

Edwin,

Just remove the rear-most cover on the transmission and you'll have the center diff in front of you. It should just come right out. You don't have to split the entire tranny open to get to it, just remove the rear "inspection plate" I guess you could call it.

Good luck,
Trey
CobbTuning.com
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