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Subaru Center Differential Problems

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This topic contains 4 replies, has 3 voices, and was last updated by Avatar Michael 6 years, 6 months ago.

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  • #508143
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    John O’Brien
    Participant

    Hey Guys,

    From some of my preliminary tests it appears that I have sheered something in my center differential of my 02 subaru impreza rx. This particular part is quite expensive in Australia to replace and I am looking at doing a rebuild myself but can’t seem to find a reliable guide or how to in order to help me achieve this. It was a high pitched whine under acceleration that has turned to a gravel like noise with a noticeable power disconnect and an almost sticky power delivery like it is binding. Any help would be appreciated.

    Regards,

    John

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  • #508179
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    dave
    Participant

    A: CONSTRUCTION

    The center differential consists of a set of bevel gears and a viscous coupling.
    The center differential has the following two functions: distributing the engine torque to the front and rear wheel drive shafts and absorbing the difference in rotating speed between the front and rear wheels.
    The engine torque enters the center differential case from the transmission’s driven shaft. The torque is then distributed through the bevel gear set directly to the drive pinion shaft and via the transfer drive and driven gears to the rear drive shaft.
    The viscous coupling limits the bevel gear set’s differential action when either front or rear wheels spin so that adequate torques are transmitted to the front and rear wheels and proper traction is obtained.

    B: MECHANISM OF VISCOUS COUPLING

    The viscous coupling consists of a number of alternately arranged inner and outer plates and air-and-silicone oil mixture filled into a sealed space that is formed by the center differential case and the rear side gear of the differential gear set. The inner plates have their inner perimeters splined to the side gear and the outer plates have their outer perimeters splined to the center differential case. The outer plates are held apart by spacer rings. There are no spacer rings between the inner rings, so the inner rings are movable slightly in axial directions. X-section rings are used to prevent leakage of silicone oil, which would otherwise occur if the oil is pressurized due to large difference in front and rear axle speeds.

    1. TORQUE CHARACTERISTICS

    When a speed difference occurs between the center differential case and the rear side gear, a shear force is generated in the silicone oil placed between the outer and inner plates. The torque is then transmitted by the silicone oil between the center differential case and the rear side gear.
    The greater the speed difference, the greater the shear force generated in the silicone oil. The relationship between the torque transmission and the speed difference is shown in the figure below. As can be seen from the figure, the smaller the speed difference, the smaller the torque transmission and the differential action.

    2. HUMP PHENOMENON
    Silicone oil is heated and expanded as differential action continues. This causes the pressure of air inside the viscous coupling to increase and the pressure of oil between plates to decrease. As a result, the inner and outer plates are pushed together. This direct plate-to-plate contact causes a non-viscous operation to occur, and this phenomenon is called “hump”.
    The hump eliminates the rotating speed difference between the center differential case and the rear side gear (or locks the differential), so soon after it has occurred, the internal pressure and temperature drop. The viscous coupling then returns to the normal shear torque transmitting operation. (The hump phenomenon does not occur under normal operating conditions.)

    C: FUNCTION
    When there is no speed difference between the front and rear wheels, the center differential delivers the engine torque to the front and rear wheels at a ratio of 50:50.
    When a rotating speed difference occurs between the front and rear wheels, the center differential operates to absorb it in a controlled way by the function of the viscous coupling.

    1. DURING NORMAL DRIVING

    During straight-line driving on a flat road at a constant speed, all the four wheels rotate at the same speed. The center differential delivers engine torque evenly to the front and rear drive axles. The viscous coupling does not generate shear torque because there is no relative movements between the inner and outer Elates

    2. DURING TURNS AT LOW SPEEDS

    During turns at low speeds, rotating speed difference occurs between the front and rear wheels, as well as between the left and right wheels. More particularly, the front wheels rotate faster than the rear wheels. The center differential then acts to absorb the speed difference to enable smooth driving.
    Although the speed difference is small under this condition, operation of the viscous coupling causes more torque to be transmitted to the rear than to the front.

    3. DRIVING ON ROUGH OR SLIPPERY ROADS

    When front wheels are on a slippery surface When the front wheels begin to spin, the resulting speed difference between the front and rear drive shafts causes the viscous coupling to generate significant amount of shear torque. As a result, the torque distributed to the rear wheels becomes much larger than that distributed to the spinning front wheels. The traction and driving stability are thus ensured on a rough or slippery road.

    When rear wheels are on a slippery surface When the vehicle is accelerated quickly from a standing start with the rear wheels on a slippery surface, the distribution of the vehicle weight on the front and rear wheels changes and the rear wheels start spinning. Due to the resulting speed difference between the front and rear drive shafts, the viscous coupling generates a significant amount of shear torque, now in the direction opposite to that generated when the front wheels are on a slippery surface. As a result, the torque distributed to the front wheels becomes much larger than that distributed to the rear wheels.

    #508183
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    dave
    Participant

    Unit Overhaul

    A: REMOVAL

    • Remove the manual transmission assembly from vehicle.
    • Remove the transfer case with extension case assembly.
    • Remove the extension case assembly.
    • Remove the transfer driven gear.
    • Remove the center differential.

    B: INSTALLATION

    • Install the center differential into transfer case.
    • Install the transfer driven gear.
    • Install the extension case assembly.
    • Install the transfer case with extension case assembly.
    • Install the back-up light switch and neutral position switch.
    • Install the manual transmission assembly to vehicle.

    C: DISASSEMBLY

    NOTE: Do not disassemble center differential because it is a non-disassemble part.

    Remove ball bearing using ST.

    CAUTION: Do not reuse ball bearing.
    ST 498077300 CENTER DIFFERENTIAL BEARING REMOVER

    D: ASSEMBLY

    Install the ball bearing to center differential assembly.

    CAUTION: Do not apply pressure in excess of 10 kN (1 ton, 1.1 US ton, 1.0 Imp ton) .

    E: INSPECTION

    • Bearings Replace the bearings in the following cases:
    • Broken or rusty bearings
    • Worn or damaged
    • Bearings that fail to turn smoothly or make abnormal noise when turned after gear oil lubrication.
    • Bearings having other defects
    • Center differential Replace the center differential assembly in the following case:
    • Worn or damaged
    #508187
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    dave
    Participant

    I am looking at doing a rebuild myself…

    NOTE: Do not disassemble center differential because it is a non-disassemble part.

    #574078
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    Michael
    Participant

    What is the situation when the car is at a stand still while in gear? (4 speed auto)
    I have a very bad vibration when ever im stopped in gear. if i do a 360 deg turn, the vibration will be completely gone after straightening up. If i move forward 100 meters more and stop the vibration is back. Seems to me that if the front wheels have a greater rotation then the rear, the vibration will go away. this has to do with the center differential right?
    If so how can it be fixed?

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