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The DaveP T-Case Primer, and Viscous Clutch / Transfer Case Test

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  • The DaveP T-Case Primer, and Viscous Clutch / Transfer Case Test

    One of the least-understood components in a SyTy is the way the Transfer case operates to split torque between the front and rear wheels. There is a planetary gear set in which the rear wheels are driven by a gear with 2.5 times the number of teeth that are on the gear that drive the front wheels. Because of the gear ratio, the torque is split 65% to the rears, and 35% to the fronts. There is a Viscous clutch between the two outputs (front and rear). This clutch will transmit torque from one output to the other IF the traction at the wheels on either axle is exceeded. Repeat: There must be a loss of traction at the tires to pavement for the V-clutch to transmit torque. Under normal conditions, the V-clutch just goes for the ride, and the torque split is accomplished in the planetary set.

    The V-clutch is a common failure item on SyTy. They can fail in two ways: either lock up solid, so no interaxle differentiation can occur, or fail "open" where it can't transmit torque .The failure mode that results in broken front axles, split boots, exploded propshafts, busted front diffs, and low-speed "parking lot jumps" is a LOCKED V-clutch. If you spin the rear wheels in the rain, or the truck won't move with the front prop shaft removed, the V-clutch has failed open.

    Years ago I developed a way to quickly test the V-clutch's health without disassembly or special tools. This test can be run with the T-case installed in the truck, or loose on the ground. What is being checked is the torque required to make the V-Clutch slip. I have checked lots of properly-functioning T-cases over the years. While this test is not scientific, or even to a manufacturer's specification, it will identify whether a V-clutch is "Locked", or "Open".

    Here are the conditions for the test:
    • Input shaft free to turn. If installed, transmission in Neutral.
    • Rear output shaft Locked. If installed, put a bar through the u-joint, and lodge it against the floor or frame.
    • Front output shaft is where you will apply the torque for the test. Use a socket on a torque wrench on the front output flange nut. If installed you can guestimate applying 40 foot lbs to a bar to turn the front shaft. One front wheel must be free to turn. Work safely under the car.
    The "DaveP Spec" is:
    • 12-15 Seconds
    • to turn the front output 90ยบ,
    • at 40 ft lbs torque applied.

    Remember: There should be resistance between the front and rear outputs. If it is very loose, or breaks away quickly after torque is applied, the V-Clutch has failed Open. If you can't budge the front output with the rear locked, be sure trans is in Neutral, and if it still won't budge, the V-Clutch has failed Locked.

    Submitted by DaveP 5/25/2017
    Adapted from a previously-written article by the same author


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