Getting some good replies... On the 35 mph cut off reply, perhaps the logic there is if you are able to go more than 35 mph, there is no need for AUTO LSD, since you were able to get moving from a stuck? At speed it is safer to have open differentials for steering ability... locked differentials are difficult to steer and could be a danger at speed (why the max. 5 mph warning on the rear locker use).
I found this on the Internet, from a Tundra discussion... but it applies to our Tacomas, too:
Why Auto LSD is more effective than Standard LSD Differentials
Posted on 26. Mar, 2009 by TJ Houghton
, Toyota Stuff
, Toyota Truck
, Toyota Tundra Brakes
Auto LSD Review – Advanced brake controls give Tundra drivers options in solving traction problems.
Auto LSD provides substantial benefits over mechanical LSD; that is, it improves traction by reducing wheel spin on slippery surfaces just like standard LSD rear ends, but Mechanical LSD needs some type of friction mechanism or gear arrangement in the differential, which can be noisy or generate other discomforts. Auto LSD works with an open differential that is quieter and smoother. Mechanical LSD may be less effective over time (wears out), plus can only transfer approximately 25% of available power to the wheel with traction whereas Toyota’s Auto LSD can transfer up to 100%! That is why the Tundra can go places in 2-Wheel Drive the competition can’t.
Automatic Limited Slip Differential (Auto LSD) is a unique feature on the Tundra that the driver must engage. It is not an automatic on/off function, as the name may imply. On the Tundra, Auto LSD is engaged by pushing the dash-mounted VSC Off switch.
In a recent media shootout between ½-ton pickups, the test drivers climbed a 20-degree incline with trucks’ right- and left-side wheels on either dry asphalt or wet basalt tile, which can simulate snow and provide a relatively consistent traction coefficient for a comparison test. While the Tundra was one of only two trucks to reach the top without shifting into 4-wheel-drive, it wasn’t the fastest. And the test driver complained that engine power was reduced too often. The driver didn’t engage Auto LSD, which would have allowed a more aggressive response.
TRAC is the Tundra’s default traction-control feature. Both TRAC and Auto LSD work in a similar manner. When the system detects a spinning the wheel on the rear axle, the brake is applied to that wheel. By slowing down the slipping wheel, power is transferred to the opposite wheel with more traction. TRAC also reduces engine power to help eliminate wheel spin. TRAC is an intergral part of Toyota’s Star Safety System
In some situations, more engine power and a little wheel spin are needed, especially when off-roading. With Auto LSD, the system doesn’t reduce engine power as much* and allows a limited about of wheel spin to the wheel with the most traction. This helps the driver “dig” out of certain conditions such as deep sand or mud, and maintain momentum up a slippery hill.
When the driver pushes the VSC Off switch once, TRAC is turned off and Auto LSD is engaged. The VSC Off switch is also used to turn off VSC, if the driver chooses. There is a specific sequence involving pushing and holding the VSC Off switch to achieve the desired setting between the different modes. This sequence has changed between model years, so drivers must consult their owner’s manual for details specific to their Tundra.
[*in the Tacoma, no engine power is reduced]