[SoNic67]

That lifted front wheel just spins. Does the traction control brake the spinning wheel at all

On a 2WD RAV4, the spinning front wheel is locked by braking and you can move (simulates a locked diff). Now, if both are spinning on ice or grass, you are stuck.
On a 4WD RAV4, because there is no central differential, braking one wheel is OK, but braking two opposite axle wheels, with different speeds on the remaining wheels, will just lock the driveshaft. So, in my experience, Toyota doesn't do it. I think it will brake ONE wheel if it loses traction, but not TWO (have not tested this).

This is a split of 55% front drive and 45% rear drive. It is disconnected at speed above 25MPH or by use of braking.

Highlander and Sienna have an open central differential and can apply brake on any wheels that lose traction:

Full-Time 4WD/AWD All Wheel Drive Sienna and Highlander gas System This systems sends variable power to both axles. It incorporates the front differential, center differential and transfer case into one compact unit. Rear differential is located at rear axle. Operation AWD system relies on a specialized Electronic Control Unit (ECU) to control wheel spin by applying the brakes to that wheel therefore directing power to other wheels with traction. Benefits This is a light duty system that allows a vehicle exceptional traction at all times while utilizing all the features of VSC and TRAC.

 

[SoNic67]

On a 2WD Rav4 it will apply the brake to a (undriven) front wheel and that will get you unstuck? 🙇‍♂️

Yes. That call that "LSD".

For a second I was thinking a 2WD Rav4 was RWD

LOL, yeah RAV4 is basically a FWD car, even the "4WD" one.

No, it has an Limited Slip Differential, it will lock both front wheels together

It is not a true LSD (differential inside transmission is still open), it is simulated via braking.

 

[SoNic67]

Correct me if I'm wrong, but does not the Limited with AWD have full time AWD? That means it has a center differential.

LOL, no. It says on the car 4WD, not AWD. And manual says it's only on-demand, up to 25 mph.

 

[SoNic67]

The center clutch is completely necessary on this car

There is no "central clutch".
Only one electromagnetic coupler at the rear diff.

 

[SoNic67]

that electro magnetic coupler, is not a bunch of discs that are compressed together?

Electromagnetic coupling
The outer part of the main clutch is attached together with the front housing. The inner part of the main clutch is attached together with the shaft. The control clutch is attached together with the control cam. The activation energy from the transfer is transferred from the propeller shaft to the front housing. However, when the linear solenoid is not operating, the main clutch and control clutch are in a free state, and the activation energy from the transfer is not transferred to the rear wheels.
If current is applied to the linear solenoid, the solenoid magnetizes, and the electromagnet pull force causes the armature to attach to the control clutch side. Or, if there is a difference between the rotation speed of the front and rear wheels and the control clutch attaches, a difference occurs in the rotation of the main cam attached to the shaft and the control cam attached to the front housing. As a result, each ball pushes its cam, and the main clutch attaches. The activation energy from the front housing passes through main clutch to the shaft, and then to the rear differential. Then the activation energy is transferred to the rear wheels.
Depending on the rotation difference between the front and rear wheels, the system controls the current flowing to the linear solenoid. Then the activation energy applied to the rear wheels is smoothly controlled. Depending on the amount of current, the restraint energy of the outer and inner side of the main clutch changes, and the activation energy from the propeller shaft is smoothly controlled from a limited condition to condition that is nearly a direct-link 4WD condition.

 

[SoNic67]

So even if the electromagnet is activated, the clutches don't get pushed unless is a difference in rotational speeds of front and rear axles. Only then the cam balls get pushed and bind the clutch surfaces.

 

[SoNic67]

Might be imagination


Joking.
Yes, when the front is slipping in mud, the cams will lock the rear to front, because there will be a delta in rpms between front and rear. And that will get 4WD rigidly coupled.
On dry asphalt, pushing the button will not get the cams to lock, because there is no rpm difference. Unless the front loses traction because of the power applied...

 

[SoNic67]

The OR in there suggests to me that the lockup occurs for two reasons. One is if there is a difference in axle speeds as discussed above. The other is if current is applied, presumably by pressing the LOCK button.

You missed the AND in that phrase: "and the control clutch attaches"

That matches my experience I mentioned before in jacking up a front wheel and turning it easily by hand. Pressing the lock button prevents turning by hand. No rotational difference in axle speeds, it just won't turn.

So the front doesn't rotate, and you are ROTATING the rear by hand. There is your difference in rotational speed.
It doesn't have to be a lot, just enough to push the ball bearing firmly in that space and grip the main clutches. See the diagrams.

 

[SoNic67]

Luckly here it doesn't snow too often. I sure had fun in other winters with my V8 AWD Explorer (the 2001 mechanical style, full time AWD, LSD central and rear, not the new 4WD on demand one).
If I knew this when I bought the RAV4, maybe I would have looked to get a Highlander, with a real central differential.
Oh, well... like I said for me it doesn't matter that much, it's just the idea that bugs me - a minivan like Sienna can have better 4WD than me.