How Does "Auto 4x4" Work?

UNSTUCK

But stuck more often.
Correct me if I'm wrong, but auto 4x4 senses rear tires spinning/slipping and transfers some/all power to the front tires. Once traction is regained, it switches back to normal rear wheel drive. It's a seamless operation.

So what's going on inside the T-case to allow this? Clutch, locking differential? Do the rear tires need to stop turning first before power is applied to the front tires? What happens when you switch from "auto 4x4" to "4x4"? Something has to lock up, right?

Lets say I want to do a 1/4 mile run in my truck.... My thinking is that if I put it in "auto 4x4" and slam on the gas, rear end breaks free spinning, the front axle can't engage until the front and rear tires match speed so things can mesh.
Now if I had put it in "4x4" to begin with I would have had front wheel drive to begin with and would not have to worry about the front axle engaging after sensing slippage.

Last scenario. I'm making a right hand 90* turn on dry asphalt. I hit "auto 4x4", turn the wheel and mash the pedal. I have plenty of HP to do damage. Did I break anything?

Maybe my head is stuck on old school T-cases, D300's or Atlas's.
 

zmotorsports

Hardcore Gearhead
Vendor
Location
West Haven, UT
I'm no expert but I think in 4a mode the transfer case uses a viscous coupler kind of like a limited slip to transfer power up front. And when in 4h or 4l switches to a mechanical connection? kind of like a torque converter locking up?


^^This. The NP247 in the WJ's and the NP249 in the ZJ's were viscous coupled and were known to have coupler issue and the replacement viscous couplers were quite expensive to purchase.
 

Kevin B.

Big hippy
Moderator
Location
Vehicular limbo
I never looked up the details, but "auto 4wd" in my '00 Explorer was just full time 4WD. I don't believe there was any automatic transfer of power to a slipping wheel or other whizbangery.
 

UNSTUCK

But stuck more often.
Nice video. Did a bit of research earlier and came up with the same idea. All clutches now days. That satisfied my curiosity. Thanks.
 
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