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I was recently replacing the brake pads and callipers on my 2002 Chevrolet Suburban 1500 (4WD), and after I’d finished the job but still had the wheels jacked up, I noticed that even though I had the vehicle in park, I was still able to freely spin all four of the wheels. I popped the SUV back down on its wheels and tested if just the parking gear would hold the SUV, and it was working just fine.

What causes the parking gear to not work when the SUV is up on jacks? Of course, my brake job wouldn’t interfere with it at all since the parking gear is part of the transmission.

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When park is selected it is only the gearbox output shaft that is locked in place, the open differentials that are fitted to the car are still able to do their job and allow relative rotation of the wheels.

When the differential input is turned by the gearbox, the differential causes the wheels to rotate in the same direction. However if you rotate a wheel with the input shaft locked, the design of the differential will cause the other wheel to rotate in the opposite direction.

When you had the wheels off the ground, you should have noticed that turning 1 wheel will have caused at least one other wheel to rotate in the opposite direction.

With the wheels on the ground, it would normally be impossible for a wheel to spin in the opposite direction, which therefore requires the differential to attempt to turn the input shaft, which in park is locked by the gearbox. In this case, the car is not able to move.

If the car is parked on ice on a slope it is possible to experience the car slide with the wheel with the least grip on the ice spinning in the opposite direction to the others due to the same reason.

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  • The lesson here is always chock the wheels!
    – GdD
    Commented May 4, 2020 at 7:40
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When the vehicle was in the air the differentials in the drive train allowed the wheels to rotate.

Once all four wheels were on the ground then each wheel was held stationary by friction relative to the ground and no one wheel could rotate as the other three were fixed.

The same idea works if the vehicle is 2wd as if the input to the differential is locked then neither wheel can turn.

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Unless you have locking differentials (most vehicles do not), each wheel can be turned independently when in park and raised in the air, you will notice when you turn a tire the opposite tire on the differential turns the opposite direction, but if the wheels are on the ground preventing that wheel from opposite rotation, park works.

You can test this by having a friend hold the opposite wheel while you attempt to turn your wheel, see what happens.

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