The idea is simple: When you put an automatic transmission in “Park,” the wheels are stopped from turning. At least that is the expectation. While it may seem that you are putting on the brakes somehow, that’s not actually what is happening. It is possibly normal if your car rolls in Park, however, it is also potentially a sign of larger issues that need addressed. Let’s take a look at the parking brake and what it means when your car rolls while in Park.
Why Your Car Rolls in Park
Deep inside almost every automatic transmission is a special component called the parking pawl. The parking pawl looks like a hooked finger, which grabs a toothed wheel inside the transmission and keeps the transmission output shaft from spinning. But there is still a lot going on between the transmission output shaft and the wheels on the ground.
Depending on how your vehicle is designed, the total length of the driveline can affect what happens when you put the shifter in park. Components like U-joints and gears have a small amount of play in them so that they can operate smoothly. Even with the transmission output shaft stopped, the slack in the driveshaft U-joints and axle gears can allow just enough movement to translate into a small amount of rolling movement at the tires.
Problems That Can Cause a Car to Roll in Park
In some cases, if a car rolls when in Park, it is a sign of other issues—here is a few of the most common problems:
When U-joints on rear-wheel drive vehicles get worn out, they get sloppy. Considering each U-joint has four points of movement and a driveshaft usually has two U-joints, there are multiple points of potential wear. U-joints depend on needle bearings with tight tolerances. You can grease some U-joints to prolong their lives, but many are simply sealed.
Throughout the years those needle joints flex twice for each revolution of the driveshaft. Add to it vibrations from the road and a constant assault of road grime, it is easy to see how things can get worn down and loose. If there is a noticeable “jerk” when changing from forward to reverse (as the slack is taken up on the driveline), that same slack can make your car roll when put in Park.
Worn Differential Gears
If the U-joints are tight, then the backlash on the differential gears are potentially out of specification. A certain amount of distance is needed between the ring gear and the pinion gear to allow for proper gear mesh. If the gear mesh is wrong, the gears can whine or howl, especially on deceleration. But a seriously worn set of differential gears can have enough slack to allow the tires to rotate a little, even if the pinion is stopped. Luckily, this kind of wear is rare, as long as the axle was not abused by carrying heavy loads or contaminated gear oil (like from off-roading in mud or water).
In the worst-case scenario, the parking pawl inside the transmission is damaged. Things like putting the transmission in “Park” before coming to a complete stop can wear down, crack or even destroy the metal parking pawl. Either the parking pawl itself breaks or the teeth on the mechanism become worn to the point where they cannot hold.
A broken parking pawl or stripped parking toothed wheel is not an easy fix. Most automatic transmission designs put these components deep inside the case, meaning a near complete teardown is needed for a repair. If the parking pawl in your transmission is completely broken, the only way to keep the car from rolling is by using the parking brake.
If everything in the driveline is within spec, then the solution is simple: Use the parking brake. Your parking brake is the most effective way to stop your vehicle’s tires from rolling because the braking force is applied directly at the wheel.
How You Park Matters
Drivers of manual transmission equipped vehicles already know the importance of using the parking brake, or else they won’t find their car in the same place they parked it! But it turns out automatic transmission equipped vehicles also need to use the parking brake. Have you ever struggled to get your vehicle out of “Park” after parking on a steep hill? The reason for this is the immense pressure put on the parking pawl to hold the vehicle in place. If the hill is steep enough, it may require manually pushing the vehicle forward to ease the pressure on the parking pawl.
Here’s how to park the right way:
- Come to a complete stop in the spot where you want to park
- With your foot firmly on the brake pedal, set the parking brake
- Put the transmission gear selector in “Park”
- Lift your foot off the brake pedal
By using the parking brake when parking an automatic-transmission-equipped vehicle, you are putting the bulk of the stress on the braking components. Brake pads and shoes do a great job of holding your car, truck or SUV in place and are easily replaced when they are worn out. Plus, you are making sure the vehicle is completely stopped before the parking pawl is engaged. Then when it is time to go, just put your vehicle in gear and release the parking brake.
A car that rolls in park is one possible sign of a bad transmission. So, if you are in doubt, visit your local NAPA Auto Care center for a proper diagnosis. Our ASE-certified technicians have the knowledge and the equipment to diagnose the signs of a failing transmission, as well as determine why your car rolls when in park. Plus, when you choose NAPA Auto Care, you are covered by our free 24-Month/24,000-Mile Peace of Mind Warranty, which spans across the entire nationwide NAPA Network and includes parts and labor on qualifying repairs and services.
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Photo courtesy of Wikimedia Commons.
With an automotive writing career spanning over two decades, Brian has a passion for sharing the automotive lifestyle. An avid DIYer he can usually be found working on one of his many project cars. His current collection includes a 1969 Olds Delta 88 convertible, BMW E46 sedan, and a slant-6 powered 1975 Plymouth Duster.