When I put my car in park what really keeps it from moving?

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You'd definitely need a parking brake here.
Isabelle Rozenbaum/PhotoAlto/Getty Images

It may seem like a simple thing to lock the transmission and keep it from spinning; but there are actually some complex requirements for this mechanism:

  • You have to be able to disengage it when the car is on a hill (the weight of the car is resting on the mechanism).
  • You have to be able to engage the mechanism even if the lever does not line up with the gear.
  • Once engaged, something has to prevent the lever from popping up and disengaging.



­The mechanism that does all this is pretty neat. Check out the next page for photos and explanations of how all those parts work together to keep you from rolling away.


The Park Mechanism

Figure 1. The output of the transmission, the square notches, are engaged by the parking brake mechanism to hold the car still.

The series of images below explain how the park mechanism on your car keeps it from rolling.

Figure 1 shows the output section of the transmission. The parking brake mechanism engages the teeth on the output to hold the car still. This is the section of the transmission that hooks up to the driveshaft. If this part can't spin, the car can't move.


Figure 2. The empty housing of the transmission with the parking brake mechanism poking through, as it does when the car is in park.

Figure 2 shows the park mechanism protruding into the housing where the gears are located. Notice that it has tapered sides. This helps to disengage the parking brake when you are parked on a hill. The force from the weight of the car helps to push the parking mechanism out of place because of the angle of the taper.

Figure 3. This rod actuates the park mechanism.

Figure 3 shows the rod that actuates the park mechanism. This rod is connected to a cable that is operated by the shift lever in your car.

Figure 4. The park mechanism.

Figure 4 shows the park mechanism from the top. When you place the shift lever in park, the rod pushes the spring against the small tapered bushing. If the park mechanism is lined up so that it can drop into one of the notches in the output gear section, the tapered bushing will push the mechanism down. If the mechanism is lined up on one of the high spots on the output, then the spring will push on the tapered bushing, but the lever will not lock into place until the car rolls a little and the teeth line up properly. This is why sometimes your car moves a little bit after you put it in park and release the brake pedal. It has to roll a little for the teeth to line up to where the parking mechanism can drop into place.

Once the car is safely in park, the bushing holds down the lever so that the car will not pop out of park if it is on a hill.

For more articles on brake types and braking, check out the links on the next page.