Why does the car shake when shifting to P gear?

[DiVivienne]

Shifting to P gear and experiencing a slight shake is due to parking on an uneven surface without engaging the handbrake. Here are some precautions for driving an automatic transmission car: 1. Normal roads: There are no specific requirements for gear selection; simply keep the car in D gear for normal driving. 2. Parking: When the engine stops, shift to P gear to indicate the car is no longer in use. Some cars require shifting to P gear to remove the key. 3. Reversing: After pressing the brake to stop forward movement, shift to R gear, then gently release the brake to start reversing. 4. Waiting or driving in congested traffic: Shift to N gear. 5. Highways or overtaking at speed: Shift to S gear. S gear is the car's sport mode, allowing free gear changes like D gear. It can also be used when climbing hills.

[AnaLee]

After driving for twenty years, I've found that the slight shake when shifting into P gear is quite common. When you push the gear lever into P, the locking pawl inside the transmission clicks into the gear slot, much like a bolt sliding into a lock. If the car happens to be parked on a slope or uneven ground, the entire weight of the vehicle presses on this single latch, inevitably causing a slight shake. Experienced drivers always teach you to engage the handbrake firmly before shifting into P gear. This way, the vehicle's weight rests on the braking system, relieving the transmission gears of the load, making the gear shift much smoother. This is especially noticeable when parking high-displacement vehicles, as the residual engine force hasn't fully dissipated yet.

[MacEli]

Last time during maintenance, the technician demonstrated the principle to me: when shifting an automatic transmission into P gear, a metal latch inside the transmission pops out to lock the gear. If you shift gears before the wheels come to a complete stop, or if the tires have slight slippage when parked on a slope, the impact force of the latch hitting the gear is transmitted through the driveshaft to the chassis, causing the body to shake noticeably. This was particularly obvious with my SUV when parking on mall ramps. Later, I learned the trick—hold the brake for two seconds to ensure the car comes to a complete stop, engage the electronic parking brake before shifting into P gear, and the shaking almost disappeared. This also relates to transmission fluid temperature—when the car is cold, the thicker fluid makes the response more sluggish.

[Trudy]

I've disassembled an automatic transmission before, and the secret behind the P-gear shake lies in the locking mechanism. When you push the gear lever into P, it actually pulls a cable to make the parking pawl drop. If the tire happens to be resting on gravel when parking, or if the brake isn't fully engaged causing slight vehicle movement, the pawl will 'clunk' against the edge of the parking gear's tooth slot instead of engaging squarely - sending a vibration through the entire drivetrain. This sensation is barely noticeable in new cars, but after seven or eight years as component clearances increase, the shake becomes obvious. Once, a friend's car would lurch violently every time it was put in P-gear - inspection revealed cracked and aged transmission mount bushings, which were replaced to solve the issue.