Transmission Self Shifts from Park to Reverse

National Highway Transportation Safety (NHTSA) refers to the "Park-to-Reverse" issue as  "Unintended Powered Roll-Away" and has opened numerous investigations of these events over the last 35 years.

WHAT NHTSA SAYS

WHAT WILL HAPPEN

WHY

NHTSA refers to the "Park-to-Reverse" issue as  "Unintended Powered Roll-Away" and has opened numerous investigations of these events over the last 35 years

Instead of immediately defaulting into gated park or reverse as a properly designed transmission should, the vehicle’s transmission can be shifted to between gears where it can self engage powered reverse after a dwell or delay period of a few seconds to several minutes or longer

History of automobile development and the mechanical cause of the defect

WHAT IS PARK TO REVERSE DEFECT?

Park to Reverse” issues are nearly always caused by a design flaw in a vehicle's transmission which makes it possible for a driver to unknowingly place the vehicle's shift selector into a position in between the Park and Reverse gear positions.  Yet rather than being in park, this area is a transitional zone between gears, which is sometimes called "false park".

 

When a vehicle's transmission is in false park, it appears to the driver that the vehicle is fully locked in park. However, on vehicles with this defect the transmission is neither in park nor in hydraulic reverse.  Instead, it is in neutral, an unstable position between the two gears.

 

From this False Park position, slight movements in the vehicle, vibration, or the build up of hydraulic pressure in the transmission can then cause the vehicle to reengage powered reverse after a delay from a few seconds to longer periods of time (what is called a "self shift"). This will cause the vehicle to suddenly and without warning move backwards unexpectedly under engine power.

If the driver has exited the vehicle with the engine running (to for example, retrieve an item, open a gate, or close a garage door, etc.), a vehicle in false park can shift into powered reverse from a few seconds to several minutes, after the driver has exited, and then run over the driver or a bystander.

 

WHAT IS THE DANGER?

False Park is a term to describe when a vehicle is between the Park and Reverse gear positions, without the parking pawl engaged.

 

Slight movements in the vehicle, vibration, or the build up of hydraulic pressure in the transmission can then cause the vehicle to reengage powered reverse

Mechanical causes of the "inadvertent movement", "Park-to-Reverse", or "False Park" problem

Manufacturers and models susceptible to inadvertent movement

Park-to-Reverse injury and death cases