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How Automatic Transmissions and Transaxles Work continued


Now that we've covered some automatic transmission basics and the two most common drivetrain layouts, we will now take a closer look at what goes on inside the transmission.

In its simplest context, here is what happens when the transmission is manually shifted into each of the transmission's gears.

Shifter in the "P" Park Positionslotted shifter

When the shifter is placed into the Park or "P" position, the transmission uses a parking pawl (or steel pin) to lock the output shaft of the transmission, which restricts the shaft from turning in either direction.

Contrary to general thinking, the primary purpose of the transmission's "Park" position is to keep the engine's power from reaching the drive wheels, not to keep the vehicle from moving/rolling when parked - this is the job of the parking brake (also known as the e-brake or emergency brake).  Relying solely on the transmission's "Park" position to keep the vehicle from moving when parked places undue stress on the parking pawl and other driveline components.  The stress on the parking pawl and drive line is much greater when the vehicle is parked on an incline.

 

> When parking your vehicle, particularly when parking on an incline, set your parking brake before shifting the transmission into "Park".  This will eliminate stress on the transmission's parking pawl and other driveline components.

> When parked on an incline, move the shifter from Park to Reverse or Drive before releasing the e-brake.  Doing so will insure the full weight of the vehicle is not suddenly placed on the parking pawl and driveline components.


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Shifter in the "R" Reverse Position

In modern automatic transmissions/gearboxes, moving the shifter lever to the reverse position triggers an electrical device (called a solenoid) to cause the reverse gear within the planetary gear set to engage thus transmitting the engine's power to the drive wheels in a reverse direction.  Placing the shifter in the "R" position also activates a switch to turn on the reverse (or backup) lights for improved visibility at night and to warn others that the vehicle is intending to backup.

For safety reasons, before the shifter lever can be moved to the reverse position, the vehicle must come to a complete stop and driver must depress the brake as well as the shift lock button located on the shifter. A different shifter design, requires the driver to move the shifter lever to one side [into a slot] before moving it to reverse.

Shifter in the "N" Neutral Position

When the shifter lever is moved to the N (or neutral) position, the gears within the transmission/gearbox are disengaged.  In this position, the vehicle's wheels turn freely - but in some vehicles not so freely that the vehicle can be towed with the drive wheels touching the road surface.   When being towed, it is advisable to lift the drive wheels up off the road surface to avoid damaging the transmission or driveline.  On a rear wheel drive (RWD) vehicle, the same thing can be accomplished by removing the driveshaft. 

Shifter in the "D" Drive Positionshifter

Moving the shifter lever to the "D" (or Drive position) allows the transmission (or gearbox) to automatically engage the full range of available gear ratios as needed, which is determined by the vehicle's Engine Control Module (ECM) or Engine Control Unit (ECU).  Some vehicles have a Power Train Control Module (PCM), which controls the functions of both the engine and transmission while others use a separate computer device for the transmission called a Transmission Control Unit (or TCU).

Vehicles equipped with automatic transmission are meant to be driven with the transmission gear selector in the Drive position.  In Drive, the transmission operates most efficiently and effectively by automatically switching to the correct gear ratio as speed and other travel factors dictate.

Automatic transmissions installed in passenger vehicles can have anywhere from three to eight forward gears.  Today, three and four-speed automatic gearboxes with overdrive are the most common but are being replaced with new five-speed and six-speed gearboxes.  Seven-speed and even eight-speed transmissions are now available in some high performance and high-end luxury vehicles.

Shifter in the "3" Third Gear Position

When the shifter is in the "3" or Third Gear position, the transmission is locked out from using any higher gears, which in many vehicles is overdrive.  In other words, first and second gear are still used just as if the shifter was in the drive position.  However, in some vehicles, even when the shifter lever is positioned in third gear, the transmission will automatically shift up to the next higher gear when a certain revolutions per minute (RPM) is reached in order to prevent damaging the engine and transmission.

Placing the shifter into third gear instead of the normal "D" position is recommended when traveling through mountainous areas as doing so prevents needless back and forth automatic shifts between third gear and drive as the vehicle struggles to determine which gear it should be in.  Third gear is also recommended when towing a trailer or other vehicle. 

In normal conditions, the correct and most efficient gear selection is "D".  Using other gears needlessly can cause damage to the engine, transmission and driveline components and increase fuel consumption.

What's the big fuss about installing a transmission oil cooler?
If you don't know, you should read this.

Shifter in the "2" Second Gear Position

When the shifter is in the "2" or Second Gear position, the transmission is locked out from using any higher gears, which in many vehicles is third gear and overdrive.  In other words, first gear will still be used just as if the shifter was in the "D" position.  In some vehicles, even when the shifter lever is positioned in second gear, the transmission will automatically shift up to the next higher gear when a certain revolutions per minute (RPM) is reached in order to prevent damage to the engine and transmission.

Placing the shifter lever in the "2" position is advisable only in a few situations, i.e., traveling in adverse conditions such as snow and ice, (especially when climbing or going down ice or snow covered hills).  Second gear can also be used when traveling down a steep grade to help control vehicle speed when the brakes become overheated and ineffective.  Warning: Continuous use of second gear as a way to control speed when traveling down a steep slope will cause the transmission to overheat.  Excessive heat can cause serious and even catastrophic transmission failure fairly quickly.

Shifter in the "1" First or "L" Low Gear Position

When the shifter is manually moved to the "1" or "L" [First or Low] position, the transmission is locked out of using any higher gears.  In this gear, the engine's RPMs rise very quickly.  Top speed being no more than 15 to 20 miles per hour.  When traveling in first (or low) gear, even at a slow speed, excessive heat quickly builds up in the engine and transmission potentially causing serious damage to both.  In some newer vehicles, the transmission will automatically shift up to the next higher gear when a certain revolutions per minute (RPM) is reached in order to prevent damage to the engine and transmission. 

Using first gear is not advisable.  However, in situations where you have no choice, you should not exceed 10 miles per hour for more than 5 to 10 minutes at a time before shutting the engine off and allowing the transmission to cool.

keep reading Automatic Transmission Components

   
Installing a transmission shift kit will:
    a) improve shifts
    b) improve performance
    c) increase fuel mileage
    d) all of the above

The answer is d) all of the above.
>Learn more about the benefits of installing an automatic transmission shift kit< 
 

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