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ASE P2 Test Prep: Automatic Transmissions


10/1/2002
By Larry Carley

 

An automatic transmission shifts itself using engine rpm, load and other inputs to regulate shift points and gear engagement. Older automatics have mechanical/hydraulic controls, while newer automatics have electronic/ hydraulic controls and are operated by a computer (the powertrain control module or a separate transmission control module). All automatics require some type of oil for the hydraulics as well as lubrication. Due to the complexity of the transmission, internal failures typically require replacing the entire transmission or transaxle with a new or remanufactured unit. Except for gaskets and filters, most internal transmission parts are dealer only parts.

VALVE BODY AND OTHER CONTROLS
The valve body is located inside the transmission oil pan and regulates gear shifts and clutch pack engagement. Other shift controls on some transmissions include a vacuum modulator and/or governor (used on older transmissions to modify the rpm at which the transmission upshifts when the vehicle is accelerating under load). The modulator is mounted on the side of the transmission and is connected by a vacuum hose to the intake manifold on the engine. Problems with either component will affect shifting. Older transmissions may also use a throttle cable or linkage for kickdown shifts when accelerating. Newer transmissions use shaft and vehicle speed sensor inputs and engine sensor inputs to regulate shifting.

ATF
Automatic Transmission Fluid (ATF) is the working fluid inside an automatic transmission. It lubricates the gears, bearings and bushings, carries hydraulic pressure to shift the gears and serves as a fluid coupling inside the torque converter to transfer engine torque to the transmission. The fluid level inside the transmission must be maintained between the FULL and ADD marks for proper transmission operation. The fluid should also be changed if it shows signs of oxidation (dark discoloration or burned odor) or at the interval recommended in the owners manual.

ATF is a lightweight mineral oil that contains special additives and friction modifiers. It is dyed red to distinguish it from motor oil. Different types of ATF are required for different makes and models of transmissions, so make sure your customer gets the correct type of fluid for his vehicle. Using the wrong ATF can cause shift problems and may damage the transmission.

GM, Ford, Chrysler, Honda, Mercedes and others all have their own specifications for ATF. There's no such thing as a "universal" ATF that works in all transmissions. Some fluids meet a variety of specifications but cannot meet them all because of the different friction additives that are required.

Ford has three automatic transmission fluid specifications: Type F (a non-friction modified formula for most 1964-81 transmissions), Mercon (a friction-modified ATF similar to Dexron II for 1988-97 transmissions) and Mercon V (Ford's latest friction-modified formula, introduced in 1997).

General Motors has two specifications: Dexron II and III. Both are friction-modified formulas and Dexron III can be used in the older GM transmissions that originally required Dexron II.

Chrysler has a number of different ATFs: MS-7176D (also known as ATF+2) is Chrysler's version of a friction-modified ATF that's similar to Dexron II. But Chrysler's fluid is more slippery than GM's, so Chrysler recommends using only ATF that meets their specifications in Chrysler transmissions. In other words, do not use Dexron or Mercon in a Chrysler transmission.

Chrysler MS-7176E (also known as ATF+3) was introduced in 1998 and supersedes ATF+2. It should only be used in 1998 and newer Chrysler transmissions, but it can also be used in earlier Chrysler transmissions. Chrysler ATF+4 is for 2000-01 model-year applications, and their newest fluid ATF+5 is for 2002 and newer models.

ATF FILTER
Located inside the transmission pan, the ATF filter traps wear particles that could damage the transmission. The filter should be replaced when the fluid is changed. A new transmission pan gasket is also required.

ATF OIL COOLER
Original equipment ATF coolers are usually located in the bottom or the side of the radiator and are connected to the transmission with a pair of lines. Fluid circulates from the transmission to the cooler to maintain and limit the temperature of the ATF. For towing or hard use, installing an aftermarket auxiliary ATF cooler can help keep ATF temperatures down to prolong the life of the fluid and transmission.

TORQUE CONVERTER
The torque converter is a fluid coupling mounted on the flywheel between the engine and transmission that transfers engine torque to the transmission and also provides "torque multiplication" much like a set of reduction gears. Inside is a three-piece set of closely spaced blades, the turbine, stator and impeller. As the torque converter rotates, fluid is thrown from one set of blades against the other, much like a propeller churning water. This pushes the blades connected to the transmission input shaft and planetary gears to drive the vehicle down the road. Torque converters in most newer vehicles have a "lockup clutch" that engages in 3rd and 4th gears to eliminate slippage for improved fuel economy. The lockup clutch is engaged hydraulically and controlled by an electronic solenoid valve. The torque converter holds approximately one third of the total fluid required by the transmission. A failure results in slugging acceleration.

TRANSAXLE
The transaxle is a transmission in a front-wheel drive car or minivan. A transaxle combines the transmission and differential into one unit.













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