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ASE PS2 Test Preparation Guide: Automatic Transmission/Transaxle


10/10/2011
By Larry Carley

 

● AUTOMATIC TRANSMISSION — A transmission that shifts itself using engine RPM, load and other inputs to regulate shift points and gear engagement. Late model automatics have electronic/hydraulic controls operated by the Powertrain Control Module (PCM) or its own Transmission Control Module (TCM). Most late model automatics have five or six gears (speeds), though some have as many as eight. Some automatics are actually Continuously Variable Transmissions (CVTs) that vary gear ratios depending on speed and load.

All automatics use a special type of oil for the hydraulic functions 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. Gaskets, filters, sensors, certain control components and other parts are usually available for repairs.

● VALVE BODY & CONTROLS — The valve body is located inside the transmission oil pan and regulates gear shifts and clutch pack engagement. Computer-controlled solenoids in the valve body can be replaced if defective. Magnetic sensors are used to monitor the speeds of the transmission input and output shafts. A separate vehicle speed sensor is usually used for speedometer inputs.

Troubleshooting automatic transmission problems requires a scan tool to access diagnostic trouble codes, and a pressure gauge to read internal pressures.

Other shift controls on older automatic transmissions include a vacuum modulator and/or governor that modifies 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. Older transmissions may also use a throttle cable or linkage for kickdown shifts when accelerating.

● 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 also should be changed if it shows signs of oxidation (dark discoloration or burned odor) or at the interval recommended in the owners manual. Many late model transmissions have no recommended service interval for ATF.

ATF is a lightweight oil (often synthetic) 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 transmission. 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, and these specifications have changed frequently over the years. Although there are “universal” ATF fluids, make sure the product meets the vehicle manufacturer’s specifications. 

● ATF FILTER — Located inside the transmission pan, the filter traps large wear particles that could damage the transmission. The filter should be replaced when the fluid is changed. Also required is a new transmission pan gasket.

● ATF OIL COOLER — A heat exchanger usually mounted inside the radiator to cool the transmission fluid. May have to be replaced if leaking. For towing or hard-use applications, installing an aftermarket auxiliary ATF cooler can help keep ATF temperatures down to prolong the life of the fluid and transmission.

● TORQUE CONVERTER — A fluid coupling bolted to the flywheel (flexplate) between the engine and transmission to transfer engine torque to the transmission. It also multiplies torque to improve acceleration. 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 above a certain speed 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. Draining the fluid from the transmission does not drain the torque converter, so a complete fluid change requires using a fluid exchange machine.

A bad torque converter will prevent the engine from accelerating normally, and may cause the engine to stall when the vehicle comes to a halt. Replacement torque converters with higher than stock stall speeds are available for performance applications. A higher stall speed (the speed at which the transmission grabs hold) improves off the line acceleration but reduces fuel economy.

On some of the latest automatics, an electromechanical clutch is used in place of a torque converter to couple the transmission to the engine.

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

 

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