|KYLE’S CORNER – AUTO REPAIR HELP|
|AUTOMATIC TRANSMISSION & TRANSAXLE – OVERVIEW
by Kyle McFadden
Automatic transmissions are highly complex hyrdomechanical devices. Diagnosing and repairing them requires nothing less than an ASE master mechanic. The automatic transmission provides automated selection of forward gears in a motor vehicle. The automatic transmission is able to select the correct forward gear for efficient engine operation based on vehicle speed, throttle position, and engine load. The automatic transmission components consist of transmission case, torque converter, fluid pump, planetary gear set, clutch packs and/or band assemblies, control valves, transmission mainshaft, extension housing and various small parts.
The transmission case is manufactured of cast aluminum. It houses the internal components of the automatic transmission and provides a mounting surface for attachment to the engine block. The torque converter is bolted to a type of flywheel called a flex plate. The flexplate in turn, is bolted to the rear of the crankshaft. The torque converter provides a means to transmit engine power into the transmission. Refer to an auto repair manual for a diagram showing the details of your vehicle’s torque converter assembly.
The torque converter is a doughnut shaped device that is filled with fluid. When the engine is running, the torque converter spins, rotating and pressurizing the fluid using internally mounted blades. The spinning fluid rotates a turbine that is connected to the transmission mainshaft. A separate internal set of blades called a stator helps to direct the fluid into the turbine. The operation of the torque converter can be compared to a powered air fan spinning a non powered air fan. The powered fan will generate moving air, directed at the non powered fan blade, causing it to rotate. The powered fan becomes the driving member, while the non powered fan becomes the driven member. If the air is moving slow enough, very little torque is transmitted from the driving member to the driven member. If you wanted, you could easily stop the rotating fan blade of the non powered fan. However, if the powered fan were to operate at high speed, the non powered fan would be rotating at a much higher speed, making it more difficult to stop. This is the same principal that allows an automatic transmission equipped vehicle to idle in gear and drive down the road without using a mechanical clutch. At idle speed, fluid pressure is low, transmitting very little engine torque through the transmission. When the engine speed is raised, fluid speed and pressure increases, allowing more engine torque to be directed to the transmission. Most torque converters contain an internal locking clutch that is applied at cruise speed. This clutch, called a torque converter clutch, eliminates the slippage that occurs with a torque converter. The torque converter clutch is used as a fuel saving device, and to reduce the amount of heat generated in the transmission. When troubleshooting poor MPG issues, one culprit is a faulty torque converter, though poor MPG would also be present with shifting issues if the torque converter is the cause.
Forward speeds and reverse are provided by a gear set called the planetary gears. The planetary gear set consists of a central gear, called the sun gear, placed inside a large gear called an internal gear. Rotating between the internal gear and the sun gear, are small gears, held in a carrier, known as planetary gears. Different gear ratios are made possible by holding one component of the planetary gear set and allowing the other to rotate. For example, if the sun gear were held, the internal gear would be rotated by the planetary gears revolving around the motionless sun gear. This would cause the internal gear to rotate at low speed, while the planetary gears move much faster. This would provide a low gear function for the transmission, since the slow moving internal gear would be used to transmit power to the driving wheels.
An internal transmission oil pump is driven by the rotation of the torque converter. The oil pump pressurizes and circulates the transmission fluid used for the operation and lubrication of the transmission. The pressure created by the pump is often referred to as line pressure. Line pressure is utilized by the transmission to signal shift points and operate various transmission components.
Bands and clutches are used to hold the components of the planetary gear set to in order to provide different forward gear ratios or reverse. They are operated by line pressure that is directed to a specific band or clutch pack by the transmission shift control valves. The shift control valves operate by responding to changes in line pressure based upon the operation of input devices that signal road speed, throttle position, and engine load. Correcting shifting issues may not involve a costly auto repair job; it may be as simply as adjusting the bands.
The input devices used for transmission shift control are the governor, throttle valve, and vacuum modulator. The governor provides road speed information to the transmission to control shift points. It works by increasing line pressure as road speed increases. The throttle valve is connected by linkage to the throttle of the engine. The throttle valve modifies line pressure based on throttle position. This information is needed to vary shift points in response to driving conditions. When the throttle is moved to wide open, the throttle valve will cause send a line pressure signal to the control valves to delay shifting until higher road speed. The vacuum modulator changes shift feel in response to engine load. Since engine intake manifold vacuum changes in response to engine load, manifold vacuum is used as an input signal to the transmission. The vacuum modulator receives vacuum signal from the engine. The vacuum modulator will increase line pressure to stiffen transmission shifting based during heavy engine loads. The increased line pressure will cause clutches and bands to hold tighter and help to diminish slipping.
Most vehicles today are equipped with automatic transmissions that use electronic shift controls. The operation of the electronically controlled transmission is similar in principle to the non electric transmission. However, the electronically shifted transmission uses input signals from the vehicle control module to control shift points, rather than a governor and throttle valve. The vehicle control module controls transmission shifting based on engine and transmission data sensors. The throttle position sensor is used in place of the mechanical throttle valve. The vehicle speed sensor is used to replace the governor. Engine load sensors, such as a manifold pressure sensor, are used to control shift feel. A vacuum modulator may still be used by some vehicle makes to assist in shift control. The vehicle control module will utilize this information to operate various shift control solenoids inside the transmission. These shift control solenoids in turn control line pressure to their respective shift control valves that in turn apply or release pressure to bands or clutches.
The result of using electronic shift controls is an automatic transmission that operates more efficiently to tailor shifting to meet engine demands. Fuel economy and vehicle emission control are enhanced by more precise control of the automatic transmission. Vehicle control modules have the ability to adapt transmission shifting to meet the individual driving patterns of the vehicle. Also internal overheat protection is provided for by the control module’s ability to monitor transmission fluid temperature and change transmission shifting and operation to minimize temperature related damage. On many vehicles, the EEC (Electronic Engine Control) will “learn” a particular driver’s driving style. This information is stored in the EEC’s memory. If the battery is disconnected (during replacement, for example), the vehicle may shift erratically for a short time until the EEC re-learns the driver’s driving style and then reprograms itself. This is important to remember and a little patience can save you a trip to the auto repair shop, ie. after your battery is replaced, drive your vehicle for a day or so and see if the erratic shifting issue is resolved (most likely it will correct itself).
(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)