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Automatic Transmissions are designed to react quickly to road conditions by shifting into different gears as required, saving power by not shifting into too high of a gear.
Hydraulic pressure provides pressure for shifting valves to engage clutches and bands that activate each gear, while your transmission’s governor notifies it when your car’s speed has changed, opening or closing shift valves proportionately with that pressure range.
The Hydraulic System
An Automatic Transmission’s hydraulic system includes a gear pump which supplies fluid to its transmission cooler and torque converter, as well as generate power that is transferred via its turbine to the transmission’s first planetary gear set.
A gear pump pulls its fluid from a sump located in the transmission, with gears rotating at equal speed as the engine, to enable smooth operation over an array of ratios.
In the fourth embodiment, a switching line-pressure passage connects with port 44e of a first switching valve 44 to provide an opposing pressure to counteract reaction force of return spring 44g and allow first switching valve 44 to operate at hydraulic pressure lower than that at which shift valves 41, 42 begin moving by spring reaction force, thus preventing interlock between first switching valve 44 and shifting mechanism 24.
Furthermore, high-clutch pressure supplied through the interlock preventing passage 103 is provided to port 44d of the first switching valve to interrupt bypass passage 45 and thus ensure that in the event that its malfunction causes it to connect directly with low clutch L/C at fourth speed, hydraulic pressure cannot flow to it and cause mechanical lock-ups.
This can be accomplished with the first switching device issuing a command to set output hydraulic pressure of the assist mechanism at its maximum value for a specified period following engine restart, thus providing smooth supply of hydraulic pressure even if vehicle starts at full throttle shortly after engine restart after idle stop and preventing select shock or similar events during normal control.
In the first switching valve is supported by a pressure modifying valve that can be adjusted to increase hydraulic pressure for biasing the return spring 44g without creating an entirely new constitution. In the third embodiment, the duty ratio of line-pressure duty solenoid 70 is set to its maximum command value MAX so that line pressure exceeds set pressure Pset during engine restart after idle stop, permitting advanced engagement of low clutch L/C at engine restart after idle stop with complete engagement occurring after complete engine explosion to eliminate start shock for vehicle start up.
The Valve Body
Valve bodies are essential components of an automatic transmission. Should one fail, your vehicle could no longer shift gears appropriately or at all – making driving challenging and potentially uncomfortable.
It plays a critical role in the overall control system that manages how your automatic transmission responds to throttle pedal pressure and shifts gears at optimal RPM to increase fuel efficiency. The valve body utilizes channels that transport hydraulic fluid directly to valves which then activate clutch packs or band servos according to driving scenarios, so gear changes occur smoothly and efficiently.
Modern automatic transmission valve bodies contain sensors to monitor speed, engine load and brake pedal position for seamless gear changes. Once this information has been sent to an engine management computer it directs an electrical solenoid to distribute hydraulic fluid to each clutch pack at just the right moment.
As soon as you notice an issue with the valve body, it should be addressed as quickly as possible to avoid more costly repair bills down the line. A failing valve body could cause your vehicle to suddenly “snap” when shifting gears causing your car to either jerk forward or slip, creating costly repair bills over time. For this reason it is wise to seek professional advice as soon as you become aware of this possible problem in order to protect yourself from it and ensure future success of its maintenance costs are kept to an absolute minimum.
Another telltale sign of a worn valve body is making clunking or knocking noises when accelerating, slowing down, or changing gears – this should serve as an early indicator that the valve body requires repair or replacement.
Many don’t realize the valve body is an integral component of an automatic transmission, yet it plays a pivotal role in how your vehicle shifts gears. Unfortunately, valve bodies often represent the first point of failure for these transmissions and should be monitored closely if your transmission stops working normally. If that occurs, symptoms associated with its malfunction must also be watched for.
Your decision on rebuild or replacement for a transmission valve body depends on a range of factors, such as its damage severity and availability of necessary tools to work on it. What ultimately works for both yourself and your customers will depend on what’s discovered when disassembly begins.
The Clutch
Clutches in automatic transmissions are mechanical devices used to engage and disengage power from an engine to driving wheels. Clutches can be found in many applications, from automobiles and electric drills to bicycles; typically these clutches allow machines’ shafts to spin at either an even speed, slip a bit slower, or disengage completely (spinning at different speeds).
A clutch is comprised of several components, including a flywheel, pressure plate, disc and throw-out bearing – along with its release system – all bolted to its flywheel and operated via lever, cable or hydraulic piston.
Autobutler reports that in a standard clutch system, a pressure plate is attached to the flywheel while friction discs sit either side. A diaphragm spring also helps allow contact between pressure plate and flywheel to form and break away as necessary.
When your foot is not on the clutch pedal, the pressure plate pushes up against the clutch disc which then presses down against the flywheel. For maximum traction against this tough material, clutch discs feature high coefficient of friction materials in their composition to allow a good hold against its surfaces.
Friction between the clutch disc and flywheel ensures engine rotation and wheel shaft speed are in sync, with both moving at equal rates. Once released by releasing clutch pedal, release bearings pull away pressure plate from clutch disc to break its connection with transmission input shaft and thus disconnecting flywheel and transmission input shaft connections.
An energy release clutch, or spring applied clutch, involves inserting an armature near to a pressure plate with engagement springs between it and the pressure plate, and acting upon this energy via transmission to squeeze and disengage its springs, disengaging its device.
An improperly maintained clutch can fail and result in slippage – this means it doesn’t firmly clamp against the flywheel as expected, leading to laggy acceleration, jerky gear changes and inconsistent RPMs while traveling at steady speed. Slippage should always be your warning sign when something’s amiss with your clutch! Slippage could mean it is losing grip with its intended target – which could result in laggy acceleration, jerky gear changes or inconsistent RPMs while driving at constant speed.
The Computer
Automatic Transmissions are complex computerized systems designed to make driving your vehicle as reliable and cost-efficient as possible. Each vehicle’s transmission control module (TCM) collects sensor information to determine when gear shifts should take place.
Most often, this component is located near your engine control module (ECM). Together with ECM, TCM ensures all parts of your car work optimally.
This part of your transmission takes in signals from various sensors, such as speed sensors, throttle position sensors, manifold vacuum sensors and shift lever position sensors, among others. Using this data to make decisions regarding when and how your gears should shift will determine when is best to change them.
Checking fluid temperature ensures your automatic transmission fluid is at an optimum level, protecting both converter clutches from being overheated or shifting slippage during shifting.
If any of the signals from sensors fall outside the accepted range, the control module will set a code and raise an alert. Depending on the severity of any discrepancies, power could be reduced or transmission put into a safe mode to protect wear components.
If this is an issue with your transmission, reflashing can bring balance back into the computer system and restore accuracy and performance levels previously seen in your transmission. Once reflashed, your transmission should perform more precisely and precisely than ever before.