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Automatic Transmission and Transaxle, Page 2 of 3

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Automatic transmission components

Torque converter
See Figures 5 and 6

The front section is called the torque converter. In replacing the traditional clutch, it performs three functions:

  • It acts as a hydraulic clutch (fluid coupling), allowing the engine to idle even with the transmission in gear.
  • It allows the transmission to shift from gear to gear smoothly, without requiring that the driver close the throttle during the shift.
  • It multiplies engine torque making the transmission more responsive and reducing the amount of shifting required.

The torque converter is a metal case that is shaped like a sphere that has been flattened on opposite sides and is bolted to the rear of the engine's crankshaft. Generally, the entire metal case rotates at engine speed and serves as the engine's flywheel.

The case contains three sets of blades. One set is attached directly to the case forming the impeller or pump. Another set is directly connected to the output shaft, and forms the turbine. The third set (stator) is mounted on a hub which, in turn, is mounted on a stationary shaft through a one-way clutch. Rollers are wedged into slots, preventing backward rotation. When the rollers are not in the slots, the stator turns in the same direction as the impeller. The pump, which is driven by the converter hub at engine speed, keeps the torque converter full of transmission fluid at all times. Fluid flows continuously through the unit to provide cooling.

A fluid coupling will only transmit the torque the engine develops; it cannot increase the torque. This is one job of the torque converter. The impeller drive member is driven at engine speed by the engine's crankshaft and pumps fluid, to its center, which is flung outward by centrifugal force as it turns. Since the outer edge of the converter spins faster than the center, the fluid gains speed. Fluid is directed toward the turbine driven member by curved impeller blades, causing the turbine to rotate in the same direction as the impeller. The turbine blades are curved in the opposite direction of the impeller blades.

In flowing through the pump and turbine, the fluid flows in two separate directions. It flows through the turbine blades, and it spins with the engine. The stator, whose blades are stationary when the vehicle is being accelerated at low speeds, converts one type of flow into another. Instead of allowing the fluid to flow straight back into the pump, the stator's curved blades turn the fluid almost 90° toward the direction of rotation of the engine. Thus the fluid does not flow as fast toward the pump, but is already spinning when the pump picks it up. This has the effect of allowing the pump to turn much faster than the turbine. This difference in speed may be compared to the difference in speed between the smaller and larger gears in any gear train. The result is that engine power output is higher, and engine torque is multiplied.

As the speed of the turbine increases, the fluid spins faster and faster in the direction of engine rotation. Therefore, the ability of the stator to redirect the fluid flow is reduced. Under cruising conditions, the stator is eventually forced to rotate on its one-way clutch and the torque converter begins to behave almost like a solid shaft, with the pump and turbine speeds being almost equal.

In the late 70's, Chrysler Corporation introduced an automatic transmission, featuring what is called a "lock-up" clutch in the transmission's torque converter. The lock-up is a fully automatic clutch that engages only when the transmission shifts into top gear or when needed based on a predetermined demand factor.

The lock-up clutch is activated by a piston. When engaged, the lock-up clutch gives the benefits of a manual transmission, eliminating torque converter slippage. In the engaged position, engine torque is delivered mechanically, rather than hydrodynamically (through fluid). This gives improved fuel economy and cooler transmission operating temperatures.

In the early 80's, Ford introduced what is known as the Automatic Overdrive Transmission (AOT). Essentially, this transmission uses a lock-up torque converter, by offering an additional refinement. The transmission is a four-speed unit, with fourth gear as an overdrive (0.67:1). Torque is transmitted via a full mechanical lock-up from the engine, completely bypassing the torque converter and eliminating hydraulic slippage.

In third gear (1:1 ratio), engine power follows a "split-torque" path, in which there is a 60% lock-up. Sixty percent of the power is transmitted through solid connections and 40% of the engine power is delivered through the torque converter.

Throughout the 90's, Subaru introduced an Electronic Continuously Variable Transmission (ECVT) and Honda introduced their version (CVT). This transmission uses a metal belt and two variable-diameter pulleys to keep smooth, uninterrupted range of gearing. Size of the pulleys is controlled through the use of hydraulics. This unit produces a miles per gallon closer to a manual transmission, while attaining a smoother shift than an automatic transmission.

Figure 5 The torque converter housing is rotated by the engine crankshaft and turns the impeller. The impeller spins the turbine, which gives motion to the turbine (output) shaft to drive the gears.
 The torque converter housing is rotated by the engine crankshaft and turns the impeller. The impeller spins the turbine, which gives motion to the turbine (output) shaft to drive the gears.

Figure 6 Sectional view of operations of a typical "lock-up" clutch and torque converter.
Click on picture to enlarge view

Torque converter clutch control
See Figures 7 and 8

Electrical and vacuum controls

The torque converter clutch should apply when the engine has reached near normal operating temperature in order to handle the slight extra load and when the vehicle speed is high enough to allow the operation of the clutch to be smooth and the vehicle to be free of engine pulses.

When the converter clutch is coupled to the engine, the engine pulses can be felt through the vehicle in the same manner as if equipped with a clutch and standard transmission. Engine condition, engine load and engine speed determine the severity of the pulsation.

The converter clutch should release when torque multiplication is needed in the converter, when coming to a stop, or when the mechanical connection would affect exhaust emissions during a coasting condition.

The typical electrical control components consist of the brake release switch, the low vacuum switch and the governor switch. Some vehicle models have a thermal vacuum switch, a relay valve and a delay valve. Diesel engines use a high vacuum switch in addition to certain above listed components. These various components control the flow of current to the apply valve solenoid. By controlling the current flow, these components activate or deactivate the solenoid, which in turn engages or disengages the transmission converter clutch, depending upon the driving conditions as mentioned previously. The components have the two basic circuits, electrical and vacuum.

Figure 7 Using electrical and vacuum controls to operate the torque converter clutch.
Using electrical and vacuum controls to operate the torque converter clutch.

Figure 8 Typical diesel engine vacuum and electrical schematic for the torque converter clutch.
Typical diesel engine vacuum and electrical schematic for the torque converter clutch.

Electrical current flow

All of the components in the electrical circuit must be closed or grounded before the solenoid can open the hydraulic circuit to engage the converter clutch. The circuit begins at the fuse panel and flows to the brake switch and as long as the brake pedal is not depressed, the current will flow to the low vacuum switch on the gasoline engines and to the high vacuum switch on the diesel engines. These two switches open or close the circuit path to the solenoid, dependent upon the engine or pump vacuum. If the low vacuum switch is closed (high vacuum switch on diesel engines), the current continues to flow to the transmission case connector, into the solenoid and to the governor pressure switch. When the vehicle speed is approximately 35-50 mph (56-80 kph) , the governor switch grounds to activate the solenoid. The solenoid, in turn, opens a hydraulic circuit to the converter clutch assembly, engaging the unit.

It should be noted that external vacuum controls include the thermal vacuum valve, the relay valve, the delay valve, the low vacuum switch and a high vacuum switch (used on diesel engines). Keep in mind that all of the electrical or vacuum components may not be used on all engines at the same time.

Vacuum flow

The vacuum relay valve works with the thermal vacuum valve to keep engine vacuum from reaching the low vacuum valve switch at low engine temperatures. This action prevents the clutch from engaging while the engine is still warming up. The delay valve slows the response of the low vacuum switch to changes in engine vacuum. This action prevents the low vacuum switch from causing the converter clutch to engage and disengage too rapidly. The low vacuum switch deactivates the converter clutch when engine vacuum drops to a specific low level during moderate acceleration just before a part-throttle transmission downshift. The low vacuum switch also deactivates the clutch while the vehicle is coasting because it receives no vacuum from its ported vacuum source.

The high vacuum switch, when on diesel engines, deactivates the converter clutch while the vehicle is coasting. The low vacuum switch on the diesel models deactivates the converter clutch only during moderate acceleration, just prior to a part-throttle downshift. Because the diesel engine's vacuum source is a rotary pump, rather than taken from a carburetor port, diesel models require both the high and the low vacuum switch to achieve the same results as the low vacuum switch on the gasoline models.

Computer controlled converter clutch
See Figure 9

With the use of microcomputers governing the engine fuel and spark delivery, most manufacturers change the converter clutch electronic control to provide the grounding circuit for the solenoid valve through the microcomputer, rather than the governor pressure switch. Sensors are used in place of the formerly used switches and send signals back to the microcomputer to indicate if the engine is in its proper mode to accept the mechanical lock-up of the converter clutch.

Normally a coolant sensor, a throttle position sensor, an engine vacuum sensor and a vehicle speed sensor are used to signal the microcomputer when the converter clutch can be applied. Should a sensor indicate the need for the converter clutch to be deactivated, the grounding circuit to the transmission solenoid valve would be interrupted and the converter clutch would be released.

Figure 9 Typical computer controlled clutch.
Typical computer controlled clutch.

Hydraulic converter clutch

Numerous automatic transmissions rely upon hydraulic pressures to sense and determine when to apply the converter clutch assembly. This type of automatic transmission unit is considered to be a self-contained unit with only the shift linkage, throttle cable or modulator valve being external. Specific valves, located within the valve body or oil pump housing, are caused to be moved when a sequence of events occur within the unit. For example, to engage the converter clutch, most all automatic transmissions require the gear ratio to be in the top gear before the converter clutch control valves can be placed in operation. The governor and throttle pressures must maintain specific fluid pressures at various points within the hydraulic circuits to aid in the engagement or disengagement of the converter clutch. In addition, check valves must properly seal and move to exhaust pressured fluid at the correct time to avoid "shudders" or "chuggles" during the initial application and engagement of the converter clutch.

Centrifugal torque converter clutch
See Figure 10

A torque converter was used that locks up mechanically without the use of electronics or hydraulic pressure. At specific input shaft speeds, brake-like shoes move outward from the rim of the turbine assembly, to engage the converter housing, locking the converter unit mechanically together for a 1:1 ratio. Slight slippage can occur at the low end of the rpm scale, but the greater the rpm, the tighter the lock-up. Again, it must be mentioned, that when the converter has locked-up, the vehicle may respond in the same manner as driving with a clutch and standard transmission. This is considered normal and does not indicate converter clutch or transmission problems. Keep in mind if engines are in need of tune-ups or repairs, the lock-up "shudder" or "chuggle" feeling may be greater.

Figure 10 Exploded view of the centrifugal lock-up converter. Click on picture to enlarge view

Mechanical converter lock-up

Another type of converter lock-up is the Ford Motor Company's AOD Automatic Overdrive transmission, which uses a direct drive input shaft splined to the damper assembly of the torque converter cover to the direct clutch, bypassing the torque converter reduction components. A second shaft encloses the direct drive input shaft and is coupled between the converter turbine and the reverse clutch or forward clutch, depending upon their applied phase. With this type of unit, when in third gear, the input shaft torque is split, 30% hydraulic and 70% mechanical. When in the overdrive or fourth gear, the input torque is completely mechanical and the transmission is locked mechanically to the engine.

Overdrive units
See Figure 11

When the need for greater fuel economy stirred the world's automakers into action, the automatic transmission/transaxles were among the many vehicle components that were modified to aid in this quest. Internal changes have been made and in some cases, additions of a fourth gear to provide the over direct or overdrive gear ratio. The reasoning for adding the overdrive capability is that an overdrive ratio enables the output speed of the transmission/transaxle to be greater than the input speed, allowing the vehicle to maintain a given road speed with less engine speed. This results in better fuel economy and a slower running engine.

The overdrive unit usually consists of an overdrive planetary gear set, a roller one-way clutch assembly and two friction clutch assemblies, one as an internal clutch pack and the second for a brake clutch pack. The overdrive carrier is splined to the turbine shaft, which in turn, is splined into the converter turbine.

Another type of overdrive assembly is a separation of the overdrive components by having them at various points along the gear train assembly and utilizing them for other gear ranges. Instead of having a brake clutch pack, an overdrive band is used to lock the planetary sun gear. In this type of transmission, the converter cover drives the direct drive shaft clockwise at engine speed, which in turn drives the direct clutch. The direct clutch then drives the planetary carrier assembly at engine speed in a clockwise direction. The pinion gears of the planetary gear assembly "walk around" the stationary reverse sun gear, again in a clockwise rotation. The ring gear and output shafts are therefore driven at a faster speed by the rotation of the planetary pinions. Because the input is 100% mechanical drive, the converter can be classified as a lock-up converter in the overdrive position.

Figure 11 Exploded and sectional views of direct drive and overdrive power flows.
Click on picture to enlarge view

The planetary gearbox
See Figures 12, 13 and 14

The rear section of the transmission is the gearbox, containing the gear train and valve body to shift the gears.

The ability of the torque converter to multiply engine torque is limited, so the unit tends to be more efficient when the turbine is rotating at relatively high speeds. A planetary gearbox is used to carry the power output from the turbine to the driveshaft to make the most efficient use of the converter.

Planetary gears function very similarly to conventional transmission gears. Their construction is different in that three elements make up one gear system, and in that the three elements are different from one another. The three elements are:

  • An outer gear that is shaped like a hoop, with teeth cut into the inner surface.
  • A sun gear mounted on a shaft and located at the very center of the outer gear.
  • A set of three planet gears, held by pins in a ring-like planet carrier and meshing with both the sun gear and the outer gear.

Either the outer gear or the sun gear may be held stationary, providing more than one possible torque multiplication factor for each set of gears. If all three gears are forced to rotate at the same speed, the gear set forms, in effect, a solid shaft.

Bands and clutches are used to hold various portions of the gear-sets to the transmission case or to the shaft on which they are mounted.

Figure 12 Planetary gears are similar to manual transmission gears, but are composed of three parts.
Planetary gears are similar to manual transmission gears.

Figure 13 Planetary gears in maximum reduction (low). The ring gear is held and a lower gear ratio is obtained.
Planetary gears in maximum Low.

Figure 14 Planetary gears in the minimum reduction (Drive). The ring gear is allowed to revolve, providing a higher gear ratio.
Planetary gears in the minimum reduction (Drive). The ring gear is allowed to revolve, providing a higher gear ratio.

Shifting gears
See Figures 15 and 16

Shifting is accomplished by changing the portion of each planetary gear set that is held to the transmission case or shaft.

A valve body contains small hydraulic pistons and cylinders. Fluid enters the cylinder under pressure and forces the pistons to move to engage the bands or clutches.

The hydraulic fluid used to operate the valve body comes from the main transmission oil pump. This fluid is channeled to the various pistons through the shift valves. There is generally a manual shift valve that is operated by the transmission selector lever and an automatic shift valve for each automatic upshift the transmission provides. Two-speed automatics have a low-high shift valve; while three-speeds will have a 1-2 shift valve, and a 2-3 shift valve; whereas four-speeds have a 1-2 shift valve, a 2-3 shift valve, and a 3-4 shift valve.

Two pressures affect the operation of these valves. One (governor pressure) is determined by vehicle speed, while the other (modulator pressure) is determined by intake manifold vacuum or throttle position. Governor pressure rises with an increase in vehicle speed, and modulator pressure rises as the throttle is opened wider. By responding to these two pressures, the shift valves cause the upshift points to be delayed with increased throttle opening to make the best use of the engine's power output. If the accelerator is pushed further to the floor the upshift will be delayed longer, (the vehicle will stay in gear).

The transmission modulator also governs line pressure, used to actuate the servos. In this way, the clutches and bands will be actuated with a force matching the torque output of the engine.

Most transmissions also make use of an auxiliary circuit for downshifting. This circuit may be actuated by the throttle linkage or the vacuum line that actuates the modulator or by a cable or solenoid. It applies pressure to a special downshift surface on the shift valve or valves, to shift back to low gear as vehicle speed decreases.

Figure 15 Servos, operated by pressure, are used to apply or release the bands, either holding the ring gear or allowing it to rotate.
Servos, operated by pressure are used to apply or release the bands, to either hold the ring gear or allow it to rotate.

Figure 16 The valve body, containing the shift valves, is normally located at the bottom of the transmission. The shift valves (there are many more than shown) are operated by hydraulic pressure.
The valve body, containing the shift valves, is normally located at the bottom of the transmission. The shift valves (there are many more than shown) are operated by hydraulic pressure.

Transaxles

When the transmission and the drive axle are combined in one unit, it is called a "transaxle." The transaxle is bolted to the engine and has the advantage of being an extremely rigid unit of engine and driveline components. The complete engine transaxle unit may be located at the front of the vehicle (front wheel drive) or at the rear of the vehicle (rear wheel drive).

The power flow through the transmission section of the transaxle is the same as through a conventional transmission.

Continue to page 3 of Automatic transmission/transaxle

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©1998 W. G. Nichols - Chilton's Easy Car Care