The torque converter housing is rotated by the engine's
crankshaft, and turns the impeller. The impeller then spins the turbine,
which gives motion to the turbine shaft, driving the gears
The automatic transmission allows engine torque and power to be transmitted to the rear wheels within a narrow range of engine operating speeds. It will allow the engine to turn fast enough to produce plenty of power and torque at very low speeds, while keeping it at a sensible rpm at high vehicle speeds (and it does this job without driver assistance). The transmission uses a light fluid as the medium for the transmission of power. This fluid also works in the operation of various hydraulic control circuits and as a lubricant. Because the transmission fluid performs all of these functions, trouble within the unit can easily travel from one part to another. For this reason, and because of the complexity and unusual operating principles of the transmission, a very sound understanding of the basic principles of operation will simplify troubleshooting.
The torque converter replaces the conventional clutch. It has three functions:
The torque converter is a metal case which is shaped like a sphere that has been flattened on opposite sides. It is bolted to the rear end 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. This set forms the torus or pump. Another set is directly connected to the output shaft, and forms the turbine. The third set is mounted on a hub which, in turn, is mounted on a stationary shaft through a one-way clutch. This third set is known as the stator.
A 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.
Under low speed acceleration, the torque converter functions as follows:
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The torque converter housing is rotated by the engine's
crankshaft, and turns the impeller. The impeller then spins the turbine,
which gives motion to the turbine shaft, driving the gears
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The torus is turning faster than the turbine. It picks up fluid at the center of the converter and, through centrifugal force, slings it outward. Since the outer edge of the converter moves faster than the portions at the center, the fluid picks up speed.
The fluid then enters the outer edge of the turbine blades. It then travels back toward the center of the converter case along the turbine blades. In impinging upon the turbine blades, the fluid loses the energy picked up in the torus.
If the fluid was now returned directly into the torus, both halves of the converter would have to turn at approximately the same speed at all times, and torque input and output would both be the same.
In flowing through the torus and turbine, the fluid picks up two types of flow, or flow 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 torus, 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 torus, but is already spinning when the torus picks it up. This has the effect of allowing the torus 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. As a result, 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 in the direction of engine rotation. Under these conditions, the torque converter begins to behave almost like a solid shaft, with the torus and turbine speeds being almost equal.
The ability of the torque converter to multiply engine torque is limited. Also, the unit tends to be more efficient when the turbine is rotating at relatively high speeds. Therefore, a planetary gearbox is used to carry the power output of the turbine to the driveshaft.
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Planetary gears work in a similar fashion to manual
transmission gears, but are composed of three
parts |
Planetary gears function very similarly to conventional transmission gears. However, their construction is different in that three elements make up one gear system, and, in that all 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; and a set of three planet gears, held by pins in a ring-like planet carrier, 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. Also, if all three gears are forced to rotate at the same speed, the gearset forms, in effect, a solid shaft.
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Planetary gears in the maximum reduction (low) range.
The ring gear is held and a lower gear ratio is
obtained |
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Planetary gears in the minimum reduction (drive) range.
The ring gear is allowed to revolve, providing a higher gear
ratio |
Most automatics use the planetary gears to provide various reductions ratios. Bands and clutches are used to hold various portions of the gearsets to the transmission case or to the shaft on which they are mounted. Shifting is accomplished, then, by changing the portion of each planetary gearset which is held to the transmission case or to the shaft.
The servos are hydraulic pistons and cylinders. They resemble the hydraulic actuators used on many other machines, such as bulldozers. Hydraulic fluid enters the cylinder, under pressure, and forces the piston to move to engage the band or clutches.
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Servos, operated by pressure, are used to apply or
release the bands, to either hold the ring gear or allow it to
rotate |
The accumulators are used to cushion the engagement of the servos. The transmission fluid must pass through the accumulator on the way to the servo. The accumulator housing contains a thin piston which is sprung away from the discharge passage of the accumulator. When fluid passes through the accumulator on the way to the servo, it must move the piston against spring pressure, and this action smoothes out the action of the servo.
The hydraulic pressure used to operate the servos comes from the main transmission oil pump. This fluid is channeled to the various servos through the shift valves. There is generally a manual shift valve which is operated by the transmission selector lever and an automatic shift valve for each automatic upshift the transmission provides.
Many new transmissions are electronically controlled. On these models, electrical solenoids are used to better control the hydraulic fluid. Usually, the solenoids are regulated by an electronic control module.
There are two pressures which affect the operation of these valves. One is the governor pressure which is effected by vehicle speed. The other is the modulator pressure which is effected 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.
Most transmissions also make use of an auxiliary circuit for downshifting. This circuit may be actuated by the throttle linkage the vacuum line which actuates the modulator, by a cable or by a solenoid. It applies pressure to a special downshift surface on the shift valve or valves.
The transmission modulator also governs the 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.
Refer to Section 1 for transmission pan removal and filter service.
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Unfasten the nut for the manual control lever and ...
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... remove the lever from the
sensor |
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Detach the connector from the TR
sensor |
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Unfasten the retaining bolts and ...
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... remove the sensor from the
transmission |
To install:
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Adjusting the MLP sensor |
Park is the last detent when the manual control lever is full forward. Return 2 detents toward the output shaft for Neutral.
The extension housing seal is located at the rear of the transmission case.
To install:
The extension housing seal on 4WD drive vehicles is located at the rear of the transfer case. Refer to the Rear Output Shaft Seal procedure in the Transfer Case portion of this section.
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On 4.6L engine equipped models, remove the rubber
access plug to ... |
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... access and remove the converter-to-flywheel
attaching nuts |
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Disconnect the shift linkage |
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Disconnect each fluid cooler line from the fittings on
the transmission |
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Detach the harness connectors from the
transmission |
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Detach the solenoid body connector
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Remove the fuel line bracket retaining bolt
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Remove the bolt that secures the transmission fluid
filler tube to the cylinder and remove the dipstick from the
transmission |
To install:
Align the orange balancing marks on the converter stud and flywheel bolt hole if balancing marks are present.
Before torque converter is bolted to the flywheel, a check should be made to ensure that the torque converter is properly seated. The torque converter should move freely with respect to the flywheel. Grasp the torque converter stud. Movement back and forth should result in a metallic clank noise if the converter is properly seated. If the torque converter will not move, the transmission must be removed and the torque converter re-position so that the impeller hub is properly engage in the pump gear.
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Disconnect the oil cooler lines from the fittings on
the transmission |
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Detach the manual lever position sensor
connector |
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Detach the solenoid body
connector |
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Detach the turbine shaft and output shaft speed
sensors |
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Remove the flywheel inspection cover retaining
bolts—gasoline engines |
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Remove the flywheel inspection cover retaining
bolts—diesel engines |
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Remove the flywheel retaining
nuts |
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Lower the transmission from the vehicle using a
suitable jack |
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Install Torque Converter Handles T81P-7902-C, or
equivalent, at the 12 o'clock and 6 o'clock
positions |
To install:
The automatic transmissions are electronically controlled, no adjustments are possible.