Main Components and Functions

Transmission, Automatic, 4R70W

Torque Converter

The torque converter (7902) is a four-element converter. It contains the standard three elements which transmit and multiply torque:

• Impeller
• Turbine
• Reactor
• Torque converter clutch

The torque converter clutch increases fuel economy in 3rd and 4th gears.

The operation of the torque converter components is as follows:

• Rotation of the converter housing and impeller set the fluid in motion.
• The turbine reacts to the fluid motion from the impeller, transferring rotation to the geartrain.
• The reactor redirects fluid going back into the impeller, allowing for torque multiplication.

The addition of a torque converter clutch to the torque converter provides a mechanical link between the converter housing and the engine, allowing for direct transfer of power from the engine to the geartrain.

The torque converter clutch is applied and released by fluid pressure, which is controlled by the powertrain control module (PCM) (12A650) through a torque converter clutch solenoid (TCC solenoid) (7G136) in the main control valve body (7A100).

Impeller and Cover

The primary functions of the impeller and cover assembly are to drive the impeller blades, contain hydraulic fluid and provide a mating surface for the converter clutch pressure plate. In addition, the impeller hub drives the fluid pump gear.

Turbine

The turbine is a sectioned, donut-shaped steel stamping that is splined to the input shaft. It is driven by fluid from the impeller and transmits this power to the geartrain. Because of its basic shape, the turbine automatically directs fluid back to the impeller.

Reactor

The reactor redirects the fluid flow returned to the impeller from the turbine so that the fluid moves in the same direction as engine/impeller rotation. This action aids in torque multiplication because it adds torque to that already supplied by the engine. The reactor contains a one-way clutch that holds it stationary when torque multiplication is necessary, but allows freewheeling at higher speeds when torque multiplication is not necessary.

Torque Converter Clutch

The pressure plate attaches to the back of the turbine and can be applied against the cover to allow for a mechanical transfer of engine torque to the geartrain. This occurs mainly in third and fourth gears when torque multiplication is no longer necessary. (Converter clutch application may also occur in second gear under certain conditions.)

The pressure plate works much like a regular mechanical clutch. Apply, release and controlled slip of the clutch are controlled by opposing fluid forces on each side of the pressure plate.

Torque Converter Sectional View

Geartrain

Power is transmitted from the torque converter to the Ravigneaux geartrain components through the input shaft and forward clutch cylinder.

• The geartrain contains a compound planetary set connected by dual pinion gears.
• By holding or driving certain members of the gearset, four forward ratios and one reverse ratio are obtained and transmitted to the output shaft (7060). The ratios are as follows.

Gear Ratios
1st gear      2.84 to 1
2nd gear      1.55 to 1
3rd gear      1.00 to 1
4th gear      0.70 to 1
Reverse      2.32 to 1

• Members of the gearset are held or driven by a series of bands or clutches.
• The 4R70W uses:
• Two bands.
• Two one-way roller clutches.
• Four friction clutches.

Planetary Gearset

The planetary gearset in the transmission is a Ravigneaux-type set consisting of the following components:

• Forward clutch sun gear.
• Reverse clutch sun gear.
• A pinion carrier.
• Long and short pinions.
• Output shaft ring gear.

Members are driven or held to produce four forward gear ratios plus one reverse ratio.

Input Shaft

The forward clutch cylinder and shaft (7F207) transfers speed and torque from the converter turbine to the geartrain. This shaft is splined to the turbine on one end and to the forward clutch sun gear and input shaft on the other end.

Stub Shaft

The input shaft transfers power from the turbine to the planetary carrier (through the direct clutch) during third and fourth gear operation.

Output Shaft

The output shaft provides torque to the propeller shaft and rear axle assembly (4200). It is driven by the ring gear (7A153) of the planetary gearset.

Forward Clutch Cylinder and Shaft

The forward clutch cylinder and shaft transfers speed and torque from the converter turbine to the geartrain. This shaft is splined to the turbine on one end and to the forward clutch sun gear and input shaft on the other end.

Apply Components

There are eight apply devices used to drive or hold the planetary gearset members. The following is a description of each apply component.

Overdrive Band

The overdrive band (7F196) holds the reverse clutch drum (7D044) stationary in fourth gear and in manual range second gear. This action causes the reverse sun gear to be held in these ranges.

Low and Reverse Band

The low and reverse band holds the pinion carrier stationary in reverse. It is also applied in 1st gear, manual 1 range, to provide engine braking.

Forward Clutch

The forward clutch couples the input shaft to the forward sun gear in 1st, 2nd and 3rd gears.

Direct Clutch

The direct clutch couples the input shaft to the planet carrier (through the stub shaft) in 3rd and 4th gears.

Reverse Clutch

The reverse clutch couples the input shaft to the reverse sun gear. It is applied in reverse range only.

Intermediate Clutch

The intermediate clutch works with the intermediate one-way clutch to hold the reverse sun gear stationary in 2nd gear. The intermediate clutch remains applied in 3rd and 4th gears, but does not transmit power.

Planetary (Low) One-Way Clutch

The planetary (low) one-way clutch is a roller clutch that holds the pinion carrier stationary in first gear, (D) and D ranges. During automatic coasting downshifts into first gear ((D) and D ranges), it freewheels so that there is no engine braking.

Intermediate One-Way Clutch

The intermediate one-way clutch is a roller clutch that works with the intermediate friction clutch to hold the reverse sun gear stationary in second gear during acceleration. This one-way clutch freewheels in 3rd gear and during coasting in 2nd gear, (D) and D ranges.

Hydraulic System

4R70W Hydraulic Components

Pump

The transmission uses a gerotor design front pump support and gear (7A103). The pump is more efficient at lower engine speeds. It is also made of aluminum. The pump provides the volume of fluid needed to charge the torque converter, main control assembly, cooling system and lube system. Pump pressure is regulated by the main regulator valve. The pump also has an internal boost circuit.

Filter

All fluid drawn from the pan by the pump passes through the filter. The filter and its accompanying seal are part of the fluid path from the sump (pan) to the fluid pump.

Main Control

The main control valve body houses four electronic solenoids:

• Two shift solenoids (7G484).
• Torque converter clutch solenoid.
• Electronic pressure control (EPC) solenoid.

Accumulators

1-2 Accumulator — The 1-2 accumulator (in the case) is used to soften the 1-2 shift by absorbing some of the pressure directed to the intermediate clutch. Constant line pressure is applied to the middle section of the 1-2 accumulator piston (7F251), opposing the intermediate clutch pressure until this pressure is enough to overcome line pressure. The top of the piston is exhausted to the sump.

2-3 Accumulator — The 2-3 accumulator is used to soften the 2-3 shift by absorbing some of the direct clutch pressure. Forward clutch pressure is applied to the top side of the accumulator piston, holding the piston down until direct clutch pressure is enough to overcome it. The middle section of the accumulator is exhausted to the sump. The 2-3 accumulator also helps control forward engagement.