1. Automatic Transmissions

2. Job
Torque multiplication/speed reduction OR Speed multiplication/torque reduction.
To increase torque when needed – such as accelerating from a stand still. Speed isn’t needed.
Increased speed when needed – such as upshifting at a cruising speed. Torque isn’t needed, but rpm is.
*** To keep the engine operating in its peak RPM range***.

3. How does it work?
Engine torque is transferred to the transmission through the torque converter – basically a fluid coupler.
Gear shifts are done by use of a planetary gearsets.
Holding or control of the planetary gearset is done by clutches, bands, and/or sprag clutches (these are hydraulic actuators – they do an action).
Control of the holding units is by the valve body (the brain of the transmission)
Throttle pressure, output shaft speed, and gear selection are 3 major input which help determine shift times.
The pump sends fluid under pressure to the valve body, which in turn, sends it to the actuators.

4. Map of fluid and power flow

5. Torque Converter
Its job is the same as the clutch in a manual transmission;
It will transmit power from the engine to the transmission.
It will engage when the driver needs
It will disengage when unloaded (sitting at a stop light while in gear)
It will also multiple torque applied to the transmission – up to 2 x the input torque.

6. Torque Converter Operation
Originally a fluid coupler
Impeller or pump is connected to the crankshaft. It spins at engine RPM.
The Turbine is splined to the input shaft of the transmission. It is driven by fluid pumped off the impeller.
In this picture the impeller is on the right side and is welded to the inside of the converter housing – this is very common.

7. Torque Converter Operation
Think of it as 2 fans facing each other.
One is plugged in and turning. The other is being turned by the drive fan. The media used to transfer rotational motion is air.
For many reasons this is efficient.
Drive fan
Driven fan
Air is the media used to transfer energy. In this situation it is very inefficient

8. Torque converter – cont.
Fluid movement is done radially. As the driving fan turns (impeller) it pushes the fluid out due to centrifugal force. Note arrows.
The fluid transfers to the driven fan (turbine) at the outside edge of the converter.
It’s like 2 fans facing each other with only 1 driving.

9. Torque Converter - Stator
A fluid coupler is about 80% efficient. One thing that differentiated a torque converter from a fluid coupler is a stator
To help control the re-direction of fluid flow, a stator is introduced. It redirects fluid flow and helps “supercharge” the fluid – thus increasing torque.

10. Torque Converter - Stator
The stator splines to a stator support. This allows the one-way clutch to lock and force fluid back into the impeller (accelerating) and freewheels when the turbine is running faster than the impeller (decelerating)

11. LOCK UP CONVERTERS Torque Converter Clutch is splined to the turbine
Torque converter clutch is used to lock the turbine to the housing of the torque converter
A TCC solenoid located in the valve body determines whether or not to send apply fluid to the torque converter clutch apply piston (between turbine and the clutch) forcing the clutch against the converter housing
Fluid sent through the turbine shaft to torque converter clutch
see visual = page 12
This results is turbine speed = impeller speed
Eliminates slippage all together which increases fuel economy and helps ATF life
Lock up conditions must be present

12. STALL TORQUE CONVERTERS
Usually higher stall speeds used with performance engines/transmissions
Converter is matched to the powerband of the engine & transmission
Predetermined speed/rpm in which the turbine will turn (fluid has to be traveling a certain speed)
Vanes will be different in stall converters

13. Planetary Gearset
Can be used to obtain reduction, overdrive, and direct drive gear ratios.
Parts: Sun gear, planetary gears, planetary carrier, ring/annulus gear.

14. But how does it work?
3 members are always in constant mesh. One always drives, one is always driven, and one is held. These can be held with a clutch, band, or sprag/one way clutch.
See videos
Pass parts.
The key for proper operation is each must drive, be driven, or be held. If you fail to have any of these, the unit will be locked and output = input.

15. How are they connected to these holding units?
Each member of the planetary is splined to something. It will then turn or be held.

16. Holding units
These are the items that work with the planetary assembly.
Clutches
Bands
Sprag clutch or one-way clutch
Cutaway of sprag and hub
Clutch discs and steel separator plates

17. Friction Clutches
Each clutch drum will hold 3 or more wet clutch discs separated buy steel plates.
The more clutch discs, the more surface area for holding the planetary member.
More clutch plates = more rotating mass = more hp drag, but stronger transmission.
This picture shows the front clutch drum, piston inside, clutches and steel separators, pressure plates, and retaining clip.

18. Band
Uses the same friction material as the clutches (organic fibers, steel, some use fiberglas and kevlar)
Basically a steel backed belt wrapped around a drum that is splined to the planetary parts.
A hydraulic servo tightens and releases it.

19. Parts of the Intermediate and Kickdown bands for the 727

20. Transmission Fluid Pumps
Operates off the neck of the converter and runs at engine speed. If the engine is running, so is the transmission fluid pump.
Most transmissions only have 1 pump off the front of the transmission – which is why an automotive transmission can not be push started.
There are 3 different kinds of pumps, but they all work similarly

21. FLUID FLOW TRAVEL Transmission fluid is stored in the transmission pan
Low pressure created by the transmission fluid pump draws ATF through the filter through a portion of the valve body which leads to the pump
The pump then pressurizes the fluid and sends it back out to the valve body
The valve body takes fluid under pressure and distributes it to individual clutches and bands

22. Types of Transmission Fluid Pumps
Rotor Pump: uses an inner rotor and an outer rotor which separates a low pressure area from the high pressure area
Low pressure = high volume
High pressure = low volume
Vane Style Pump: output is altered by moving the outer vane cylinder
As the inner rotor and the vanes rotate, the area between the vanes increases and fluid volume is positively displaced creating a vacuum at the intake port.
Internal/External Pump: uses an internal gear with a crescent or half-moon inside of an outer gear to separate the suction cavity from the pressure cavity
Suction cavity= low pressure, high volume
Pressure cavity= high pressure, low volume

23. Valve Body The brain of the system
Inputs: pump, governor, throttle valve, and gear shift
Outputs: servos to bands and pistons to clutches
Newer transmissions use electronic inputs and outputs to help control the hydraulic actuation.
This component is very sensitive to dirt.

24. Sprag and/or One-way clutch
A sprag clutch will lock when turned in one direction and free wheel when reversed.
It is a holding unit that is always automatically operated.

25. Other Hydraulic Inputs
Governor – sends input to the valve body with changes in output shaft speed
The faster speed, the stronger the signal to the valve body to UPSHIFT.
Throttle Valve
Can be cable, lever, or electronically operated.
The more throttle input, the stronger the signal to STAY in gear.

26. Electronic Additions Torque converter clutch solenoid
1-2 shift solenoid
2-3 shift solenoid
3-4 shift solenoid
Fluid temp and level sensor
Line pressure sensor