chap100 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 100, the reader should
be able to:

•

•
•

Prepare for the ASE Automatic
Transmission/Transaxle (A2) certification test

content area “A” (General
Transmission/Transaxle Diagnosis)
Explain how a torque converter can transmit
and multiply engine torque
Describe how a planetary gear set can be
used for gear reduction and reverse
Continued

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OBJECTIVES:
After studying Chapter 100, the reader should
be able to:

•

Explain how automatic transmission fluid is
circulated through the system and how it is
cooled

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
compound planetary gear set • coupling phase
flexplate
impeller • input member

lapelleties gear set • lube oil
output member
pump
ravigneaux gear set • reaction member
Continued
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KEY TERMS:
simpson gear set • stall speed • stator
torque converter clutch (TCC) • torque multiplication
phase • turbine

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition

By James D. Halderman

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Automatic transmissions were first used on a large scale
in the late 1940s, and now about 85% of the vehicles in
North America are so equipped.
Unlike manual transmissions, most automatic
transmissions do not actually “shift gears” but apply
clutches (or bands) to hold various sections of two or
more planetary gear sets.
The torque converter is attached between the engine
and the transmission/ transaxle and transmits and
multiplies engine torque.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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TORQUE CONVERTERS
The impeller, also known as the pump, is the driving member and rotates with the engine. The impeller 
vanes pick up fluid in the converter housing and direct it toward the turbine.
Fluid flow drives the turbine, and when the flow between the impeller and turbine is adequate, the 
turbine rotates and turns the transmission input shaft. A torque converter contains the stator, or reactor, 
a reaction member mounted on a one­way clutch.
The vanes used in each of the three elements of a torque converter are curved to increase the diversion 
angle of the fluid. This also increases the force exerted by the fluid and improves the hydraulic 
advantage. See Figure 100–1.

Continued
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Figure 100–1 A cross-sectional view of a Chrysler Power Flight 2-speed automatic
transmission used in the 1950s. Most of the heat generated in an automatic transmission is
created in the torque converter. This air-cooled unit has a vent to allow hot air to escape.

Continued
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The outlet side of the impeller vanes accelerates the fluid as it leaves the impeller to 
increase torque transfer to the turbine.
See Figure 100–2.
The inlet side of the turbine vanes absorb shock and limit power loss that occurs when 
flow between the impeller and turbine suddenly changes. The curve of the stator vanes is 
opposite to the curve of the impeller and turbine vanes.
See Figure 100–3.
Since the stator is located between the impeller and turbine, it adds to the original 
impeller flow and multiplies the force delivered to the turbine. See Figure 100–4.

Continued
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Figure 100–2 Fluid flow within a torque converter. The stator redirects the fluid that is thrown
out by the turbine, thereby improving efficiency.

Continued
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Figure 100–3 Fluid pumped into
the turbine by the impeller not
only creates rotary fluid flow but
also vortex flow that increases
the efficiency of the torque
converter.

Continued
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Figure 100–4 The vane curvature of each
element in a torque converter helps maintain
efficient fluid flow

Torque converters are single­piece, 
welded assemblies that cannot be 
disassembled or repaired easily.
Although they can be rebuilt using 
specialized equipment, most shops 
simply replace welded converters as 
a unit if they fail.

Continued
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Torque Converter Attachments The torque converter normally attaches to the engine 
through a flexplate that mounts on the crankshaft flange of the engine. 
The flexplate replaces the heavy flywheel used with a manual transmission. An important 
function of a flywheel is to smooth out engine pulsations and dampen vibrations.
An automatic transmission does not require a conventional flywheel because the fluid in 
the torque converter provides
enough mass to dampen engine vibrations.
See Figure 100–5. 

Continued
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Figure 100–5
The torque converter bolts to the
flexplate, which is attached to the
crankshaft and rotates at engine
speed. The hub of the converter
drives the oil pump directly on
most rear wheel-drive
transmissions.

An external ring gear 
generally attaches to the 
outer rim of the flexplate.
This ring gear engages the 
starter motor pinion gear
to turn the engine during 
starting.
Continued
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NOTE: On some older applications, such as the Ford C4 and Chrysler 
Torqueflite, the ring gear may be welded to the outside of the torque 
converter cover.

To ensure the pump will deliver fluid to the transmission whenever the engine turns, an 
integral hub is located on the converter housing and directly engages the pump. 
Oil pump drive shafts generally pass through the converter inside
a hollow input or transfer shaft and internally connect to the converter housing by splines.
Most rear­wheel­drive transmissions use an inline method to drive the converter and 
provide a direct mechanical connection ­between the turbine and the transmission input 
shaft.
See Figure 100–6.

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Figure 100–6 The inner oil pump gear is keyed to the hub of the torque converter, which drives
the pump. Notice that the hub does not engage the full depth of the gear. This is the major
reason why the torque converter must be fully installed. Failure to fully engage the oil pump
gear can cause serious damage to both the pump and the torque converter.

Continued
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In a typical design, splines on the turbine connect it to the transmission 
input shaft and the stator hub mounts on a one­way overrunning clutch.
The one­way clutch mounts on splines to a stationary extension
of the oil pump called the stator support, or reaction shaft. The converter 
drive hub at the rear of the torque converter housing passes over the 
stator support and through the front oil seal to drive the oil pump.
See Figure 100–7. 

Continued
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Figure 100–7
The transmission input shaft
connects directly to the
turbine through splines in
most rear-wheel-drive
transmissions.

Continued
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Many transaxles, and a few rear­wheel­drive transmissions, use an offset drive 
arrangement to conserve space.
An offset drive design generally uses a drive chain to provide the mechanical 
connection between the turbine and the input shaft.
The oil pump may be driven directly by the converter housing or by a separate 
drive shaft.
See Figure 100–8. 

Continued
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Figure 100–8 On a transaxle, the turbine drives the input shaft through a drive chain assembly.

Continued
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Torque Converter Operation  Fluid sent to the torque converter 
from the transmission oil pump is picked up by the rotating vanes of 
the impeller and transferred to the turbine vanes through rotary and 
vortex flow paths.

Torque Multiplication Phase  Fluid leaving the turbine vanes 
strikes the concave, or front side, of the stator vanes during the 
torque multiplication phase.
The force of the fluid from the stator adds to the force of the fluid 
flowing from the impeller to increase the overall torque being 

transferred from the impeller to the turbine. 
Torque multiplication occurs whenever the vortex flow makes a 
full cycle from impeller to turbine, then through the stator and back 
to the impeller. See Figure 100–9. 
Continued

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Figure 100–9 Torque multiplication occurs when
fluid leaving the turbine strikes the front of the
stator vanes and is redirected back to the impeller.

A torque converter multiplies 

torque in relation to speed ratio.
At low ratios, the impeller is 
turning much faster than the 
turbine, so vortex flow is high 
and torque multiplication occurs.
As the speed ratio approaches 
90%, torque multiplication 
becomes minimal and a torque 
converter functions like a fluid 
coupling.
Continued
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The stator redirects fluid flow because it remains stationary during 
the torque multiplication phase. The stator hub mounts on a one­
way clutch, freewheeling in a clockwise direction, but locking 
when driven in a counterclockwise direction. 

Figure 100–10 A stator contains a one-way
roller clutch, which locks it from rotating in
one direction and allows it to rotate freely in
the opposite direction.

When fluid from the turbine strikes the concave face of the stator 
vanes, it tries to drive the stator counterclockwise. By locking the 
stator it can redirect the fluid back to the impeller.
Continued
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Coupling Phase  When the speed ratio is 90% or more, fluid flow in the toque converter is 
mostly rotary flow and the angle of flow from turbine to stator increases.
Fluid eventually strikes the convex side, or backside, of the stator vanes rather than the concave. 
As the force of fluid striking the backside becomes great enough to drive the stator clockwise, 
the one­way clutch overruns.
With the clutch overrunning the turbine, impeller, and stator, all rotate in the same direction and 
at approximately the same speed. This is called the coupling phase.
The stator unlocks and freewheels once the angle of fluid flow changes enough to strike the 
opposite side of the stator vanes and rotate the stator clockwise.

Continued
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Torque multiplication drops as the torque converter approaches 
the coupling phase because the stator no longer redirects fluid to 
increase the flow from impeller to turbine.
When the torque converter reaches coupling speed, the turbine is 
traveling at nearly the same speed as the impeller, rotary flow is 
much greater than vortex flow, and the torque converter simply 
transmits torque like a fluid coupling.

Continued
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