After studying this chapter, you will be able to: Explain the basic purpose of a transmission. Identify the differences between manual and automatic transmissions. Describe the differences between automatic transmissions and automatic transaxles. Identify major automatic transmission and transaxle components. Explain the basic operation of an automatic transmission or transaxle. Trace the development of modern automatic transmissions and transaxles.

11

Technical Terms

Chapter 1

Introduction toAutomaticTransmissions andTransaxles

Gear ratio

Reduction gear

Direct drive

Overdrive

CV axles

Continuously variabletransaxles (CVTs)

Fluid couplings

Torque converters

Input shafts

Output shafts

Planetary gears

Holding members

Case

Oil pans

Bushings

Ball bearings

Hydraulic pump

Pressure regulator

Hydraulic control system

Electronic control system

Gaskets

Seals

Manual linkage

Throttle linkage

Transmission fluid

Transmission fluid cooler

Transmission fluid filter

Hybrid vehicle

Infinitely variable transmission (IVT)

Toroidal transmission

This sample chapter is for review purposes only. Copyright The Goodheart-Willcox Co., Inc. All rights reserved.

Introduction

Automatic transmissions and transaxles have beenused for more than 60 years. They have been consistentlymodified and improved, evolving from early inefficientdesigns to the smooth-shifting, efficient units of today.Most modern transmissions and transaxles are controlledby an onboard computer and provide almost the same fueleconomy as manual models.

To service late-model automatic transmissions ortransaxles, the technician must possess considerableknowledge and skill. This chapter will introduce you to thefundamentals of automatic transmissions and transaxles.The basic principles covered here will be expanded uponin later chapters.

The Purpose of Transmissions

All transmissions, whether manual or automatic, havethe same basic purposes:

To transmit power from the engine to the drive wheelswhen necessary.

To disconnect the running engine from the drivewheels during gear changes and when the vehicle isnot moving.

To reverse the direction of power flow when the vehicle must be backed up.

To multiply engine torque as needed.In simplest terms, a transmission modifies engine

torque and speed to match the vehicles needs. For exam-ple, moving a vehicle from a stop requires a great deal ofengine torque, or turning force. At low speeds, however,an engine produces relatively little torque. The transmis-sion must multiply engine torque to get the vehicle mov-ing. It does this by reducing speed to increase torque.

The relationship of the speed of the transmissionsinput shaft to the speed of its output shaft is called the gearratio. The transmission uses a set of at least two gears thatcause the output shaft speed to be much lower than theinput shaft speed. This set of gears is called a reductiongear. Figure 1-1 shows a simple reduction gear.

At higher speeds, the vehicle does not require asmuch engine torque to keep it moving. The engine wouldbe turning very fast if the transmission output speedremained slower than the input speed. High engine speedwill cause poor fuel economy and rapid engine wear.Therefore, the transmission must be shifted into succes-sively higher gears as vehicle speed increases. Shifting intohigher gears changes gear ratios, so the speed of the out-put shaft approaches and eventually equals or exceeds thespeed of the input shaft.

Most modern transmissions have at least four forwardgears, with the highest gear being either direct drive oroverdrive. A direct drive gear causes the input and output

shafts to turn at the same speed. The overdrive gear causesthe output shaft to turn faster than the input shaft, Figure 1-2.Overdrive allows the engine to turn at a relatively slowspeed, increasing fuel economy and reducing engine wear.

Both manual and automatic transmissions canaccomplish any of the previously mentioned jobs. Thereare many similarities between manual and automatictransmissions. All transmissions have a way of keeping theengine from stalling when the vehicle is stopped, all usegears and shafts to obtain different ratios, and all have away to reverse the direction of vehicle travel. However,with a manual transmission, the gear selection decisionmust be made by the vehicles operator. The driver slidesthe transmission gears in and out of engagement using agearshift lever. The gears are meshed in different combina-tions to achieve the desired gear ratios. The driver mustalso operate a manual clutch to connect and disconnectthe engine from the transmission when stopping or chang-ing gears.

12 Automatic Transmissions and Transaxles

3 Revolutions

Drive gear

90-tooth gear

Drivengear

Power out1 Revolution

30-toothgear

Power in

Figure 1-1. Gear reduction produces the torque needed tomove the vehicle from a stop. In this illustration, a 30-tooth gearis turning a 90-tooth gear. The 30-tooth gear must make threerevolutions to turn the 90-tooth gear once.This multiplies torquethree times but cuts speed to one-third.

30-toothdriven gear

3 ft-lb oftorque

Power in

90-toothdrive gear

1 ft-lb of torque Power out

Figure 1-2. In this example, overdrive is accomplished by usingthe 90-tooth gear to turn the 30-tooth gear. This increasesspeed three times but cuts torque to one-third.

With automatic transmissions, on the other hand, gearselection decisions are made by an automatic control sys-tem. Instead of a manual clutch to connect and disconnectthe engine from the transmission, automatic transmissionsuse fluid couplings or torque converters to transfer powerfrom the engine to the transmission.

Automatic transmissions use planetary gearsets,which do not slide in and out of engagement. In operation,one of the gears in the gearset is locked in place. Theremaining unlocked gears are driven by engine power andcomprise the input and output. Different gear ratios areachieved by different combinations of locked andunlocked gears. The gears are operated by holding mem-bers called clutches and bands. The clutches and bandsare controlled by a hydraulic control system. Late-modelautomatic transmissions have hydraulic systems controlled byon-board computers. Vehicles with automatic transmissionsare easier to drive than those with manual transmissions.They are also more durable for heavy-duty operation, suchas trailer towing. The major differences between manualand automatic transmissions/transaxles are shown inFigure 1-3.

The ideal transmission will transmit engine powerwith no slipping. Slipping can be defined as failure totransmit all engine power to the other drive train compo-nents. In other words, a slipping transmission will loseboth speed and torque between its input and output shafts.

Early automatic transmissions were so inefficient andslipped so much that they were called slush boxes.Modern automatics are efficient and smooth. Except forfirst and reverse gears, modern automatic transmissionspermit no slippage. They transmit as much engine poweras manual transmissions. It is now possible for an auto-matic transmission to be more efficient than a manual.With an automatic transmission, there is no need to releasethe accelerator pedal during shifts, and then reaccelerateto maintain vehicle speed.

Transmissions and Transaxles

Until about 20 years ago, nearly all vehicles had arear-wheel drive arrangement that used a transmission totransfer power to the rest of the driveline. The rear-wheel

drive transmission transmits power in a straight line, fromthe front of the vehicle to the back. The differential and thefinal drive assembly are contained in a separate housing atthe rear axle.

Today, most automobiles use front-wheel drive sys-tems equipped with transaxles. Transmissions andtransaxles perform the same function. The major differ-ences include the arrangement of the parts and the fact thatthe differential and the final drive assembly (sometimescalled the ring-and-pinion assembly) are an integral part ofthe transaxle. Transaxles have two output shafts, one foreach wheel. These shafts are attached to the CV axles.Engine power is transmitted sideways through a chain orgears at some point in the transaxle.

The advantages of transaxles include reduced weightand increased fuel economy. On trucks, however, weightand fuel economy are less of a factor than durability. Therear-wheel drive train is usually used on these vehicles.Therefore, it is important that both transmissions andtransaxles be understood completely.

Four-wheel drive vehicles have a transfer caseattached to the rear of the transmission. The transfer casesends power to the front wheels. Figure 1-4 illustrates thelayouts of modern rear-wheel drive vehicles with auto-matic transmissions and front-wheel drive vehicles withautomatic transaxles.

Automatic Transmission andTransaxle Development

The modern automatic transmission was not the resultof a single invention. Some components used in automatictransmissions were developed long before the automobileitself. Planetary gear principles were known during thetime of the Roman Empire and eventually appeared on theFord Model T. Fluid couplings were used to drive machin-ery in 19th century mills. The first automotive fluid coup-lings were used on English cars in the 1920s and onChrysler vehicles in the mid 1930s.

The 1938 Oldsmobile is widely considered the firstcar to have an automatic transmission. These earlyOldsmobile Hydra-Matics had planetary gears operated by

Chapter 1 Introduction to Au