Automatic Transmission Diagnosis and Service

1
Automatic Transmission Diagnosis and Service

• Chapter 74

2
Objectives

• Perform maintenance service on automatic
  transmissions
• Diagnose automatic transmission problems and
  recommend repairs
• Change the transmission fluid and filter
• Repair automatic transmission leaks
• Perform basic automatic transmission tests
• Understand what is involved in a transmission
  overhaul

3
Introduction

• Major transmission repairs
• Done by specialty dealers or businesses
• All technicians should be knowledgeable about
  repairs and diagnosis
• Transmission needs major repair
• Must be removed
• Complicated specialty area
• Special schooling is recommended

4
Automatic Transmission Identification and
Maintenance

• Important considerations
• Transmission identifications
• Locate identification label
• Use wall chart provided by transmission part
  manufacturers
• Automatic transmissions generally require very
  little maintenance
• Transmission should last 100,000 miles or more

5
Transmission Fluid Service

• Automatic transmission fluid (ATF)
• Most are dyed red
• Low viscosity
• Contains additives
• Several types
• Long lasting
• Severe service
• Anything that causes heat
• Example trailer towing

6
Fluid Level

• Important considerations
• Dipstick is in tube extending from transmission
  pan
• Level should be between the add and full marks
• Low fluid level is caused by a leak
• High fluid level can result in foamed fluid
• Conditions for checking fluid level
• Car on level ground
• Engine and transmission warm
• Parking brake set
• Gear selector is in park position

7
Check Fluid Condition

• Conditions
• Milky fluid coolant and fluid mix together
• Varnish dipstick will not wipe clean
• Fluid has become oxidized
• Burned fluid dark with burnt smell
• Adding fluid
• Be sure to use the correct type
• Use a clean funnel

8
Changing Transmission Fluid and Transmission
Filter Service

• Important considerations
• Check the service manual before changing fluid
• Electronic transmissions are more sensitive to
  problems with fluid condition
• Transmission flush machines are a common
• Pan must be removed to clean or replace filter
• Draining the fluid requires removal of the pan
• Inspect inside of pan for pieces of friction
  material or metal particles
• Check the screw holes in pan

9
Remove and Replace the Filter and Refill the
Transmission

• Important considerations
• Important pump must suck only fluid and not air
• Some filters have a gasket
• Be sure to remove old O-ring or sleeve
• Lubricate new O-ring with ATF
• Replace the pan and tighten screws evenly
• Check the service information for fluid capacity
  and refill the transmission
• Do not overfill

10
Diagnosis and Repair of Leaks

• Causes of leaks
• Front pump seal
• Rear seal to drive shaft yoke
• Shift lever shaft seal
• Pan gasket
• Extension housing gasket
• External leaks
• Obvious
• Difficult leaks
• Use fluorescent dye and black light

11
Leaks from the Converter Housing

• Remove access cover on housing
• Disconnect battery ground cable and remove
  starter
• Oil leak from engine crankshaft seal could be the
  cause

12
Transmission Cooler Line Leak

• Important considerations
• Some mechanics cut out bad section and replace it
  with a length of fuel hose
• Use transmission oil cooler hose
• Both ends of the line must be flared
• Better repair
• Cut old section of line and replace with a union
  or install flare fittings with a short piece of
  reinforced hose with crimped threaded ends

13
VSS/Speedometer Drive Gear Leak and Shift Lever
Seal Replacement

• Important considerations
• Mechanical speedometers have an O-ring seal that
  can leak
• Many late-model cars have an electronic
  speedometer pickup
• Shift lever to manual valve is sealed with a lip
  seal or O-ring
• Sometimes it is necessary to remove the pan and
  valve body to remove the shift lever

14
Pump Seal Replacement

• Transmission has a full round seal that rides on
  front of torque converter
• Exposed to a good deal of heat
• Transmission fluid pours out when seal fails
• Pump seal replacement
• Requires transmission be removed from vehicle
• FWD procedures
• Vary
• RWD procedures
• More straightforward

15
Front Seal Replacement and Pump Bushing
Replacement

• Front seal replacement major steps
• Remove and drain pan under torque converter
• Remove old seal
• Install seal on installer and drive it into the
  pump
• Pump bushing replacement
• Bushing in transmission pump supports snout of
  torque converter
• Check service manual for procedure replacement
• Often requires removal and disassembly of pump

16
Reinstalling the Transmission

• Major steps and considerations
• Install and bolt to engine
• Engine and transmission must be touching each
  other completely before tightening bolts
• Torque converter must be correctly engaged
• Should be at least 1/8" between converter studs
  and flexplate

17
Rear Oil Seal and Bushing Replacement

• Removal of extension housing seal and bushing on
  RWD vehicles
• Remove drive shaft
• Old bushing can be removed without removing
  extension housing
• Use special puller
• Bushing sometimes has an oil return hole
• Must be aligned

18
Automatic Transmission Problem Diagnosis

• Before condemning a transmission
• Check for diagnostic trouble codes
• Check condition of engine
• Quadrant indicator might not be properly aligned
• Perform a visual check of electrical connections
• Talk to customer before test-driving
• How transmission operates when cold
• Check fluid levels
• Use a road test checklist

19
Slippage

• Electronic transmissions
• Have sensors input and output ends that can
  calculate slippage
• Slippage causes
• Low fluid level
• Leakage in clutch pack or servo
• Plugged pump inlet screen or filter
• Problem with valve body

20
Transmission Drainback

• Worn input shaft
• Transmission slips on initial takeoff
• Shift feel
• Mushy or harsh shifts
• Mechanical transmission problems
• Shift feel
• Incorrect shift timing
• Shift chatter or torque converter clutch chatter
• Sticking governor
• Vacuum modulator diagnosis and adjustment

21
Transmission Drainback (contd.)

• Additional potential problems
• No upshifts
• Wrong gear starts
• Engine dies in gear
• No vehicle movement
• Backward one-way clutch
• Internal hydraulic system cross leaks

22
Noises

• Causes of noise
• High or low fluid level
• Gear or bearing wear or damage
• Bad torque converter bushing
• One-way clutch
• Engine accessories can mimic transmission noise

23
Flexplate

• Characteristics
• Flexes inward toward engine
• Converter normally slides forward during
  deceleration
• Flexplate will allow movement of up to 0.080 to
  0.100"
• Cracked flexplate causes a knocking sound

24
Transmission Tests

• Include
• Stall test used to determine if transmission is
  slipping or torque converter is defective
• Pressure test tells if transmission is
  experiencing internal leakage
• Air test used to check operation of various
  clutch packs and servos

25
Valve Body Removal and Transmission Adjustments

• Valve body removed while transmission is in
  vehicle
• Be sure it is cold
• Check balls must be replaced in same position
• Transmission adjustments
• Linkage adjustment
• Neutral safety switch/park neutral position
  switch
• Throttle cable adjustment
• Band clearance adjustment
• Speedometer adjustment

26
Electronic Transmission Service

• Important considerations
• Some generic scan tools can work on different
  makes of vehicles
• Electronic systems can self-diagnose electrical
  components and circuits
• Be certain of cause of transmission failure
  before removing an electronic transmission
• Computers in most vehicles use ground side
  switching with current limiting devices

27
Electronic Transmission Service (contd.)

• Electronic transmission shift control line
  pressure is changed to accommodate shifts
• Current flow varies depending on number of
  solenoids energized at one time

28
More Tests Before Transmission Removal

• Some tests are done in the service bay or on the
  road or dyno
• Other tests done on the lift
• On the lift
• Tighten the band all the way
• Run without ATF for a few seconds
• Perform air test of hydraulic circuits
• Computer-controlled transmission
• Do not remove until transmission is determined to
  be the problem

29
Hybrid Automatic Transmission Service

• Important considerations
• Many hybrids use conventional automatic
  transmissions
• Serviced in traditional ways
• High-voltage electrical system
• Must be disabled before removing transmission
• Unique automatic transmissions needing repair
• Replaced with a complete replacement unit

30
Remanufactured Automatic Transmissions and
Transaxles

• Important considerations
• Many dealers do not perform rebuilds
• Remanufactured transmissions have become readily
  available
• It is important that the correct transmission is
  acquired
• Especially with imports

31
Transmission Rebuilding

• Important considerations
• Internal damage has occurred
• Hard parts will be necessary
• All soft parts are good use inexpensive seal kit
• Valve body is disassembled lay out each piece
• Remove snap ring from clutch pack and clutch
  discs
• Replace old seals on piston
• Seal rings fit into grooves on shaft that
  supports center of clutch drums
• Check all bushings for visible signs of wear

32
Transmission Reassembly

• Important considerations
• Do not use moly assembly lube on a one-way clutch
• Spring in the roller clutch is bimetal
• Heat is an enemy
• End play is checked with a dial indicator
• Selective washers or shims adjust end play
• Many transmissions have more than one end play
  adjustment
• Torque bolts to transmission pump after final end
  play check