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 (cont’d.) • 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 (cont’d.) • 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