Automatic Transmission Diagnosis and Service Chapter 74

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

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

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

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

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

Check Fluid Condition Conditions Adding fluid 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

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

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

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

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

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

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

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

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

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

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

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

Slippage Electronic transmissions Slippage causes 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

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

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

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

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

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

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

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

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

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

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

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

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

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