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Manual Transmission Diagnosis and Repair

Objectives. Diagnose transmission-related problemsRemove a transmission safely in a professional mannerDisassemble transmissions correctlyUse presses and pullers competently to disassemble and reassemble transmissionsDiagnose and service synchronizersReassemble and install a transmission in a vehicle.

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Manual Transmission Diagnosis and Repair

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Manual transmission diagnosis and repair l.jpg

Manual Transmission Diagnosis and Repair

Chapter 72

Objectives l.jpg


  • Diagnose transmission-related problems

  • Remove a transmission safely in a professional manner

  • Disassemble transmissions correctly

  • Use presses and pullers competently to disassemble and reassemble transmissions

  • Diagnose and service synchronizers

  • Reassemble and install a transmission in a vehicle

Introduction l.jpg


  • Important considerations in transmission repair

  • Proper press and puller use

  • Locating service information

  • Disassembling transmissions

    • Requires controlled force

    • Each transmission has its own peculiarities

    • Synchronizers are mostly of the same design

  • Transmission diagnosis l.jpg

    Transmission Diagnosis

    • Clutch problem

      • Transmission doesn't go into gear

    • Transmission failure

      • Results from abuse or misapplication

      • Determine cause before removal

    • Some typical symptoms:

      • Gear clashing during shifting

      • Hard shifting and jumping out of gear

      • Unusual noise

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    Lubricant Checks

    • Lubricant level

      • Checked by removing the fill plug and inserting your little finger inside the hole

    • Check for leaks

      • Front and rear seals

      • Any switches

      • Plug or vents

      • Gaskets

    • Oil leakage can result in transmission failure

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    Transmission Removal

    • Major steps

      • Removing a transmission can be dangerous

        • Drain gear oil before installing jack

        • May be necessary to remove exhaust pipes

      • Disconnect battery

      • Remove speedometer cable

      • Disconnect external shift linkage

      • Remove and clean transmission

        • Identify transmission before ordering parts

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    Transmission Disassembly

    • Major steps

      • Check service manual before disassembling

      • Look for damage

        • Gears develop normal wear pattern with use

      • Remove front bearing retainer and snap rings

      • Remove countershaft and mainshaft

        • Mainshaft removed from rear or top of transmission

      • Remove the input shaft

        • Be careful not to damage gears

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    Disassemble the Mainshaft Assembly

    • Major steps

      • Lay parts out as they are disassembled

        • Keep parts in order

      • High-gear synchro clutch hub

        • Held on mainshaft with a snap ring

        • Remove snap ring with snap ring pliers

      • Press or puller pulls synchronizer clutch hub

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    Synchronizer Service

    • Synchronizer will be reused

      • Scribe line on outside to reassemble in same position

        • Some are marked from factory

    • Look for wear

      • Gears and shift sleeves

    Synchronizer inspection l.jpg

    Synchronizer Inspection

    • Best way to tell if synchro is not working

      • Test-drive the vehicle

    • Worn synchro

      • Causes gear clashing

    • Inspect synchronized gears

      • Rarely experience excessive wear

    • Inspect synchro sleeve

      • Even width across length

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    Inspect Blocker Rings

    • Inspect brass blocker rings if they are to be reused

      • Inside surface should have sharp edges

    • Push blocker ring against polished tapered surface on gear that it rides on

      • Should grab against chrome surface

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    Inspect Input Shaft and Mainshaft

    • Inspect mainshaft splines

      • May be twisted from abusive driving

      • Twisted spline prevents drive shaft slip yoke from sliding in and out of transmission

    • Inspect drive gear for VSS or speedometer

      • Located on the output shaft

      • Rarely suffers damage

      • A special puller is available to pull these gears

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    Replace Worn Bearings

    • Use a press and support inside of bearing with bearing separator

      • Reassembling: apply pressure on inside of bearing only

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    Reassemble the Transmission

    • Before assembling

      • Locate and assemble any new replacement parts

    • Two gears operating in mesh

      • Create wear pattern

    • Gear is replaced

      • Corresponding gear also replaced

      • Using an old gear with a new gear can result in unacceptable gear noise

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    Reassemble the Synchronizers

    • Reassemble synchronizer hub parts

      • Three inserts are held in place with a round spring

    • Install springs so they rotate in opposite directions

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    Reassemble the Mainshaft

    • Major steps

      • Install gears

      • Lubricate parts liberally

      • Install synchro hub

      • Check end play of assembled parts

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    End Play

    • Thrust washers

      • Control end clearance

      • Determine where gear will be positioned

    • Check gear clearance after all gears are assembled

      • Adjust clearance by changing thickness of snap ring

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    Needle Bearing Installation

    • Needle bearings are sometimes caged

      • Can also be loose and installed one at a time

      • Count needle bearings during disassembly

    • Service manual

      • Specifies number of needle bearings

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    Install New Gaskets

    • Purchase a new transmission gasket set

      • Gaskets are not reusable

    • Gasket working fine previously could leak after reassembly

      • Replacing this gasket would be a complicated repair

        • Would be done at no charge to the customer

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    Complete the Transmission Reassembly

    • Install shift mechanism

    • Replace extension housing bushing and seal

      • Use a seal installer to replace the seal without distorting it

    Install the transmission l.jpg

    Install the Transmission

    • Major steps

      • Be sure clutch fork is properly seated on its pivot

      • Align transmission with engine

      • Shake transmission while rotating it back and forth

      • Install bolts in transmission

      • Torque to specifications

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    Add Lubricant

    • Important considerations

      • Add recommended lubricant before driving the vehicle

      • Lubricant level should come to bottom of fill hole

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    Test Drive

    • Important considerations

      • Be sure shifts are smooth in all gears

      • Neutral gate should be accurately aligned

      • Check outside of transmission for leaks after returning to shop