Replace a single synchronizer assembly in the drive unit reduction gear assembly, requiring drive unit removal and internal disassembly to access the damaged synchro.

Warnings

Tools required

Parts

• Synchronizer assembly (specific gear) × 1 — Use OEM specification for specific gear position
• Synchronizer blocking ring × 1 — Use OEM specification
• Synchronizer insert springs × 3 — Use OEM specification
• Drive unit gasket kit × 1 — Use OEM specification
• Drive unit seal kit × 1 — Use OEM specification
• Drain plug washer × 1 — Use OEM specification
• Axle seals (both sides) × 2 — Use OEM specification
• RTV silicone sealant (GM approved) × 1 — Use OEM specification

Fluids

• ACDelco Dexron VI ATF — 14 qt

Preparation

1. Park vehicle on level surface and apply parking brake
2. Disconnect 12V battery negative terminal and secure away from post
3. Access high voltage disconnect per service manual procedure and verify zero voltage
4. Raise vehicle on lift and secure with safety locks
5. Remove front wheels and retain lug nuts
6. Remove engine undershield and belly pan fasteners
7. Place drain pan beneath drive unit drain plug
8. Allow drive unit to cool to ambient temperature if recently driven

Procedure

1. 1

   Drain drive unit fluid
   Remove the drain plug from the bottom of the drive unit and allow fluid to drain completely into collection pan. Remove fill plug to allow complete drainage. Inspect drained fluid for metal particles or debris which may indicate additional damage. Clean magnetic drain plug and inspect for excessive metal accumulation.
   Torque spec

   Drain Plug34 Nm (25 lb-ft)

   Fill Plug34 Nm (25 lb-ft)
2. 2

   Remove halfshafts and disconnect components
   Remove both front halfshafts by removing hub nuts and suspension components as needed. Disconnect electrical connectors from drive unit including motor position sensors and temperature sensors. Support drive unit with transmission jack positioned securely under unit. Disconnect ground straps and wiring harness brackets.
3. 3

   Remove drive unit from vehicle
   Remove drive unit mounting bolts while supporting unit with transmission jack. Carefully lower drive unit away from motor assembly, being careful not to damage coolant lines or remaining electrical connections. Transfer drive unit to clean workbench or assembly stand. Clean exterior of unit thoroughly before disassembly.
   Torque spec

   Mounting Bolts61 Nm (45 lb-ft)
4. 4

   Separate drive unit case halves
   Remove all case bolts following proper sequence (typically outside-in pattern). Use case spreader tool or carefully apply pry pressure at designated pry points only. Never pry on machined mating surfaces. Tap case with soft mallet if needed. Once separated, carefully document positions of all shims, bearings, and components. Photograph assembly for reference.
   ⚠Do not damage precision machined mating surfaces. Case damage will cause fluid leaks and require case replacement.
   Torque spec

   Pan Bolts16 Nm (12 lb-ft)
5. 5

   Remove gear assemblies and locate damaged synchro
   Remove any retaining clips or snap rings securing gear assemblies. Carefully extract gear shafts and assemblies, keeping all components organized by position. Measure and record all bearing preloads and endplay values with dial indicator before disassembly. Locate the specific gear position requiring synchro replacement. Remove snap ring retaining the synchro assembly to the gear.
6. 6

   Disassemble and inspect synchro components
   Mark orientation of synchro sleeve relative to hub before disassembly. Slide synchro sleeve off hub and remove insert springs and inserts. Remove blocking ring from gear cone. Inspect gear cone surface for wear, scoring, or heat damage. Measure blocking ring-to-cone clearance on mating gear using feeler gauge - should be within specification (typically 0.8-2.0mm). If cone is damaged, gear must be replaced.
   ⚠Synchro insert springs must be installed in opposite directions to provide proper tension. Note original orientation.
7. 7

   Install new synchro assembly
   Install new blocking ring onto gear cone, ensuring proper engagement with gear teeth. Assemble synchro hub with new sleeve, aligning marks made during disassembly (or per manufacturer alignment marks). Install inserts into hub slots, then install insert springs in opposite directions, ensuring spring ends locate in different inserts. Verify synchro sleeve slides smoothly on hub and blocking ring engages properly. Install synchro assembly onto gear shaft and secure with snap ring, ensuring snap ring is fully seated in groove.
8. 8

   Measure and adjust gear train endplay
   Install gear assemblies back into case half with all original shims in place. Measure endplay of each gear using dial indicator while applying force to move gears axially. Compare measurements to service specifications (typically 0.05-0.25mm depending on gear). Adjust shim thickness as needed to achieve proper endplay. This is critical for proper gear engagement and bearing life.
   ℹ️Endplay specifications vary by gear position. Refer to service manual for specific values for each gear in the assembly.
9. 9

   Install new seals and prepare case for assembly
   Install new axle seals into case using appropriately sized seal driver, ensuring seals are square and seated to proper depth. Clean all gasket surfaces thoroughly with appropriate solvent. Inspect case mating surfaces for damage or imperfections. Apply thin bead of RTV sealant to case mating surface per manufacturer specifications, typically 2-3mm bead on inside of bolt holes. Install new gaskets where specified.
10. 10

    Reassemble drive unit case
    Carefully align case halves, ensuring all gears, bearings, and shafts remain properly positioned. Install case bolts hand-tight initially. Torque case bolts in proper sequence (typically criss-cross pattern from center outward) to specification. Verify case halves are fully seated with no gaps. Rotate input shaft by hand to verify smooth operation with no binding or unusual resistance.
    ⚠Allow RTV sealant to cure per manufacturer recommendation before filling with fluid if specified in service manual.
    Torque spec

    Pan Bolts16 Nm (12 lb-ft)
11. 11

    Reinstall drive unit in vehicle
    Position drive unit on transmission jack and carefully raise into position, aligning with motor assembly and mounting points. Install mounting bolts and torque to specification in star pattern. Reconnect all electrical connectors, ensuring positive engagement and no damaged pins. Reconnect ground straps. Install both halfshafts with new retaining hardware as specified.
    Torque spec

    Mounting Bolts61 Nm (45 lb-ft)
12. 12

    Fill drive unit with fresh fluid
    Install drain plug with new washer and torque to specification. Fill drive unit through fill plug opening with ACDelco Dexron VI ATF until fluid reaches bottom of fill plug hole (approximately 13.5 quarts total capacity, but fill to level not volume). Install fill plug with new washer if equipped and torque to specification. Wipe any spilled fluid from drive unit exterior.
    Torque spec

    Drain Plug34 Nm (25 lb-ft)

    Fill Plug34 Nm (25 lb-ft)

Reassembly

1. Install engine undershield and belly pan fasteners
2. Install front wheels and torque lug nuts to specification in star pattern
3. Lower vehicle to ground
4. Reconnect high voltage system per service manual procedure
5. Reconnect 12V battery negative terminal
6. Clear any diagnostic trouble codes that may have set during repair

Verification

• Start vehicle and verify no warning lights related to drive unit or propulsion system
• With vehicle in Park and parking brake applied, select Drive and verify smooth engagement with no unusual noise
• Perform test drive including gentle acceleration and deceleration to verify proper synchro operation and smooth gear transitions
• Listen for any unusual noises during operation including whining, grinding, or clicking
• Verify no fluid leaks from drive unit case, drain plug, fill plug, or axle seals
• Re-check fluid level after test drive and top off if needed
• Scan for diagnostic trouble codes and verify no new codes present
• Monitor drive unit temperature during normal operation to ensure proper cooling

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