Transmission Service

This procedure covers draining and refilling the automatic transmission fluid and replacing the transmission pan gasket on the 8L90 8-speed automatic transmission.

Warnings

Tools required

Parts

• Transmission pan gasket × 1 — ACDelco 24282528 or equivalent
• Transmission filter assembly × 1 — ACDelco 24278243 or equivalent

Fluids

• ACDelco Dexron VI ATF — 6.5 qt

Preparation

1. Park vehicle on level surface and apply parking brake
2. Place wheel chocks behind rear wheels
3. Allow transmission to cool completely if vehicle was recently operated
4. Raise front of vehicle using floor jack and support on jack stands at frame rails
5. Verify vehicle is stable and secure before working underneath

Procedure

1. 1

   Access transmission pan
   Position drain pan underneath transmission oil pan. Inspect the area and identify the transmission pan location behind the engine oil pan. The transmission pan is the rearmost pan on the underside of the vehicle.
2. 2

   Loosen transmission pan bolts
   Locate all transmission pan bolts around the perimeter. Loosen all bolts 2-3 turns but do not remove completely. Start at one corner and work around the pan to break the seal evenly.
   ⚠Fluid will begin draining as bolts are loosened. Position drain pan to capture fluid.
3. 3

   Drain transmission fluid
   Carefully loosen bolts on one side of the pan more than the other to allow the pan to tilt and drain fluid in a controlled manner. Once majority of fluid has drained, remove all bolts completely and lower the pan. Be prepared for additional fluid spillage when pan is removed.
4. 4

   Remove and inspect filter
   Remove the transmission filter assembly from inside the transmission housing. The filter is typically held by bolts or clips depending on design. Inspect the old filter and pan for metal debris or excessive clutch material, which could indicate internal transmission problems.
   ℹ️Small amounts of fine material are normal. Large metal particles or chunks of friction material indicate potential transmission damage requiring further diagnosis.
5. 5

   Clean transmission pan and mating surfaces
   Thoroughly clean the transmission pan using solvent and rags. Remove all old gasket material from both the pan and transmission housing mating surface using a razor blade scraper. Be careful not to gouge the aluminum surfaces. Wipe clean with a lint-free cloth and inspect for damage.
   ⚠Do not allow debris or gasket material to fall into the open transmission housing.
6. 6

   Install new transmission filter
   Install the new transmission filter assembly into the transmission housing. Ensure the filter is fully seated and any mounting bolts or clips are properly secured. Verify the filter seal is correctly positioned.
   Torque spec

   Sensor Bolts14 Nm (10 lb-ft)
7. 7

   Install new pan gasket
   Position the new transmission pan gasket onto the pan or transmission housing according to gasket design. Most gaskets are rubber molded and do not require sealant. Ensure gasket is properly aligned with all bolt holes.
8. 8

   Reinstall transmission pan
   Carefully raise the transmission pan into position, ensuring the gasket remains properly aligned. Thread all pan bolts by hand first to prevent cross-threading. Tighten bolts in a crisscross pattern to specified torque to ensure even gasket compression.
   Torque spec

   Cover Screws14 Nm (10 lb-ft)
9. 9

   Refill transmission fluid
   Locate the transmission fluid fill plug on the driver side of the transmission, near the front. Remove the fill plug. Using a fluid transfer pump, add approximately 5.5 quarts of ACDelco Dexron VI ATF through the fill hole. This is an initial fill; final level will be checked with transmission at operating temperature.
10. 10

    Check for leaks
    Lower vehicle from jack stands. Start the engine and allow it to idle. With foot on brake, shift through all gear positions, pausing 2-3 seconds in each position. Return to Park. Raise vehicle and inspect transmission pan for leaks.
    ⚠Keep hands and tools clear of moving parts and hot exhaust components while engine is running.
11. 11

    Verify fluid level at operating temperature
    With engine running and transmission at operating temperature (approximately 180-200°F after 15-20 minutes of driving), the fluid level should reach the bottom of the fill plug hole. Add fluid as needed until fluid just begins to drip from fill hole, then reinstall fill plug. Recheck for leaks after test drive.
    ℹ️The transmission must be at proper operating temperature for accurate fluid level. Cold or hot fluid levels will be incorrect.
    Torque spec

    Oil Drain Plug35 Nm (26 lb-ft)

Reassembly

1. Ensure all bolts are torqued to specification in proper sequence
2. Verify transmission fill plug is securely installed after final fluid level check
3. Clean any spilled transmission fluid from frame and suspension components

Verification

• Test drive vehicle for 15-20 minutes, cycling through all gears multiple times
• Verify smooth shifting with no slipping, flaring, or harsh engagements
• Recheck transmission pan and fill plug for leaks after test drive
• Verify final fluid level at operating temperature with engine running
• Monitor transmission operation over next several days for any changes in shift quality

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