Differential Fluid Change - Rear

Drain and refill the rear differential with fresh 75W-85 synthetic gear oil to maintain proper lubrication and extend component life.

Warnings

Tools required

Parts

• Rear differential drain plug washer/seal × 1 — Use OEM specification
• Rear differential fill plug washer/seal × 1 — Use OEM specification

Fluids

• 75W-85 synthetic gear oil — 2.1 qt

Preparation

1. Park vehicle on level surface and apply parking brake.
2. Place wheel chocks at front wheels.
3. If differential was recently driven, allow 15-20 minutes for components to cool to a safe handling temperature.
4. Lift rear of vehicle using floor jack at rear axle lift point, then secure on jack stands rated for vehicle weight at manufacturer-specified chassis lift points.
5. Remove both rear wheels using 21mm socket to access differential housing.

Procedure

1. 1

   Locate differential drain and fill plugs
   Position yourself beneath the rear axle. The rear differential housing is centered between the rear wheels. Identify the drain plug on the bottom surface of the differential housing (verify the correct hex/Allen or Torx plug size against MB documentation). The fill plug is located on the side or rear face of the housing, using the same plug type. Clean the area around both plugs with a wire brush and brake cleaner to prevent contamination during service.
2. 2

   Loosen fill plug first
   Before draining, loosen the fill plug using an 8mm hex socket. Turn counterclockwise to break it free, but do not remove completely yet. This confirms you can refill the differential before draining it. If the fill plug is seized, do not proceed with draining until it can be removed.
   ⚠Always verify the fill plug can be removed before draining. A seized fill plug with an empty differential will leave the vehicle immobile.
3. 3

   Drain differential fluid
   Position drain pan directly beneath the drain plug with at least 3 quarts capacity. Using the 8mm hex socket, remove the drain plug completely by turning counterclockwise. Allow fluid to drain completely; this may take 5-10 minutes. Old fluid will appear dark and may contain metallic particles (normal wear). Once draining has stopped, wipe the drain plug and inspect the sealing washer for damage.
4. 4

   Inspect and replace drain plug seal
   Remove the old crush washer or seal from the drain plug. Install a new OEM drain plug washer onto the plug. Wipe the drain plug sealing surface on the differential housing with a clean rag to remove any debris or old gasket material.
5. 5

   Reinstall drain plug
   Thread the drain plug with new washer into the differential housing by hand to avoid cross-threading. Once seated, tighten to the MB-specified drain plug torque (verify against workshop documentation) using a torque wrench. Do not overtighten, as this can strip the aluminum housing threads. Note: Differential Drain Plug
6. 6

   Prepare fill plug and seal
   Fully remove the fill plug from the differential housing. Remove the old sealing washer from the fill plug and install a new OEM washer. Set the plug aside in a clean location.
7. 7

   Refill differential with gear oil
   Using a fluid pump or squeeze bottle with extension tube, insert the tube into the fill plug opening. Slowly pump in the MB-approved synthetic gear oil (verify correct viscosity, typically 75W-85 or 75W-90, against MB spec 235.x for this axle). The differential is full when oil reaches the bottom of the fill plug hole and begins to seep out. Fill slowly to allow air to escape. Verify the rear differential fluid capacity against MB documentation for this specific axle; fill until oil reaches the bottom of the fill plug hole and begins to seep out regardless of stated capacity.
8. 8

   Reinstall fill plug
   Wipe any excess oil from around the fill hole. Thread the fill plug with new washer into the housing by hand until seated. Tighten to the MB-specified fill plug torque (verify against workshop documentation) using a torque wrench. Note: Differential Fill Plug
9. 9

   Clean and inspect for leaks
   Wipe down the entire differential housing with shop rags to remove any spilled gear oil. Inspect both drain and fill plugs to ensure they are properly seated with no visible seepage.
10. 10

    Reinstall wheels and lower vehicle
    Reinstall both rear wheels, threading lug nuts by hand first to avoid cross-threading. Tighten wheel bolts in a star pattern to the MB-specified torque (verify against documentation, typically ~150-180 Nm) using a torque wrench. Raise vehicle slightly with floor jack, remove jack stands, and lower vehicle to ground.
    Torque spec

    Wheel Lug Nuts142.4 Nm (105 lb-ft)

Reassembly

1. Ensure both drain and fill plugs are torqued to specification with new sealing washers installed.
2. Wheels must be torqued in a star pattern to ensure even seating against the hub.
3. Dispose of used gear oil at an appropriate recycling facility; do not pour down drains or discard with regular trash.

Verification

• Start the vehicle and allow it to idle for 1 minute, then drive forward and backward slowly to circulate new gear oil through the differential gears.
• After a short test drive (5-10 minutes), re-inspect the differential drain and fill plugs for any signs of leakage.
• Check that no tools or materials were left beneath the vehicle.
• Re-torque wheel lug nuts after 50-100 miles of driving to verify proper seating.

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