Rear Main Seal

This procedure involves removing the transmission to access and replace the rear main crankshaft seal on the 5.9L Cummins diesel engine, preventing oil leaks between the engine and transmission.

Warnings

Tools required

Parts

• Rear main crankshaft seal × 1 — Use OEM Cummins specification
• Transmission to engine gasket or RTV sealant × 1 — Use OEM specification
• Oil pan gasket (if disturbed) × 1 — Use OEM specification
• Engine oil × 1 — 15W-40 diesel oil or 5W-40 synthetic
• Transmission fluid (automatic or manual as equipped) × 1 — ATF+4

Fluids

• engine_oil — 12 qt
• transmission_auto — 20 qt

Preparation

1. Disconnect both battery cables, negative first, then positive
2. Raise vehicle on a lift or with floor jack and support securely on jack stands at all four corners
3. Remove skid plates and belly pans if equipped
4. Drain engine oil completely and remove oil filter
5. Place transmission jack under transmission and secure with safety chains
6. Place engine support bar across engine bay or attach engine hoist to support engine weight
7. Mark driveshaft position relative to differential flange and transfer case output with paint pen

Procedure

1. 1

   Remove transfer case and driveshafts
   Drain transfer case fluid into drain pan. Remove front and rear driveshafts by removing U-bolt straps or flange bolts. Support and remove transfer case from transmission by removing mounting bolts. Set transfer case aside safely.
2. 2

   Disconnect transmission connections
   Disconnect transmission electrical connectors including speed sensors, neutral safety switch, and any solenoid connectors. For automatic transmission, disconnect transmission cooler lines and plug openings. Remove shift linkage at transmission. For manual transmission, remove slave cylinder and hydraulic line assembly.
3. 3

   Remove crossmember and support transmission
   Remove exhaust crossover pipe if it interferes with transmission removal. Support transmission firmly with transmission jack. Remove transmission crossmember mounting bolts and lower crossmember. Ensure transmission jack is positioned at balance point of transmission.
4. 4

   Remove starter motor
   Disconnect starter electrical connections. Remove starter mounting bolts and extract starter from bell housing. This provides access to some lower bell housing bolts and allows clearance during transmission removal.
5. 5

   Remove bell housing bolts
   Remove all bell housing to engine block bolts. There are multiple bolts of varying lengths around the perimeter (verify exact count and locations in the service manual). Note bolt locations as different lengths are used in different positions. Support transmission weight with transmission jack as you remove the last bolts.
6. 6

   Separate transmission from engine
   Carefully slide transmission straight back away from engine on transmission jack. Keep transmission aligned with engine to avoid binding. For automatic transmission, ensure torque converter stays with transmission. Continue sliding back until input shaft clears bell housing, approximately 12-18 inches of movement required.
7. 7

   Inspect flywheel and remove rear main seal
   Inspect flywheel or flexplate for cracks, heat damage, or excessive wear. Check ring gear teeth for damage. Using seal puller or pick, carefully pry out old rear main seal from seal bore. Be extremely careful not to score or damage the crankshaft sealing surface or the bore in the block. Clean seal bore thoroughly with solvent and lint-free cloth.
8. 8

   Prepare and install new rear main seal
   Inspect new seal for any damage or defects. Apply a thin film of clean engine oil to the outer diameter of the seal and the seal lip. Using the proper rear main seal installation tool (Cummins rear main seal installer, verify correct tool number in the service manual, or equivalent), carefully drive the seal into the bore evenly and squarely. The seal must be installed flush with the rear face of the block or slightly recessed per specification. Do not drive the seal in at an angle or use improper tools such as hammers or sockets, which will damage the seal.
9. 9

   Verify crankshaft and seal condition
   Apply a thin film of clean engine oil to the crankshaft sealing surface. Inspect crankshaft surface for grooves, scratches, or damage where seal rides. Minor imperfections can be polished with fine emery cloth. Ensure seal lip is properly oriented toward engine interior and not flipped or damaged.
10. 10

    Reinstall transmission
    Clean bell housing mating surface on engine block and transmission. For automatic transmission, verify torque converter is fully seated in transmission pump (should have multiple clicks when pushed in). Apply thin bead of RTV sealant or install new gasket to bell housing mating surface if required. Carefully align transmission with engine using transmission jack, ensuring input shaft aligns with clutch disc (manual) or torque converter hub (automatic). Slide transmission forward until bell housing contacts engine block.
11. 11

    Install bell housing bolts and starter
    Install all bell housing to engine block bolts in their correct locations. Tighten bolts in a cross pattern to draw transmission evenly to engine. For automatic transmission, install torque converter to flexplate bolts through starter opening and rotate engine as needed to access all bolts. Reinstall starter motor and connect electrical connections.
12. 12

    Reinstall crossmember and transfer case
    Raise transmission to proper height and reinstall transmission crossmember. Tighten crossmember bolts securely. Remove transmission jack. Reinstall transfer case to transmission with new gasket or sealant. Reconnect all transmission electrical connectors, shift linkage, and transmission cooler lines. Install front and rear driveshafts aligning marks made during disassembly.
13. 13

    Refill fluids and final checks
    Refill transfer case with approximately 2.0 quarts ATF+4 (verify exact capacity for your specific transfer case model). Install new engine oil filter and refill engine with approximately 12 quarts of appropriate diesel engine oil (verify exact capacity, typically 11-12 quarts with filter, for the 5.9L Cummins). For automatic transmission (48RE), add ATF+4 through the dipstick tube; initial fill of approximately 8-10 quarts, then check and top off, as total capacity with the torque converter drained can approach 14-17 quarts. For manual transmission (G56 6-speed), refill through the fill plug with the manufacturer-specified manual transmission fluid (verify exact fluid type and capacity, typically around 4-5 quarts, in the service manual). Reconnect battery cables, positive first then negative. Double-check that all connections, fasteners, and shields are reinstalled.

Reassembly

1. Torque all fasteners to manufacturer specifications using a calibrated torque wrench (specific torque values not available in database, consult service manual)
2. Ensure driveshafts are aligned properly to avoid vibration
3. Verify all electrical connectors are fully seated and locked
4. For automatic transmission, check fluid level with engine running at operating temperature and add additional fluid as needed to reach proper level on dipstick
5. Reinstall skid plates and belly pans if removed

Verification

• Start engine and verify no oil leaks from rear main seal area while engine idles
• Check transmission fluid level with engine at operating temperature (automatic) or verify proper fill level (manual)
• Test drive vehicle through all gears and verify smooth shifting
• Check for unusual noises or vibrations that could indicate improper driveshaft alignment
• Recheck oil and transmission fluid levels after test drive
• Inspect rear main seal area after 100 miles of driving for any signs of oil seepage
• Verify transfer case engages properly in all ranges (2WD, 4WD high, 4WD low)

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