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Gear oil weeping from a differential seal

Both differentials on this truck use seals at the pinion nose and where the shafts leave the housing, and the specialist supplier lists four separate part numbers to cover those positions — which is a fair map of where the oil comes out. The failure is slow and quiet, which is exactly the danger: the level falls over months and there is no gauge, no warning light and no noise until the gears have already been running short. A forum thread about the same axle notes that the distinctive smell of gear oil is usually what tells an owner first, before any stain is obvious. One cause worth checking before you condemn a seal is the housing breather. If it is blocked, the housing pressurises as it heats and pushes oil past seals that were doing their job perfectly well, and clearing it costs nothing.

Gen 32001–2006Damages other partsCommunity consensus

Drive gently — repair soonManeje con cuidado — repare pronto

Community consensus. This entry is not backed by factory documentation — treat its values and steps as a starting point, not as if they came from the factory manual.

Consenso de la comunidad. Esta entrada no se apoya en documentación de fábrica — tome los valores y los pasos como punto de partida, no como si vinieran del manual de fábrica.

Symptoms

  • An oily film around the pinion nose or the side of a differential housing
  • Oil flung along the underside or onto the inner face of a wheel
  • The oil level is below the filler plug when you check it
  • A whine that rises and falls with road speed rather than with engine speed

Likely causes

  1. The pinion seal at the nose of the differential has hardened and is passing oil
  2. A side shaft seal has hardened and is passing oil
  3. The housing breather is blocked and the pressure is pushing oil past the seals
  4. The differential has already been damaged by running short of oil

Diagnostic steps

  1. Clean both differential housings, drive for a week and look again at where the oil beginsPoints toThe pinion seal at the nose of the differential has hardened and is passing oil · A side shaft seal has hardened and is passing oil
  2. Work out whether the wet trail begins at the pinion nose or where a shaft leaves the housingPoints toThe pinion seal at the nose of the differential has hardened and is passing oilRules outA side shaft seal has hardened and is passing oil
  3. Check the breather is clear, its hose is not crushed, and it is routed upwardPoints toThe housing breather is blocked and the pressure is pushing oil past the seals
  4. Check the level at the filler plug, and if you drain it look for metal in the oilPoints toThe differential has already been damaged by running short of oil

Fix paths

Clear the breather and correct the level

$Difficulty 2/5

The cheapest possible outcome, and it costs an hour to confirm. Do it before ordering seals, not after.

Replace the seal the trail points at

$$Difficulty 4/5

The positions take different part numbers, so identify which one is leaking before ordering. Refill to the level the manual specifies rather than to where it looks right.

Partsgen3-differential-seal

Have the differential opened and assessed

$$$–$$$$Difficulty 5/5

What is left if the level was low for a long time and the whine has already started. Ask for the drained oil to be shown to you — metal in the pan changes what a rebuild is worth.

Sources

  1. Luso Overland — MONTERO Gen3 Differential Seal, four positions (MB393883, MB664285, MR350883, MR580530)Vendor cataloguearchived copy
  2. Pajero 4WD Club of Victoria — «NP Diff Problems» (oil escaping past a rear shaft, and the smell as the first sign)Owner forumarchived copy