How to check a rear differential

Written by robert moore
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How to check a rear differential
The gear assembly inside the differential changes the rotation of power coming from the transmission. (maquette image by Dominique LUZY from

All rear-wheel-drive cars and trucks have a rear differential. The rear differential uses a gear set to change the rotation of the transmission and driveshaft by 90 degrees so the wheels will rotate forwards and backwards. The rear differential also allows the rear wheels to rotate at different speeds during turns. The rear differential must be lubricated to prevent excessive wear on the internal gears.

Skill level:

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Things you need

  • Flashlight
  • 3/8-inch drive ratchet

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  1. 1

    Park the vehicle on as level a surface as possible. If you must lift the vehicle to get underneath it, you will need to lift the entire vehicle to keep it level.

  2. 2

    Inspect the differential cover plate with the flashlight, and look for any leaks around the edges of the plate where it meets with the rear differential.

  3. 3

    Remove the check bolt by placing the three-eighths-inch-drive ratchet into the square hole on the check bolt.

  4. 4

    Check the level of fluid by placing a finger into the hole. The fluid should be level with the bottom of the check hole. Place a finger into the hole to get a bit of fluid. The colour should be light tan. If it is a different colour, the differential fluid needs to be changed.

  5. 5

    Thread the check bolt back into the differential cover. Tighten the bolt down with the three-eighths-inch-drive ratchet; do not overtighten.

Tips and warnings

  • If the differential fluid is low, this may indicate a leak at the axle seals leading to the rear wheels.
  • A howling sound coming from the rear end is a common sign of either low differential fluid or extremely worn gears.
  • Do not drive the vehicle if there is no differential fluid or there is a howling noise. If the differential locks up during operation, both rear wheels will lock up, and it could cause an accident.

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