How to Inspect Differential Gears for Wear

by Valerie David
Photos c2008 by Valerie David.

When changing the differential fluid, it's always a good idea to inspect the gears while you've got the differential cover off. The gears work in a precise way, and if any piece is set wrong it will throw off the entire system. Inspecting the differential gears for wear can help you diagnose a host of howling, whining, rattling and knocking noise problems at the rear of the vehicle.

Step 1

Place an oil drain pan underneath the differential. To inspect the gears you need to see inside, so you'll have to remove the differential cover and drain the system first. Keep shop rags handy for any spills.

Step 2

Use a flashlight or shop light for a clear view of the interior. The large outer section shown here is the ring gear. Check that there are no chips or damage to the ends and edges of the teeth. The surface of the ring gear teeth should be smooth. If it looks like wood grain, with patterns of scoring all across, this is NOT normal wear and the gear needs to be replaced.

Step 3

Inspect the interior gears, referred to as the differential side gears or "spider gears". Here you can see the shine down the center of the gears which is a normal wear pattern.

Step 4

Check the side gears for any breaks or cracks. Here we may have a bit of chipping at the edges.

Step 5

Check the edges, ends, and surface of this smaller exterior gear, called the pinion gear. Look for any scoring, cracks or breaks.

Step 6

Examine the teeth on the pinion gear as they fit into the larger ring gear. Normal patterns of wear for both these gears is perfectly down the center of the gear teeth. If you see wear near the edges or on alternating ends of the gears, then there is an adjustment problem. If one gear is set too far back or too close, the gears won't fit together properly and will continually collide in the wrong spot as they turn. There will be irregular wear patterns and eventually damage to the teeth.

Step 7

Slowly spin one of the back wheels to turn the gears and bring the rest of the teeth into view. (Do NOT turn the wheel if you've removed any part of the differential assembly.) Only part of each gear is visible at one time, so you'll need to spin the wheel in increments to examine every part of the gear assembly.

Step 8

Make note of each gear with irregular wear patterns, chips and breaks, so you know what adjustments and replacement parts are needed. Unless you have experience working with differentials, you should consult a professional auto repair technician.

Take the car for a quick test drive around the block. When you return, check the fill plug and cover to be sure there are no leaks.


  • After inspection you'll have to replace the cover, seal the system, and refill with differential fluid. NEVER attempt to drive the vehicle without differential fluid. Get more information on service for your differential with my eHow articles "How to Drain a Rear Differential" and "How to Fill a Rear Differential".
  • Learn about the engineering of differentials and how each component and gear of the assembly works in the textbook "Automotive Manual Transmissions and Power Trains" by William H. Crouse and Donald L. Anglin, published by McGraw-Hill.


  • Follow all manufacturer's instructions and warnings when dealing with fluids, cleaners, and sealants.
  • Differential fluid has a noxious odor. Be sure to work in a well-ventilated area!

Items you will need

  • Oil catch pan
  • Shop rags
  • Flashlight or shop light

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