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Rear Wheel Differential: Diagnosis Rear Wheel Differential: Diagnosis From "Trade School" episode DTRS-104

Figure A

In this episode of Trade School, we follow the education of Sarah Powell, an auto technician-in training (figure A). With today's modern vehicles, auto technicians often rely on computer diagnostics to isolate problems within a vehicle. Becoming a certified auto technician, however, still requires identifying and fixing many problems without the aid of a computer....

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Figure B

In this segment, with guidance from her instructor, Sarah performs a number of checks to diagnose a noisy rear-wheel differential. This differential is located between the two rear wheels of the car and acts as the final distribution of engine power (figure B). This power, or engine torque, travels through the transmission, which turns the drive shaft. The driveshaft connects to the differential and through a specific configuration of gears turns the wheels. The differential earned its name because it divides the power to the wheels while allowing them to rotate at variable speeds.

Basic Tools:

White Correcting Fluid
Dial Indicator
Torque Wrenches
Socket Wrenches
Blue Transfer Fluid
Brass drifts
Hammers

• After removing the differential from its case under the car, instructor Al Dragoo helps Sarah to secure it in a table vise (figure C).

• Sarah first checks the ring gear for backlash. The ring gear is the largest gear in a differential. Backlash is the amount of clearance between the teeth of the ring gear and the pinion gear hidden inside the case. She secures a dial indicator to the component and positions the stylus so that it barely touches one tooth on the ring gear (figure D).

  Figure C

  Figure D

  
  

• A dial indicator is a precision measuring tool that can measure tiny distances. By rocking the ring gear by hand, Sarah measures the clearance between the teeth of the ring and pinion gears. She finds that the clearance exceeds the manufacturer specs. Her diagnosis: the ring gear is worn and needs replacing.
  

  Figure E

• Next, Sarah performs a lateral run-out check on the ring gear. This measures any side-to-side movement of the ring gear. She moves the dial indicator so that the stylus is flat and perpendicular to the flat surface of the ring gear (figure E). This time, she pre-loads the dial indicator by pressing the stylus onto the gear. Then, using white correcting fluid, she marks a starting spot on the gear.

• She rotates the gear 360 degrees, noting any variance measured with the dial indicator. By adding the amount of positive change to the amount of negative change, she calculates a run-out of 6/1000ths of an inch, a little too much based on the specs. Her diagnosis: the ring gear could be warped.
  

  Figure F

• Before disassembling the differential, Sarah checks the ring tooth contact pattern. This test determines how the ring gear teeth engage the pinion gear teeth. She brushes on a coat of blue marking fluid between six adjacent teeth (figure F).
  

  Then she rotates the ring gear several times while Al applies pressure to the ring gear using a screwdriver (figure G). She inspects the blue ink and finds that the pinion gear teeth are engaging the ring gear teeth near the toe, or lower-most section of each tooth (figure H). This engagement should happen in the middle section of the teeth. This indicates that the ring gear is severely worn.

In the next segment, Sarah disassembles the differential and furthers her diagnosis ....

Figure G

Figure H