3.2 CV Joints, Boots, and Axles

CV Joints, Boots, and Axles

Quick Answer: CV axles transfer power to the drive wheels while accommodating steering and suspension movement. Outer CV joints click during sharp, accelerating turns when worn. Inner CV joints produce a side-to-side body shudder under acceleration load when worn. Torn CV boots must be serviced immediately to prevent dirt from grinding down the joint's internal bearings.

Constant velocity (CV) axle assemblies, or half-shafts, transfer torque in FWD, AWD, and 4WD vehicles. They transmit engine power smoothly while allowing wheels to travel vertically and steer.

CV Joint Construction and Mechanics

A standard CV axle consists of an axle shaft with two joints: an inner joint and an outer joint, protected by flexible boots that seal in grease and keep out road contaminants.

• Outer CV Joints: Typically Rzeppa-type joints. Six spherical balls are positioned in a cage between inner and outer races. This allows the joint to bend up to 45 degrees or more for steering, but does not allow axial length changes (plunge).
• Inner CV Joints: Typically tripod plunge joints with a three-legged spider and rollers that slide inside a housing. This allows the axle shaft to slide in and out (plunge) as distance changes during suspension movement.

CV Joint Diagnostics and Failure Symptoms

Due to differing designs and steering loads, inner and outer CV joints fail with distinct symptoms.

1. Outer CV Joint Failure: Worn outer joints develop excess clearance in their ball tracks.
   • Symptom: A worn outer joint produces a rhythmic clicking, popping, or snapping noise during sharp turns while accelerating (e.g. cornering at intersections). Noise frequency increases with wheel speed. Drive the vehicle in tight circles in both directions to diagnose.
2. Inner CV Joint Failure: Worn inner joints experience roller binding in their plunge tracks under acceleration.
   • Symptom: A worn inner joint does not click on turns. Instead, it produces a heavy side-to-side body shudder or vibration under acceleration or load (climbing hills). This vibration disappears when coasting. Worn inner joints can also clunk when engaging gears.

CV Boot Inspection and Repack Service

CV boots are the lifeline of the CV joint. Visually inspect them during every service.

• Torn Boot Inspection: A torn boot slings thick, dark grease in a radial pattern onto wheels, brakes, and frame. If cracked, split, or missing clamps, it must be serviced immediately.
• The Failure Process: A torn boot lets centrifugal force throw out grease, while road dust and water enter. This creates an abrasive grinding paste that destroys the precision steel balls and races in a short time.
• Boot Replacement Procedure: If caught early before noise begins, the joint can be saved:
  1. Remove CV axle from vehicle.
  2. Disassemble joint and wash away all old grease in a solvent tank.
  3. Inspect bearings and races. Replace the axle assembly if there is pitting, scoring, or blueing.
  4. If smooth, slide the new boot onto the shaft.
  5. Repack with specified high-temperature molybdenum disulfide ("moly grease"). Chassis grease is unacceptable.
  6. Secure clamps using a CV boot banding tool for a liquid-tight seal.

Front Axle Seal Inspection

Where the inner joint stub shaft plugs into the transaxle, a rubber lip seal prevents transmission fluid leakage.

• Leaking Seal Symptoms: Wetness or fluid drips at the transaxle-axle junction. Leaks lead to low fluid levels, causing slipping or transaxle failure.
• Replacement: Inspect the seal when replacing an axle. Install the new axle shaft carefully; sharp splines can nick the rubber lip or dislodge the seal's garter spring, causing immediate leaks.

CV Axle Diagnostics and Maintenance Guide

graph TD
    A[Vehicle Drivetrain Symptom] --> B{Noise or Vibration?}
    B -->|Noise| C{When does noise occur?}
    B -->|Vibration| D{When does vibration occur?}
    C -->|Clicking on Sharp Turns| E[Failed Outer CV Joint]
    C -->|Clunking on Deceleration| F[Failed Inner CV Joint Plunge Track]
    D -->|Shudder under Acceleration| G[Failed Inner CV Joint]
    D -->|Vibration at Cruise Speed| H[Axle Shaft Out-of-Balance or Bent]
    E --> I[Replace Outer Joint or Axle Assembly]
    F --> J[Replace Inner Joint or Axle Assembly]
    G --> J
    H --> K[Perform Axle Runout Test]