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Axle Noise Diagnosis

From MasterTech issue #10 October 1999
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Whenever a vehicle is in motion, the driveline produces sounds that can be a normal part of operation or sounds that may indicate trouble. Customers can sometimes find the sounds objectionable, regardless of whether or not there is a condition.

Sounds that seem to be coming from an axle (Fig. 1) can be especially tricky to diagnose, and if there is a condition in the axle, you need in-depth knowledge of axle adjustment procedures to perform the proper service.

Figure 1

Sounds that appear to be coming from an axle should be thoroughly investigated before adjustments are made.

Using the Six-Step Troubleshooting Procedure (Fig. 2), you will see how to verify and define an axle sound complaint by first asking key questions of the customer, which all service advisors should know. You will also become familiar with sound isolation methods. In addi- tion, we will take a look at the special tools you will need for axle service, and you will also see the steps involved in an axle inspection and analysis, which will help you find and fix any conditions.

We will be using a 1999 Grand Cherokee as an example, though much of the information covered can be applied to any rear-wheel-drive vehicle.

  • Step 1 Verify the Complaint
  • Step 2 Verify Related Symptoms
  • Step 3 Analyze the Symptoms
  • Step 4 Isolate the Condition
  • Step 5 Service the Condition
  • Step 6 Verify the Adjustment

Figure 2

The Six-Step Troubleshooting Procedure.

Verify the Complaint

You probably already know that verifying the customer's complaint is the first step in making a repair, but when dealing with axle sound concerns, this step is particularly important in determining whether there is an issue and if so, exactly what it is. Technicians sometimes get a description of the condition that lacks the detail they need, which makes diagnosis and service more difficult and time-consuming.

Verification begins with detailed input from customers, and since they are usually the first point of contact, service advisors must translate customer descriptions into inforation that is usable to the technicians. The service advisor should ask the customer very specific questions about the nature of the concern and the conditions under which it occurs, especially when objectionable axle-like sounds are suspected.

First, ask the customer if the condition is a sound, which is heard, a vibration, which can be felt, or a combination of both. Be sure to ask if the symptom is constant or intermittent, and ask if there are any associated symptoms.

Do not forget to ask if the sound is heard while the vehicle is stationary. If so, it would tell you right away that the sound is not axle-related. The engine speed and vehicle speed at which the symptom occurs are also important to the technician, so be sure to ask.

You also need to ask the customer about other vehicle conditions associated with the symptom, including the operating temperature, load, and vehicle maneuvering, such as a turn. Do not neglect to ask about the road conditions, such as dry or wet, or smooth or rough. They can have an effect as well.

Ask about any other conditions you think might be contributing to the concern. Remember, the technician does not always get to talk to the customer, so you can never give him too much information.

In order to verify that a sound concern exists, the next course of action is a road test to try to duplicate the condition. Remember that the road test should be performed under safe road and traffic conditions. A subjective rating system should be used during your road test to determine if a sound is present, and, if so, how bad the sound is. You can compare this rating to one made after performing an axle adjustment to determine if the adjustment was successful in reducing the sound.

First, operate the vehicle in the same way that the customer said produced the condition. Note whether or not a sound is detected.

Road test the vehicle under different drive modes (Fig. 3) and speed ranges (Fig. 4) and record your sound level ratings. The different drive modes include light drive, which is a slight, steady acceleration through the target speed; float, where the target speed is maintained as the gears load and unload on the drive side; heavy drive, which is rapid acceleration through the target speed without inducing a downshift; coast, or deceleration through the target speed; and cruise, where the target speed is maintained with light throttle pressure - not accelerating or decelerating - and the gears are loaded on the drive side.

Perform all drive modes within the different speed ranges, which are shown in Figure 4.

ROAD-TEST DRIVE MODES

  • LIGHT DRIVE
  • FLOAT
  • HEAVY DRIVE
  • COAST
  • CRUISE
 

ROAD-TEST SPEED RANGES

  • 25 mph - 35 mph
  • 35 mph - 45 mph
  • 45 mph - 55 mph
  • 55 mph - 65 mph
  • 65 mph and above
Figure 3 - Different drive modes.
Figure 4 - Different speed ranges.

If you find no reason for concern, return the vehicle. You may need to talk with the customer and explain that certain sound levels are normal, especially with trucks and SUVS. If you verify that a sound concern is present, you will need to check for any related symptoms and isolate the condition.

Part 1 Verify the Complaint
Part 2 Verify Related symptoms
Part 3 Analyze the symptoms.
Part 4 Isolate the Condition
Part 5 Axle Inspection and Analysis
Part 6 Service the Condition
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Last Update: December 20, 1999