GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2005 🢒 XLR 🢒 General Information 🢒 Vibration Diagnosis and Correction 🢒 Vibration Analysis - Road Testing

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Obtaining rotational speed for the components rotating at both tire/wheel speed and driveline speed is critical to systematically eliminating specific vehicle component groups. These component rotational speeds can be generated by using the J 38792-VS , Vibrate Software, or through calculating them manually.

  2. Important: Be certain to OBSERVE for disturbances that match the customer's description FIRST, then look at the J 38792-A frequency which corresponds with that disturbance.

    Proper location of the J 38792-A , Electronic Vibration Analyzer (EVA) 2, sensor onto the component which is most excited by the vibration disturbance is critical to obtaining an accurate frequency reading.

    This test will duplicate virtually any disturbance which occurs while the vehicle is in motion.

  3. Accelerate to a speed high enough above the speed of the disturbance to allow for the time needed to shift into NEUTRAL and for the engine to decrease in RPM to idle speed, before coasting down through the disturbance range.

    4.

  4. This test will either eliminate or confirm the engine as a contributing cause of the customer concern.

Step

Action

Yes

No

Caution: Road test a vehicle under safe conditions and while obeying all traffic laws. Do not attempt any maneuvers that could jeopardize vehicle control. Failure to adhere to these precautions could lead to serious personal injury and vehicle damage.

 

1

Did you review the Diagnostic Starting Point - Vibration Diagnosis?

Go to Step 2

Go to Diagnostic Starting Point - Vibration Diagnosis and Correction

2

Visually inspect the tire and wheel assemblies, steering components and suspension components for any possible faults.

Are the tire and wheel assemblies, steering components and suspension components in good working condition?

Go to Step 4

Go to Step 3

3

Important:: Do NOT operate the vehicle until the faults are corrected. Correct the faults with the tire and wheel assemblies, steering components, and/or suspension components before proceeding.

Did you correct the faults with the tire and wheel assemblies, steering components, and/or suspension components?

Go to Step 4

Go to Step 2

4

  1. Obtain the drive axle ratio.
  2. If the J 38792-VS , Vibrate Software, IS available, obtain the transmission gear ratios.
  3. If the J 38792-VS is NOT available, take note of the tire size on each axle, then calculate the tire rotational speed for each size tire used and calculate the driveline rotational speed. Refer to Component Rotational Speed Calculation .

Did you obtain the powertrain ratio for use with the J 38792-VS , or calculate the component rotational speeds, if J 38792-VS is NOT available?

Go to Step 5

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5
  1. Install a scan tool.
  2. With the scan tool, bring up the Powertrain Control Module data list and select Engine Speed.

Is the scan tool operating properly?

Go to Step 6

Go to Scan Tool Does Not Communicate with Class 2 Device in Computer/Integrating Systems

6

Install the J 38792-A , Electronic Vibration Analyzer (EVA) 2.

Is the J 38792-A operating properly?

Go to Step 7

Go to Electronic Vibration Analyzer (EVA) Description and Operation

7

Did the customer concern indicate that the vibration occurs ONLY while the vehicle is standing still?

Go to Vibration Analysis - Engine

Go to Step 8

8

  1. Install the J 38792-A sensor to the component identified by the customer as most respondent to the vibration. If no component was identified, install the J 38792-A sensor to the steering column. You may have to move the sensor to other locations later.
  2. Select a smooth, level road and slowly accelerate the vehicle up to highway speed.
  3. Observe the vehicle for disturbances that match the customer's description and note the following conditions:
    4. • The vehicle speed
    • The engine RPM
    • The transmission gear range and the specific gear
    • The vibration frequency reading, if detected by the J 38792-A
    • The feel and/or sound of the disturbance
  4. If the vibration seems to excite a particular component of the vehicle more than the steering column, then move the J 38792-A sensor onto that component and repeat steps 2 and 3.

Were you able to duplicate the customer's concern?

Go to Step 9

Go to Vibration Diagnostic Aids

9

  1. Accelerate the vehicle to a speed higher than the speed at which the disturbance occurs.
  2. Shift the vehicle into NEUTRAL and allow the vehicle to coast down through the disturbance range.

Does the disturbance still occur while coasting-down in NEUTRAL?

Go to Step 11

Go to Step 10

10

  1. Select a smooth, level road and slowly accelerate the vehicle up to the speed at which the disturbance occurs.
  2. Decelerate and safely downshift by one gear range.
  3. Operate the vehicle at the same VEHICLE SPEED at which the disturbance occurs.

Does the disturbance still occur while going the same vehicle speed in a lower gear range?

Go to Vibration Analysis - Driveline

Go to Vibration Analysis - Engine

11

Did the J 38792-A detect a dominant frequency?

Go to Step 12

Go to Symptoms - Vibration Diagnosis and Correction

12
  1. If the J 38792-VS IS available, use the drive axle ratio, the specific transmission gear ratio, and the engine RPM to make a comparison to the dominant frequency reading recorded.
  2. If the J 38792-VS is NOT available, compare the dominant frequency reading recorded to the component rotational data which you calculated previously.

Does the frequency data clearly fall within the tire/wheel parameters ONLY?

Go to Vibration Analysis - Tire and Wheel

Go to Symptoms - Vibration Diagnosis and Correction