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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 1462 (Vibration Symptoms Diagnosis And Correction) >> Diagnostic Information and Procedures >> Vibration Analysis - Road Testing >> Test Description

Test Description

WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2004 Oldsmobile Bravada, 2004 GMC Envoy XUV, 2004 GMC Envoy XL, 2004 GMC Envoy, and 2004 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.

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

  1. 4:  Obtaining rotational speed for the components rotating at both tire/wheel speed and propeller shaft 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. See Special Tools and Equipment .
  2. 8: 
    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. See Special Tools and Equipment .
  3. 11:  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. 12:  This test will either eliminate or confirm the engine as a contributing cause of the customer concern.
Vibration Analysis - Road Testing

Step Action Yes No
CAUTION: Refer to Road Test Caution in Cautions and Notices.
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 Correct the fault(s) before proceeding.
Did you correct the fault(s)?		 Go to Step 4  Do NOT Operate the Vehicle
Go to Step 2 
4 
  1. Obtain the drive axle(s) (final drive) ratio(s).
  2. If the J 38792-VS  , Vibrate Software, IS available, obtain the transmission gear ratios. See Special Tools and Equipment .
  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 propeller shaft(s) rotational speed(s). See Special Tools and Equipment . Refer to Component Rotational Speed Calculation  .
Did you obtain the powertrain ratios for use with the J 38792-VS  , or calculate the component rotational speeds (if J 38792-VS  is NOT available)?		 Go to Step 5  -
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 (W/O Immobilizer) or Scan Tool Does Not Communicate with Class 2 Device (W/Immobilizer) in Data Link Communications
6 Install the J 38792-A  , electronic vibration analyzer (EVA). See Special Tools and Equipment .
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. See Special Tools and Equipment . If no component was identified, install the J 38792-A  sensor to the steering column. See Special Tools and Equipment . (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:
    • The vehicle speed (km/h, mph)
    • The engine speed (RPM)
    • The transmission gear range and the specific gear
    • The vibration frequency reading (if detected by the J 38792-A  ). See Special Tools and Equipment .
    • 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. See Special Tools and Equipment .
Were you able to duplicate the customer's concern?		 Go to Step 11  Go to Step 9 
9 Is the vehicle equipped with four-wheel drive or all-wheel drive? Go to Step 10  Go to Vibration Diagnostic Aids 
10
  1. With the J 38792-A  sensor still installed in the same position, activate/engage all-wheel drive. See Special Tools and Equipment .
  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:
    • The vehicle speed (km/h, mph)
    • The engine speed (RPM)
    • The transmission gear range and the specific gear
    • The vibration frequency reading (if detected by the J 38792-A  ). See Special Tools and Equipment .
    • 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. See Special Tools and Equipment .
Were you able to duplicate the customer's concern?		 Go to Step 11  Go to Vibration Diagnostic Aids 
11 
  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 13  Go to Step 12 
12 
  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 
13 Did the J 38792-A  detect a dominant frequency? Go to Step 14  Go to Symptoms - Vibration Diagnosis and Correction 
14
  1. If the J 38792-VS  IS available, use the drive axle(s) (final drive) ratio(s), the specific transmission gear ratio, and the engine speed to make a comparison to the dominant frequency reading recorded. See Special Tools and Equipment .
  2. If the J 38792-VS  is NOT available, compare the dominant frequency reading recorded to the component rotational data which you calculated previously. See Special Tools and Equipment .
Does the frequency data clearly fall within the tire/wheel parameters ONLY?		 Go to Vibration Analysis - Tire and Wheel  Go to Step 15 
15 Does the frequency data clearly fall within the propeller shaft parameters ONLY? Go to Vibration Analysis - Driveline  Go to Symptoms - Vibration Diagnosis and Correction 
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