GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2008 🢒 Silverado - 4WD 🢒 General Information 🢒 Vibration Diagnosis and Correction 🢒 Vibration Analysis - Engine/Accessory Isolation

Test Description

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

  1. A loose, damaged, misaligned, or defective powertrain insulator and/or bracket may create a transfer path into the passenger compartment.

    2.

  2. A loose, damaged, misaligned, or defective exhaust system insulator and/or bracket may create a transfer path into the passenger compartment.

    3.

  3. Incorrectly seated and/or aligned powertrain components and/or exhaust system components may create a transfer path into the passenger compartment.

    When loosening powertrain mounts in order to re-bed the powertrain observe the following:

    Do not loosen the mount bracket-to-engine bolts/nuts, do not loosen the mount bracket-to-vehicle frame bolts/nuts if mount brackets are used.

    Loosen the mount-to-mount bracket bolts/nuts if mount brackets are used, or loosen the mount-to-slotted holes in vehicle frame bolts/nuts if a direct-mount design is used.

    4.

  4. Non-rotating engine driven accessory component systems can no longer produce a unique disturbance.

    5.

  5. Non-rotating engine driven accessory components can no longer produce a unique disturbance. If a disturbance is still present, but the characteristics have been altered, it is possible that these component systems are acting as a transfer path for engine firing frequency or a first order engine disturbance.

    If a disturbance is still present, but the characteristics have NOT been altered, it is NOT likely that these component systems are acting as a transfer path for engine firing frequency or a first order engine disturbance.

    6.

  6. If the mark placed on the face of an engine driven accessory pulley seems to stand still while running this test, then that accessory system is either responding to an existing frequency, such as engine firing pulses, or creating a disturbance.

    7.

  7. A loose, damaged, misaligned, or defective engine driven accessory system insulator and/or bracket may create a transfer path into the passenger compartment.

    8.

  8. Removing the engine driven accessory and bracket, or brackets from the engine allows a thorough inspection to determine if any conditions are present that may create a transfer path into the passenger compartment.

Step

Action

Yes

No

Caution: Refer to Work Stall Test Caution in the Preface section.

 

1

Were you sent here from the Vibration Analysis - Engine table?

Go to Step 2

Go to Vibration Analysis - Engine

2

Is the J 38792-A , Electronic Vibration Analyzer (EVA) 2, available?

Go to Step 3

Go to Step 5

3
  1. Using the engine RPM and frequency data recorded for the customer's vehicle, determine the order of engine rotation to which the disturbance is related.
  2. Determine the possible causes of the disturbance as it relates to a specific order of engine rotation.

Refer to Engine Order Classification .

Does the Engine Order Classification table indicate that the disturbance is of the same order as the engine firing frequency?

Go to Step 5

Go to Step 4

4

Does the Engine Order Classification table indicate that the disturbance is likely related to engine driven accessories?

Go to Step 8

Go to Step 5

5

Inspect the powertrain mounts--engine, transmission, transfer case, and direct-mount drive axle, if equipped--and any powertrain braces for the following conditions:

    • Loose and/or missing fasteners
    • Improper alignment
    • Cracked, dry-rotted, and/or oil-soaked insulators
    • Twisted, broken, torn, and/or collapsed insulators
    • Bent, twisted, and/or deformed brackets

Realign or replace powertrain mounts as indicated by the inspection.

Did you find and correct a condition?

Go to Step 13

Go to Step 6

6

Inspect the exhaust system components for the following:

    • Loose and/or missing fasteners
       - Heat Shields
       - Joints and/or couplings:
       - Nuts, bolts, studs, clamps, straps
       - Bracket and/or insulator mounting
    • Inadequate clearance to body and/or chassis components
        Inspect with the exhaust system both COLD and HOT; in NEUTRAL, FORWARD and REVERSE gears
    • Improper alignment
    • Disconnected and/or missing insulators
    • Cracked, dry-rotted, and/or oil-soaked insulators
    • Stretched, twisted, broken, torn, and/or collapsed insulators
    • Bent, twisted, cracked, and/or deformed brackets

Repair, replace, and/or realign exhaust system components as indicated by the inspection.

Did you find and correct a condition?

Go to Step 13

Go to Step 7

7

Perform the Powertrain Mount Balance Procedure if available or perform the following procedure to re-bed the powertrain:

  1. Loosen, but do not remove, all powertrain mounts and exhaust system hangers.
  2. Ensure that the exhaust flexible coupling, if equipped, moves freely.
  3. Start the engine.
  4. Settle the powertrain by shifting the transmission from DRIVE to REVERSE.
  5. Place the transmission into NEUTRAL.
  6. Turn OFF the ignition.
  7. Tighten all of the loosened fasteners with the powertrain in a relaxed position.

Did you complete the operation?

Go to Step 13

--

8

Notice: Do not run the engine for longer than 60 seconds with the accessory drive belt, or belts removed, or overheating and/or damage may result.

  1. Remove the engine accessory drive belt, or belts.
  2. Block the front wheels.
  3. Apply BOTH the service brakes and the park brake.
  4. With the scan tool still installed, start the engine.
  5. Place the transmission in NEUTRAL or PARK.
  6. Increase the engine RPM to the level recorded during duplication of the disturbance.
  7. Allow the engine to idle, then place the transmission in DRIVE.
  8. Increase the engine RPM to the level recorded during duplication of the disturbance.
  9. Turn OFF the ignition.
  10. Install the engine accessory drive belt, or belts.

Was the disturbance significantly reduced or eliminated?

Go to Step 10

Go to Step 9

9

Were the characteristics of the disturbance altered but still present?

Go to Step 11

Go to Step 15

10

  1. Mark the face of the suspected accessory pulleys, including any related idler pulleys, near the outer edge with a paint mark.
  2. Install the J 38792-25 , Inductive Pickup Timing Light to the J 38792-A .

    3. For information on the use of the EVA features, refer to Electronic Vibration Analyzer (EVA) Description and Operation .

  3. Block the front wheels.
  4. Apply BOTH the service brakes and the park brake.
  5. With the scan tool and the J 38792-A , still installed, start the engine.
  6. Select the Smart Strobe feature on the J 38792-A .
  7. Enter the recorded frequency of the disturbance as the initial frequency for strobe operation.
  8. Have an assistant place the transmission in NEUTRAL or PARK.
  9. Slowly increase the engine RPM to the level recorded during duplication of the disturbance, then maintain that speed.
  10. Using the J 38792-25 , check each of the suspected accessory pulleys to determine if any of them is related to the frequency of the disturbance.

    11. Check each of the accessory systems, both engaged and under maximum load and disengaged or under minimum load.

  11. Allow the engine to idle, then place the transmission in DRIVE.
  12. Slowly increase the engine RPM to the level recorded during duplication of the disturbance, then maintain that speed closely.
  13. Using the J 38792-25 , check each of the suspected accessory pulleys to determine if any of them is related to the frequency of the disturbance.

    14. Check each of the accessory systems, both engaged and under maximum load and disengaged or under minimum load.

  14. Turn OFF the ignition.

Did you identify an engine driven accessory system as being related to the frequency of the disturbance?

Go to Step 11

Go to Vibration Diagnostic Aids

11

Inspect the components of the engine driven accessory system for the following:

    • Loose and/or missing fasteners
       - Heat Shields, if equipped
       - Joints and/or couplings:
       - Nuts, bolts, studs, clamps, straps
       - Bracket and/or insulator mounting
    • Inadequate clearance to body and/or chassis components
        Inspect with the accessory system both under a LOAD and NOT loaded
    • Improper alignment
    • Bent or damaged pulleys
    • Disconnected and/or missing insulators
    • Cracked, dry-rotted, and/or oil-soaked component insulators
    • Stretched, twisted, broken, torn, and/or collapsed component insulators
    • Bent, twisted, cracked and/or deformed component brackets

Repair, replace, and/or realign the engine driven accessory system components as indicated by the inspection.

Did you find and correct a condition?

Go to Step 13

Go to Step 12

12

  1. Remove the engine driven accessory and bracket, or brackets from the engine.
  2. Thoroughly inspect the accessory bracket, or brackets, bolts/nuts/studs, and the accessory itself for signs of the following:

    3. Bent, twisted, cracked and/or deformed conditions

  3. Replace any of the components found to exhibit any of these conditions.
  4. Reinstall the components to the engine.

Did you find and correct a condition?

Go to Step 13

Go to Step 17

13

Check the vehicle to determine if the disturbance is now significantly reduced or eliminated. Perform the following steps:

  1. Install a scan tool into the customer's vehicle.
  2. Install the J 38792-A , if available, into the customer's vehicle; place the sensor in exactly the same location as it was originally placed in the vehicle.
  3. Block the front wheels.
  4. Apply BOTH the service brakes and the park brake.
  5. Start the engine.
  6. Place the transmission in NEUTRAL or PARK.
  7. Slowly increase the engine RPM to the level at which the disturbance was most noticeable.
  8. Record the engine RPM obtained on the scan tool and the most dominant frequency reading if obtained on the J 38792-A , if available.
  9. Place the transmission in DRIVE.
  10. Slowly increase the engine RPM to the level at which the disturbance was most noticeable.
  11. Record the engine RPM obtained on the scan tool and the most dominant frequency reading if obtained on the J 38792-A , if available.
  12. If the disturbance has been significantly reduced or eliminated, confirm the results by placing the transmission into REVERSE, then repeat steps 10 and 11. Reverse-loading of the powertrain may increase or change the characteristics of the vibration.

Has the disturbance been significantly reduced or eliminated?

Go to Step 18

Go to Step 14

14

Have you investigated powertrain isolation as a possible cause of the disturbance?

Go to Step 15

Go to Step 5

15

Have you investigated engine driven accessories as a possible cause of the disturbance?

Go to Vibration Analysis - Engine Balance

Go to Step 16

16

Is the J 38792-A available?

Go to Step 8

Go to Vibration Diagnostic Aids

17

Replace the engine driven accessory component causing the disturbance.

Did you complete the replacement?

Go to Step 18

--

18
  1. Install or connect any components that were removed or disconnected during diagnosis.
  2. Perform the Vibration Analysis - Road Testing table. Refer to Vibration Analysis - Road Testing .

Is the disturbance still present?

Go to Step 2

System OK