GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2005 🢒 Bonneville 🢒 General Information 🢒 Vibration Diagnosis and Correction 🢒 Vibration Analysis - Engine Balance

Test Description

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

  1. If sufficient clearance exists to separate the transmission torque converter from the engine flywheel/flexplate, then further tests can be used to isolate the transmission from the engine.

    2.

  2. An engine flywheel/flexplate that has excessive lateral runout, when combined with the mass of the transmission torque converter, can produce a disturbance.

    3.

  3. An engine flywheel/flexplate that is loose at the engine crankshaft or that is cracked or damaged, when combined with the mass of the transmission torque converter, can produce a disturbance.

    4.

  4. This step is designed to isolate the transmission from the engine to determine if the disturbance is related to the engine ONLY.

    5.

  5. Re-indexing the transmission torque converter to the engine flywheel/flexplate alters the balance relationship between the torque converter and the rear of the engine.

    6.

  6. Placing the J 38792-A sensor onto the underside of the engine oil pan along the FRONT and the REAR edge allows for a determination to be made, which will help to narrow down the cause of the disturbance.

  7. An engine flywheel/flexplate that is damaged, misaligned, and/or imbalanced, can produce a disturbance.

    8.

  8. An engine crankshaft balancer that is damaged, misaligned, and/or imbalanced, can produce a disturbance.

Step

Action

Yes

No

Caution: One or more of the following guidelines may apply when performing specific required tests in the work stall:

   • When a test requires spinning the drive wheels with the vehicle jacked up, adhere to the following precautions:
      - Do not exceed 56 km/h (35 mph) when spinning one drive wheel with the other drive wheel stopped. This limit is necessary because the speedometer indicates only one-half the actual vehicle speed under these conditions. Personal injury may result from excessive wheel spinning.
      - If all of the drive wheels are spinning at the same speed, do not exceed 112 km/h (70 mph). Personal injury may result from excessive wheel spinning.
      - All persons should stay clear of the rotating components and the balance weight areas in order to avoid possible personal injury.
      - When running an engine in the repair stall for an extended period of time, use care not to overheat the engine and the transmission.
   • When a test requires jacking up the vehicle and running with the wheels and brake rotors removed, adhere to the following precautions:
      - Support the suspension at normal ride height.
      - Do not apply the brake with the brake rotors removed.
      - Do not place the transmission in PARK with the drive axles spinning.
      - Turn Off the ignition in order to stop the powertrain components from spinning.
   • When running an engine in the work stall, use the exhaust removal system to prevent breathing dangerous gases.

 

1

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

Go to Step 2

Go to Vibration Analysis - Engine/Accessory Isolation

2

  1. Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  2. Remove the flywheel/flexplate-to-torque converter bolts access cover, if equipped.
  3. Determine if sufficient clearance exists to separate the transmission torque converter away from the engine flywheel/flexplate and safely secure the torque converter from accidentally engaging with the flywheel/flexplate.

Is there sufficient clearance to separate and safely secure the transmission torque converter away from the engine flywheel/flexplate?

Go to Step 3

Go to Step 9

3
  1. With the flywheel/flexplate-to-torque converter access cover still removed, and with the vehicle still raised, mark the position of the transmission torque converter in relation to the engine flywheel/flexplate.
  2. Disconnect the torque converter and move it away from the flywheel/flexplate.
  3. Secure the transmission torque converter away from the engine flywheel/flexplate.
  4. Lower the vehicle, start the engine and allow the engine to idle.
  5. Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  6. Visually inspect the flywheel/flexplate for excessive lateral runout.
  7. Lower the vehicle.
  8. Turn OFF the ignition.

Did the flywheel/flexplate exhibit excessive lateral runout?

Go to Step 6

Go to Step 4

4
  1. Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  2. Inspect the flywheel/flexplate for the following:
    3. • Looseness at the engine crankshaft
    • Cracks and/or damage
    • Missing balance weights

Did the flywheel/flexplate exhibit any of the conditions listed?

Go to Step 6

Go to Step 5

5

  1. With the transmission torque converter still secured away from the engine flywheel/flexplate to avoid accidental engagement with the flywheel/flexplate, lower the vehicle.
  2. Block the front wheels.
  3. Apply BOTH the service brakes and the park brake.
  4. With the scan tool and the J 38792-A , Electronic Vibration Analyzer (EVA) 2, still installed, start the engine.
  5. Place the transmission in NEUTRAL or PARK.
  6. Slowly increase the engine RPM to the level at which the disturbance is most noticeable.
  7. Record the engine RPM obtained on the scan tool and the most dominant frequency reading if obtained on the J 38792-A .
  8. Turn OFF the ignition.

Has the disturbance been significantly reduced or eliminated?

Go to Step 7

Go to Step 9

6
  1. If the flywheel/flexplate is loose at the engine crankshaft, tighten the flywheel/flexplate mounting bolts in sequence and to specification.
  2. If the flywheel/flexplate is cracked, damaged, and/or has missing balance weights, replace the damaged flywheel/flexplate.

Refer to Engine Flywheel Replacement in Engine Mechanical - 3.8L.

Did you complete the tightening or replacement?

Go to Step 12

--

7

  1. Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  2. Re-index the transmission torque converter to the engine flywheel/flexplate, 120 degrees from its original position.
  3. Reconnect the transmission torque converter to the engine flywheel/flexplate.
  4. Lower the vehicle.
  5. Block the front wheels.
  6. Apply BOTH the service brakes and the park brake.
  7. With the scan tool and the J 38792-A still installed, start the engine.
  8. Place the transmission in NEUTRAL or PARK.
  9. Slowly increase the engine RPM to the level at which the disturbance is most noticeable.
  10. Record the engine RPM obtained on the scan tool and the most dominant frequency reading if obtained on the J 38792-A .
  11. If the disturbance is still noticeable, re-index the torque converter again to obtain the least amount of disturbance.

Has the disturbance been significantly reduced or eliminated?

Go to Step 12

Go to Step 8

8

Replace the out-of-balance transmission torque converter.

Did you complete the replacement?

Go to Step 12

--

9
  1. Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  2. Position the J 38792-A , electronic vibration analyzer (EVA) sensor onto the underside of the engine oil pan, along the FRONT edge.
  3. Lower the vehicle.
  4. Block the front wheels.
  5. Apply BOTH the service brakes and the park brake.
  6. With the scan tool and the J 38792-A , still installed, start the engine.
  7. Place the transmission in NEUTRAL or PARK.
  8. Slowly increase the engine RPM to the level at which the disturbance is most noticeable.
  9. Record the engine RPM obtained on the scan tool and the most dominant frequency reading if obtained on the J 38792-A from the underside of the engine oil pan.
  10. Repeat steps 1 through 9, placing the J 38792-A sensor onto the underside of the engine oil pan, along the REAR edge.

Is the disturbance greater along the FRONT of the engine?

Go to Step 11

Go to Step 10

10

Replace the imbalanced engine flywheel/flexplate. Refer to Engine Flywheel Replacement in Engine Mechanical - 3.8L.

Did you complete the replacement?

Go to Step 12

--

11

Replace the engine crankshaft balancer. Refer to Crankshaft Balancer Replacement in Engine Mechanical - 3.8L.

Did you complete the replacement?

Go to Step 12

--

12
  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 Vibration Diagnostic Aids

System OK