GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Cobalt 🢒 Preface 🢒 Cautions and Notices 🢒 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 Electronic Vibration Analyzer 2 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.

  7. An engine flywheel that has excessive lateral runout, when combined with the extra mass of the clutch pressure plate and clutch driven plate, can produce a disturbance.

    8.

  8. The clutch pressure plate and the engine flywheel are marked for proper indexing of the heavy-spot of one to the light-spot of the other. Improper indexing of the pressure plate to the flywheel can produce a disturbance.

    9.

  9. An engine flywheel that is loose at the engine crankshaft or that is cracked, damaged, and/or missing balance weights, and/or a clutch pressure plate and clutch driven plate that has loose springs, cracks, warpage, damage, and/or missing balance weights, can produce a disturbance when their mass is combined.

    10.

  10. An engine flywheel that is loose at the engine crankshaft or that is cracked, damaged, and/or missing balance weights, and/or a clutch pressure plate and clutch driven plate that has loose springs, cracks, warpage, damage, and/or missing balance weights, can produce a disturbance when their mass is combined.

    11.

  11. Re-indexing the pressure plate to the engine flywheel alters the balance relationship between the pressure plate/flywheel assembly and the rear of the engine.

    12.

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

    13.

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

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/Accessory Isolation table?

Go to Step 2

Go to Vibration Analysis - Engine/Accessory Isolation

2

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

Go to Step 3

Go to Vibration Diagnostic Aids

3

Is the vehicle equipped with a manual transmission?

Go to Step 11

Go to Step 4

4

  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 5

Go to Step 11

5

  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 to avoid accidental engagement with the 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 8

Go to Step 6

6

  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 8

Go to Step 7

7

  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 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 9

Go to Step 11

8
  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 - 2.0L (LSJ) or Engine Flywheel Replacement in Engine Mechanical - 2.2L (L61).

Did you complete the tightening or replacement?

Go to Step 21

--

9

  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 21

Go to Step 10

10

Replace the out-of-balance transmission torque converter.

Did you complete the replacement?

Go to Step 21

--

11

  1. Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  2. Position the J 38792-A 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 1-9, 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 20

Go to Step 12

12

Is the vehicle equipped with an automatic transmission?

Go to Step 18

Go to Step 13

13

  1. Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  2. Remove the flywheel inspection cover.
  3. Start the engine. Allow the engine to idle.
  4. Visually inspect the engine flywheel clutch surface for excessive lateral runout.

Does the engine flywheel clutch surface exhibit excessive lateral runout?

Go to Step 19

Go to Step 14

14

Inspect the clutch pressure plate to engine flywheel mounting for proper factory indexing.

Is the clutch pressure plate properly indexed to the engine flywheel?

Go to Step 16

Go to Step 15

15

  1. Remove the clutch pressure plate and clutch driven plate from the engine flywheel. Refer to Clutch Pressure and Driven Plate Replacement in Clutch.
  2. Inspect the engine flywheel for the following conditions:
    3. • Looseness at the engine crankshaft
    • Cracks, warpage, and/or damage
    • Missing balance weights
  3. Inspect the clutch pressure plate and clutch driven plate for the following conditions:
    4. • Loose and/or damaged clutch driven plate damper springs
    • Loose and/or damaged clutch pressure plate diaphragm springs
    • Cracks, warpage, and/or damage
    • Missing balance weights

Do any of the above conditions exist?

Go to Step 19

Go to Step 17

16

  1. Remove the clutch pressure plate and clutch driven plate from the engine flywheel. Refer to Clutch Pressure and Driven Plate Replacement in Clutch.
  2. Inspect the engine flywheel for the following conditions:
    3. • Looseness at the engine crankshaft
    • Cracks, warpage, and/or damage
    • Missing balance weights
  3. Inspect the clutch pressure plate and clutch driven plate for the following conditions:
    4. • Loose and/or damaged clutch driven plate damper springs
    • Loose and/or damaged clutch pressure plate diaphragm springs
    • Cracks, warpage, and/or damage
    • Missing balance weights

Do any of the above conditions exist?

Go to Step 19

Go to Vibration Diagnostic Aids

17

Re-index the pressure plate to the engine flywheel.

Did you complete the re-indexing?

Go to Step 21

--

18

Replace the imbalanced engine flywheel/flexplate. Refer to Engine Flywheel Replacement in Engine Mechanical - 2.0L (LSJ) or Engine Flywheel Replacement in Engine Mechanical - 2.2L (L61).

Did you complete the replacement?

Go to Step 21

--

19

Replace the engine flywheel. Refer to Engine Flywheel Replacement in Engine Mechanical - 2.0L (LSJ) or Engine Flywheel Replacement in Engine Mechanical - 2.2L (L61).

Did you complete the replacement?

Go to Step 21

--

20

Replace the engine crankshaft damper. Refer to Crankshaft Damper Installation in Engine Mechanical - 2.0L (LSJ) or Crankshaft Damper Installation in Engine Mechanical - 2.2L (L61).

Did you complete the replacement?

Go to Step 21

--

21
  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