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For 1990-2009 cars only
Makes 🢒 Buick 🢒 1999 🢒 Century (China) (CNG) 🢒 General Information 🢒 Vibration Diagnosis and Correction 🢒 Types of Road Test

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.

Important: Before performing any road test, inspect the tires and wheels. Refer to Tire and Wheel Inspection .

The following road test procedures are the most informative and the most used:

    • Tire and wheel inspection
    • Slow acceleration test
    • Neutral coast-down test
    • Downshift test
    • Neutral run-up test
    • Brake torque test
    • Steering input test
    • Standing start acceleration test (launch shudder)

These tests will help to pinpoint the vibration. Perform all of the tests on a smooth, level road.

Slow Acceleration Test

This test will identify those conditions which are related to the engine-speed or to the vehicle-speed. Additional tests may be necessary in order to determine the exact cause of the vibration.

    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. On a smooth, level road, slowly accelerate up to highway speed.
  2. Look for disturbances that match the customer's description.
  3. Observe the following readings where the disturbance occurs:
    4. • The vehicle speed, km/h (mph)
    • The engine speed (RPM)
    • The frequency (if possible)

Now perform the neutral coast-down test and the downshift test.

Neutral Coast-Down Test

    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. On a smooth level road, accelerate to a speed slightly higher than the speed at which the vibration occurs.
  2. Shift the vehicle into Neutral gear and coast down through the vibration range.

    3. Observe whether the vibration is present in Neutral gear.

If the vibration still occurs in NEUTRAL gear, then the vibration is definitely sensitive to vehicle-speed. At this point, the following components have been eliminated as a cause of the vibration:

    • The engine
    • The clutch disc (manual transmission)
    • The transmission flex plate (automatic transmission)
    • The torque converter

Depending on the symptoms or the frequency, the repair will concentrate on one of the following components:

    • The tire and wheel assemblies
    • The transmission output shaft

Downshift Test

    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. On a smooth, level road, accelerate to the speed at which the complaint vibration occurs.

    2. Observe the engine RPM.

  2. Decelerate and safely downshift to the next lower gear.
  3. Operate the vehicle at the previous engine RPM.

If the vibration returns at the same engine RPM, the following conditions are the most probable causes of the vibration:

    • The engine
    • The clutch disc (manual transmission)
    • The transmission flex plate (automatic transmission)
    • The torque converter

Repeat this test is still smaller gears, and in Neutral gear, in order to confirm the results.

In some cases, a vibration may also be sensitive to torque or engine load, as well as being related to a specific engine speed or vehicle speed. These vibrations can be most difficult to diagnose, and require additional testing. A systematic approach usually leads to isolating the problem.

Neutral Run-Up Test

This test is designed to identify vibrations which are related to the speed of the engine. Use this test when the customer complains of vibration at idle, or as a follow-up to the downshift test. This test probably doesn't apply when the complaint is related to vehicle speed only (appearing at the same vehicle speed regardless of the engine speed).

    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. Slowly increase the engine speed while looking for disturbances that match the customer's complaint.
  2. Observe the engine speed (RPM) and the frequency (if possible) where the vibration occurs.

Brake Torque Test

This test is designed to identify engine-related vibrations that were not uncovered with the neutral run-up test. This test also works for vibrations that are sensitive to the engine load or to the torque. This test will probably not apply to vibrations which are related only to the speed of the vehicle.

    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. Apply the parking brake.
  2. Block the front wheels.
  3. Step firmly on the brake pedal.
  4. Place the vehicle in Drive.
  5. Slowly increase the engine speed while looking for vibrations that match the customer's description.
  6. Observe the engine speed (RPM) and the frequency (if possible) at which the disturbance occurs.
  7. If necessary, place the vehicle in Reverse gear and repeat steps 5 and 6.

Steering Input Test

This test is intended to determine how much the wheel bearings and other suspension components contribute to the vibration, especially a vibration relating to noise, such as growl, grinding, and roaring.

    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. With the vehicle at the vibration speed (mph), drive through slow sweeping turns, first in one direction and then in the other direction.
  2. If the vibration gets worse or if the vibration goes away, inspect the following components as possible causes of the vibration:
    3. • The wheel bearings
    • The hubs
    • The tire tread

Standing Start Acceleration (Launch Shudder) Test

The purpose of this test is to duplicate a vibration called launch shudder. In some cases, a powertrain mount or an exhaust ground-out may also be the cause of the vibration, depending on the symptoms.

    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. With the vehicle at a complete stop and in gear, remove your foot from the brake pedal.
  2. Accelerate to 48-64 km/h (30-40 mph) while looking for vibrations that match the customer's description.

Other possible causes of launch shudder include the following conditions:

    • Incorrect trim height. Refer to Trim Height Inspection in Suspension General Diagnosis.
    • A worn or damaged drive axle CV joint
    • A ground-out engine/transmission mount
    • Faulty exhaust hangers and mounts
    • If the vibration goes away with the propshaft removed, the problem is probably in that component.
    • If the vibration does not go away, put the propshaft back in. Remove the other propshaft. Repeat the road test.
    • If the vibration is still present, use an accurate reed tachometer or EVA information in order to test the tire/wheel assembly or the related components (for example, the hub, the brake rotors, the drum). Remember, accurate vibration frequency readings are important.