For 1990-2009 cars

### Tire and Wheel Inspection

The tires on all new GM models have a tire performance criteria (TPC) rating number molded on the sidewall. The TPC rating will appear as a four-digit number preceded by TPC SPEC on the tire wall near the tire size. A replacement tire should have the same TPC rating.

(1) | Hard Cornering, Under Inflation
(2) | Excessive Toe on Non-Drive Axle
(3) | Heavy Acceleration on Drive Axle, Excessive Toe on Drive Axle, Over Inflation

This test should be performed for ALL vibration complaints unless the disturbance occurs only with the vehicle at a standstill.

Inspect the tire and wheel assemblies for the following conditions:

• | Unusual wear such as cupping, flat spots, and heel-and-toe wear

| (These conditions can cause tire growl, howl, slapping noises, and vibrations throughout the vehicle.)

• | Proper inflation

• | Bulges in the sidewalls

| (Not to be confused with normal ply splices, commonly seen as indentations in the sidewall).

• | Bent rim flanges

Inspecting these characteristics of the tire and wheel assemblies may lead to the cause of the vibration. At the very least, the inspection will provide assurance that the vehicle is safe for road testing.

### Slow Acceleration Test

This test will identify the engine-speed or vehicle-speed related conditions. Additional tests may be necessary in order to determine in which category the vibration belongs.

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. Observe the necessary safety precautions.
2. On a smooth, level road, slowly accelerate up to highway speed.
3. Look for disturbances that match the customer's description.
4. Note the following readings where the disturbance occurs:

• | The vehicle speed, km/h (mph)

• | The engine speed (RPM)

• | The frequency (if possible)

Following this test, 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. Observe the necessary safety precautions.
2. On a smooth level road, accelerate to a speed slightly higher than the speed at which the vibration occurs.
3. Shift the vehicle into NEUTRAL and coast down through the vibration range.

Note if the vibration is present in NEUTRAL.

If the vibration still occurs in NEUTRAL, then the vibration is definitely vehicle-speed sensitive. At this point, the engine, the clutch disc (manual transmission), the propeller shaft (located inside the driveline support assembly), the transmission flex plate (automatic transmission) and the torque converter have been eliminated as a cause. Depending on the symptoms or frequency, the repair will concentrate on the tire and wheel assemblies, the transmission output shaft, the rear axle differential pinion, or the rear drive axle shafts.

### 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. Observe the necessary safety precautions.
2. On a smooth, level road, accelerate to the speed at which the complaint vibration occurs.

Note the engine RPM.

3. Decelerate and safely downshift to the next lower gear (from OVERDRIVE to DRIVE, or from DRIVE to SECOND, etc.)
4. Operate the vehicle at the previous engine RPM.

If the vibration returns at the same engine RPM, the engine, the clutch disc (manual transmission), the propeller shaft (located inside the driveline support assembly), the transmission flex plate (automatic transmission), or the torque converter are the most probable causes. You may repeat this test is still smaller gears, and in NEUTRAL, in order to confirm the results.

In some cases, a vibration may be sensitive to torque or engine load, as well as being related to a specific engine (RPM) or vehicle (km/h / mph) speed. These vibrations can be most difficult to diagnose, and require additional testing. Still, following a systematic approach usually leads to pinpointing the problem.

### Neutral Run-Up Test

This test is designed to identify engine-speed related vibrations. Use this test when the customer complains of vibration at idle, or as a follow-up to the downshift test. This test more than likely doesn't apply when the complaint is vehicle-speed related 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. Observe the necessary safety precautions.
2. Slowly increase the engine speed while looking for disturbances that match the customer's complaint.
3. Note at which engine speed (rpm) and frequency (if possible) that 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 more than likely not apply to vehicle-speed related only vibrations.

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

### Additional Tests

You can perform one or more of the following tests for unique vibration complaints such as those that are torque/load sensitive, in addition to vehicle-speed or engine-speed sensitive:

• | Steering Input Test

• | Standing Start Acceleration Test

### Steering Input Test

This test is intended to determine how much the wheel bearings and other suspension components contribute to the vibration, especially those relating to noises 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. Observe the necessary safety precautions.
2. Drive through slow sweeping turns with the vehicle at the vibration speed (mph) in one direction, then in the other.
3. If the vibration gets worse or goes away, inspect the following components as possible causes of the vibration:

• | 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 suspected, 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. Observe the necessary safety precautions.
2. With the vehicle at a complete stop and in gear, remove your foot from the brake pedal.
3. 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 in Suspension General Diagnosis.

• | A worn or damaged drive axle CV joint.

• | A ground-out engine/transmission mount.

• | Faulty exhaust hangers and mounts.