The following are the most often used and informative vibration diagnosis test procedures:
These tests will help to pinpoint the vibration. Perform all of the tests on a smooth, level road. Refer to
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.
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.
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:
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.
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.
Following this test, perform the Neutral Coast-Down Test and the Downshift Test.
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.
Note the engine speed (RPM).
If the vibration returns at the same engine RPM, the engine, the clutch disc (manual transmission), the propeller shaft, the flywheel (automatic transmission), or the torque converter are the most probable causes. You may repeat this test in lower 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. However, following a systematic approach usually leads to pinpointing the problem.
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).
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 vehicle-speed related only vibrations.
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:
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.
The purpose of this test is to duplicate a vibration called launch shudder.
Other possible causes of launch shudder include the following conditions:
