Chevrolet Cobalt Service & Repair Manual: Vibration, Diagnosis

The designs and engineering requirements of vehicles have undergone drastic changes over the last several years. Vehicles are stiffer and provide more isolation from road input than they did previously. The structures of today's stiffer vehicles are less susceptible to many of the vibrations which could be present in vehicles of earlier designs, however, vibrations can still be detected in a more modern vehicle if a transfer path is created between a rotating component and the body of the vehicle.
There are not as many points of isolation from the road in many vehicles today. If a component produces a strong enough vibration, it may overcome the existing isolation and the component needs to be repaired or replaced.
Rotating components will cause vibrations when excessive imbalance or runout is present.
A vibrating component operates at a consistent rate (mph or RPM). Measure the rate of vibration in question. When the rate/speed is determined, relate the vibration to a component that operates at an equal rate/speed in order to pinpoint the source. Vibrations also tend to transmit through the body structure to other components. Therefore, just because the seat vibrates does not mean the source of vibration is in the seat.

Visual

When diagnosing vibrations it is imperative to notice when and where the vibrations occur. Test drive the vehicle with this in mind. You need to determine whether the vibrations are a tire issue, an engine or drive train problem or exhaust issue. Feel for the vibrations at an idle state and whether they are felt in the steering wheel. Put the car in gear and see if the vibrations lessen or increase. Drive the car at 10 to 20 miles per hour and feel for side to side shifts and vibrations. Inspect to see if the vibrations get faster as speed increases. Inspect to see if there are vibrations going over bumps. Drive the car over 45 miles per hour and notice if you feel the vibrations increasing in the steering wheel. Inspect the appropriate steps that apply to the vibrations felt in your test drive for the proper order of diagnosis.
1. Inspect front tires for a lateral or side to side vibrations at slow speeds by jacking up each tire individually. Grasp tire on top and bottom and try to rock tire. If there is play when moving tire in and out, bearing/hub bearing is bad and needs to be replaced.
2. Inspect for significant damage to tire. Visually inspect if there are any bulges on inside or outside of tire. Look for any deformities as you rotate tire, such as uneven wear or steel protruding out of tread.
3. Inspect all tires for cupping of tread. This is an unevenness of tread that you feel as your hand is run across tread. Look for flat spots on tires created by a panic stop where tires were locked up and slid removing rubber on that spot. This will generate vibrations at all speeds.
4. Inspect front tires for balancing if vibration is only at speeds in excess of 45 miles per hour.
5. Inspect engine for vibrations. Test for broken or worn motor/transmission mounts with aid from an assistant.
6. Put transmission in neutral, leave door open, and rev engine up to about 3000 RPM. If noise and vibration can be heard in middle of car it is most likely shield on catalytic converter, which often gets bent and contacts either converter or body.
7. Inspect exhaust by shaking pipe and see if it contacts body.

Electronic

An Electronic Vibration Analyzer (EVA), is a hand-held device, similar to a scan tool, which receives input from an attached vibration sensor or accelerometer and displays the most dominate input frequencies on its liquid crystal display. The vibration concern frequencies are obtained through the use of the electronic vibration analyzer while following the vibration analysis diagnostic tables. The frequencies obtained, when applied to the vibration analysis diagnostic tables, are used as a primary input to help determine the source of the vibration concern.
Proper placement of the EVA vibration sensor (accelerometer) is critical to ensure that proper vibration readings are obtained by the EVA. The vibration sensor should be placed on the specific vehicle component identified as being the most respondent to the vibration. If no component has been identified, install the sensor to the steering column as a starting point.
The vibration sensor of the EVA is designed to pickup disturbances which primarily occur in the vertical plane, since most vibrations are felt in that same up and down direction. The EVA vibration sensor is therefore directional sensitive and must be attached to vehicle components such that the side of the sensor marked UP is always facing upright and the sensor body is as close to horizontal as possible. The sensor must be installed in the exact same position each time tests are repeated or comparisons are made to other vehicles.
The EVA vibration sensor can be attached to vehicle components in various ways. For non-ferrous surfaces, such as the shroud of a steering column, the sensor can be attached using putty, or hook and loop fasteners. For ferrous surfaces, the sensor can be attached using a magnet supplied with the sensor.
1. Attach EVA vibration sensor to area where vibration occurs.
2. Recreate vibration condition.
3. Record readings from EVA unit.
4. Compare readings to vibration analysis diagnostic tables.
5. If results are inconclusive, reposition vibration sensor and test again.
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