Feature December 1, 2011 by
Tom Hamilton, Curriculum Developer CARS
Vehicle Vibration Diagnosis
Diagnosing a vibration concern on a vehicle can be frustrating, time consuming and often very difficult to isolate. Understanding the relationship between when the vibration occurs and which component of a vehicle may be rotating or moving in...
Diagnosing a vibration concern on a vehicle can be frustrating, time consuming and often very difficult to isolate. Understanding the relationship between when the vibration occurs and which component of a vehicle may be rotating or moving in relation to the vibration can be a great asset in pinpointing the cause of the problem. The first step is to determine when and under which conditions the vibration occurs. By separating vibrations into categories, we can narrow down the possible causes of the concern. The conditions under which the vibration occurs can be divided into four different categories: engine speed related, vehicle speed related, engine load related or random vibrations. An engine speed related vibration can occur in any gear, including park or neutral, at a set engine RPM. This type of vibration can be associated with either the engine itself, any component that is driven by the crankshaft pulley or components that are rotating at the same speed as the engine including the harmonic balancer, flex plate and torque converter or flywheel and pressure plate assembly. To determine if a vehicle has an engine speed related vibration, try to maintain vehicle speed at the point of which the vibration occurs then shift into neutral and allow engine RPM to drop. If the vibration changes or is not present in relation to the engine RPM changes, this is indicative of an engine speed related vibration. To help confirm that the vehicle has an engine speed related vibration, with the vehicle stopped, put it into park or neutral and raise the engine speed to the RPM at which the vibration occurred to see if the vibration can be reproduced. If reproduced, diagnosis should begin with engine speed related components. A vehicle speed related vibration will usually occur at certain vehicle speeds and will change or disappear in relation to vehicle speed. Changes in engine RPM will not affect a vehicle speed related vibration. To determine if a vehicle has a vehicle speed related vibration, perform the same test as discussed above. If the vibration is still present when shifting into neutral and coasting through the speed at which the vibration occurred, any component from the transmission output shaft to the tires can be suspect and diagnosis should begin there. Keep in mind that these components are still turning at the maintained speed regardless which gear is selected. An engine load related vibration will usually occur during acceleration or deceleration and may not be noticeable at a cruising speed. This type of vibration is generally associated with worn or damaged driveline components and may increase in severity as the load is increased. When under load, stress on the driveline is dramatically increased allowing the driveline components to shift within their mounts. If engine or transmission mounts are worn or damaged, axle shaft and/or driveshaft angles may be altered beyond specifications creating a vibration. Worn or damaged mounts can also allow the engine, transmission, or exhaust components to come in contact with the frame or body of the vehicle causing a vibration and noise. Random vibrations are typically vibrations caused by the vehicle driving over bumps, pot holes or rough road surfaces. These driving conditions can cause loose or improperly mounted components to momentarily vibrate or rattle. This type of vibration can usually be located by raising the vehicle on a lift and thoroughly inspecting for loose or worn steering, suspension and exhaust system components. Another location, often overlooked, that should be inspected for random vibrations is in storage areas of the vehicle. Unsecured items moving around in the storage areas or even a loose spare tire may cause this issue. Be aware that intermittent vibration concerns may be caused by system components being cycled ON and OFF at certain intervals such as electric cooling fans, air conditioning compressors and electric air pumps. If an intermittent vibration concern is the issue, operate the vehicle within the conditions that will allow for the cycling of the components or command them ON and OFF with a compatible scan tool to isolate the concern. These are some basic tips that can be utilized when diagnosing vibration concerns. Isolating a vibration to a specific area or the operation of a particular system on the vehicle will assist in narrowing down the possible causes.
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