Monday, May 21, 2018 Detailed Auto Topics
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Detailed Topics

To solve a vibration, we must know what causes it.  With so many possible causes, people often waste a great deal of money on trial and error.  Using a few simple methods  eliminates unnecessary expenses and leads us to the problem quickly.  This article begins where part one left off.

      

Is this an engine or suspension vibration?


A few simple tests help to eliminate almost half the possible causes.  First, see if the vibration can be duplicated with the vehicle sitting still.  In park or neutral, and with the brake applied, raise engine speed to the RPM that corresponds to the MPH where the vibration occurs.  If the vibration is present, the cause is in the engine or torque converter, and not the suspension or drive-line.


If the vibration is not present, with the vehicle sitting still, it is more likely a suspension or drive-line problem.  The next test is to drive above the speed where the vibration occurs.  Place the shifter in neutral and coast through the speed of the shake.  If the problem still occurs at the same speed, suspect a wheel or tire issue.  A vibration that does not show up in neutral suggests a drive-line problem or an engine misfire.


What tests will help to find a wheel or tire vibration?


Rotate the tires and see if the vibration changes


A simple test method is to rotate the tires, from front to rear.  A vibration that changes means a tire or a wheel problem is likely.  Raise the vehicle and give the tire a spin.  Watch the edge of the wheel as it spins.  Any noticeable movement suggests a bent wheel.  Next watch the tire as it rotates.  Run-out that is visible, side-to-side or up and down is likely too much.  If the wheels and tires run true, a balance problem could still exist.


Another simple test helps to isolate if the problem is a tire or balance issue.  Any change, when driving on different road surfaces, means a tire problem.  For instance, if the shake changes on concrete or on blacktop, suspect a tire. The tire is the only thing that touches the road.   A professional can check the balance of the tires.


Rarely, a tire will have a defect in the sidewall.  This may cause a vibration, though properly balanced.  This is not common and rotating the tires will show the issue.  Improper mounting can also cause a tire vibration problem.  Such a vibration will be different when the tire position changes.  


What tests will show drive-shaft vibrations?

The drive shaft has to run at the proper angle to avoid vibration


Rear-wheel-drive driveshafts spin very fast and can vibrate.  This will be felt in the seat, rather than the steering wheel.  Acceleration and deceleration will often affect such a vibration.  Driveshaft vibration results if there are tight universal joints, any dent in the tube or an improper shaft angle.  


Watching the drive-shaft rotate, will often reveal the problem.  Any noticeable run out will cause a vibration.  Removing the shaft and moving the universal joints is a good way to check them.  Any resistance to motion, in either plane, or slack in the joint means it is bad.


In another Detail Topic, we discuss drive-shaft angles.  A simple test is to place a shim, under the transmission mount or the center carrier bearing.  This changes the angle of the shaft.  A drive-shaft problem exists if the vibration changes.  Checking the balance of a drive-shaft is best left to a drive-line specialist with the necessary equipment.


Front-wheel-drive axles turn very slow by comparison.  These will not generally cause a noticeable vibration, unless they are badly bent.  An exception is a bad inner C.V. joint, which will cause a vibration on acceleration and will quit on deceleration.  


Tests for vibrations, when the vehicle is sitting still


We usually relate vibrations when the vehicle is not moving to the engine or a torque converter.  If the vehicle shakes at an idle, and gets better with engine speed, try shifting from park to reverse.  Additionally, move from reverse to drive and see if the vibration changes.  Vibrations that change, when shifting the transmission, suggest bad engine mounts.  The engine-torque direction changes, from reverse to drive.  This will load and unload the mounts, creating the change.


Increased engine speed may also make the vibration worse.  Temporarily remove the drive belt(s).  If the shaking quits, a belt driven accessory is the cause.  A common culprit, is a harmonic balancer, which is beginning to separate.


An out-of-balance or damaged torque converter will also shake the vehicle.  This is most common after engine or transmission repair or after a collision.  Removing the bolts that hold the converter in place will allow it to slide back into the transmission.  If the vibration stops, with the torque converter disconnected, the problem is found.  Replacing a bad converter is the only practical repair.


With these few no-cost tests, most vibrations are found.  Any shake not found, after these procedures, requires a professional for diagnosis.  Allowing a vibration to continue will cause damage and is unnecessary.  Frame and alignment shops have equipment and experience to test many other things.  


 





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