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Use your scope to pinpoint causes of NVH

Monday, October 1, 2018 - 06:00
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The world of NVH (noise, vibration, harshness) diagnostics can be rather interesting. Over the years, many different pieces of equipment have been used to point the technician in the right direction when tackling noise and vibration complaints.

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Before I go too far on the subject, we need to understand the causes of NVH that cause our customers to bring their vehicles in for diagnosis. When it comes to a vibration, the only difference between the slamming of a door (which creates a noise and a vibration in the vehicle) and a vibration that will make the dashboard rattle, is the frequency of the vibration and the amplitude (harshness) of the vibration. Keeping these two things in mind can make the finding of the problem a lot easier.

Albin Moore using his hands and a stethoscope to find a rough wheel bearing.

Applying technology to NVH

Over the years, I have used several different types of tooling to find the causes of NVH; reed tachometers, the Chassis Ear®, stethoscopes and the Pico NVH kit, for example. Each of these tools has their place and should be used as the need arises. NVH problem analysis starts out much as any other problem analysis by gathering a lot of information over a wide area. This might start with a test drive and listening and observing the operation of the vehicle while it is being driven around corners, over bumps and on smooth roads, all while trying to duplicate the concerns of the customers. We can call this “getting a direction” so the vibration or noise can then be pinpointed with other tooling or testing.

I think most all of us have had our fair share of wheel bearings that would talk as the vehicle is driven. Many times it is easy to use a stethoscope or even grab hold of the coil spring and turn the wheel to both hear and feel the vibration that is caused by the rough wheel bearing, but what about a wheel bearing on a vehicle with torsion bar or leaf spring suspension? This adds to the difficulty of finding the problem. There is no one simple way to accomplish the task.  

When it comes to vibrations in the drivetrain (engine, transmission, driveline and differential), these vibrations can all be synchronized with engine RPM or tire RPM. It all boils down to the frequency of the vibration.

Over the years, I have had vibrations that gave me a run for my money. I remember one on a Mercedes Benz E320 that came in for a rear wheel bearing noise. I found the problem with the right engine mount. The mount had deteriorated, which let the engine lean over to the right, which let the exhaust pipe bump on the frame, which transferred the noise down to the right rear wheel bearing. Stopping the wheel would make the noise go away. Vibrations can be transmitted throughout the vehicle easily, which can make the problem of finding the root cause of the NVH hard at times.

For those of us that have done NVH work for a while, I think I can safely say, “There is no one part that vibrates the same way, or makes the same noise when it fails.” Take for instance a wheel bearing; I have heard them squeak, growl, moan or not even make a noise when they fail. Having a tool to give a good direction to NVH is a way cool tool to have.

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