Shake, Rattle & Roll: DIAGNOSING NVH COMPLAINTS

Motor,  Jun 2006  by Warren, Mark

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The screen shown in Fig. 1, from the same recording, shows the wheel frequencies. You can tab through multiple screens for all the possible sources-engine, driveshaft, alternator, water pump, a/c compressor, etc. Note that the bottom of the display is labeled "Wheels," and has four arrows for wheel frequencies. Any out-of-balance or bent assembly will vibrate at a specific frequency relative to rpm. Consider a wheel with a single lump; it will vibrate relative to wheel speed. Now consider a wheel that is egg-shaped. This wheel will vibrate at twice the frequency as the single-lump wheel. The arrows in Fig. 1 show the frequencies associated with wheel vibration.

Fig. 2 is a bar chart of the vibration energy distribution. These screen captures are single events; the MTS 4100 records a series of events over time. Yon page through the events to see the vibration at various rpm, load and speed combinations.

On a front-drive vehicle, the wheels and drive axles turn at the same speed. When necessary, the exact vibration source can lie isolated using a strobe light. The accelerometer inputs the timing and amplitude of the vibration. The timing loop sends out a trigger signal in time with the highest energy of the vibration. All rotating assemblies have some vibration. When the strobe illuminates the offending assembly it freezes motion. Other assemblies not in time with the vibration will show movement under the strobe. The MTS 4100 also has an on-the-car driveshaft balance function using the strobe.

To get a feel for factory-recommended late-model vehicle NVH diagnostic procedures, check out Motor/Alldata TSBs 04-25-7 and 04-25-8 for a 2004 Ford F-150.

A big thank-you to all the tool manufacturers that assisted with research on this article. Special thanks to Dave Crippen and the Vetronix-Bosch Group for their technical assistance and expertise.

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Copyright Hearst Business Publishing Jun 2006
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