Inspection and Verification

How To Use This Diagnostic Procedure Section

Noise, vibration and harshness (NVH in engineering shorthand) is becoming more and more important as the vehicles are becoming more sensitive to these vibrations. This section is designed to give a working knowledge of the process of diagnosing noise and vibration situations. It is divided into several sections based on the description of the condition. If the condition occurs at high speed, for instance, the most likely place to start is under High Speed Shake. The Road Test Section will tell how to sort out the conditions into categories and how to tell a vibration from a shake. It will give a few hints on quick checks to make sure that a cause is either pinpointed or eliminated.

Become familiar with the terms. Use the Glossary section to find the descriptive name of conditions not encountered before. After naming it, proceed to the section identifying the condition, and locate the proper diagnosis section. Remember, just by beginning at a particular point, most other systems in the vehicle have been eliminated. When the proper section is identified, the job is partly done. Follow the steps within the section. Quick Checks are described within the step, while more involved tests and adjustments are found in Service and Adjustment. Always follow each step exactly and make notes along the way to recall important findings.

Road Test

NVH Diagnosis should always start with the road test. Noise, vibration and harshness (NVH) usually occur in four areas: tires, engine accessories, suspension and driveline. It is important, therefore, that a NVH concern be isolated into its specific area as soon as possible. The easiest and quickest way to do this is to perform a Road Test as outlined.

NVH Locator Index

A gear-driven unit, especially an automotive drive axle, will produce a certain amount of noise. Some noise is acceptable and may be audible at certain speeds or under various driving conditions, as on a newly paved blacktop road. The slight noise is in no way detrimental to the operation of the rear axle housing (4010) and must be considered normal. Camshaft belt whine is most often associated with new belts used on smaller engines. Whine will usually go away after belt break-in.

NOTE: It is important that the rear axle noise on vehicles equipped with either a manual or automatic overdrive transmission be evaluated in direct drive rather than overdrive.

The road test and customer interview (if available) provide information needed to identify the condition and give direction to the correct starting point for diagnosis.

1. Make notes throughout diagnosis routine. Use a Road Test form that includes space for comments. Make sure to write down even the smallest bit of information. It may turn out to be the most important.

2. Road test the vehicle and define the condition by reproducing it several times during the road test.

3. Perform the Road Test Quick Checks as soon as the condition is reproduced. This will identify the proper section of the diagnosis procedure. Run through the quick checks more than once to ensure they are providing a usable result. Remember, the Road Test Quick Checks may not tell where the concern is, but they will tell where it is not.

4. Do not touch anything until a road test and a thorough visual inspection of the vehicle have been performed. Leave the tire pressures and vehicle load just where they were when the condition was first observed. Adjusting tire pressures, vehicle load, or making other adjustments, may reduce the condition's intensity to a point where it cannot be identified clearly. It may also inject something new into the system, preventing proper diagnosis.

   Make a visual inspection as part of the preliminary diagnosis routine, writing down anything that does not look right. Note tire pressures, but do not adjust them yet. Note leaking fluids, loose nuts and/or bolts, or bright spots where components may be rubbing against each other. Check the luggage compartment for unusual loads.
   
   
   
   

Road Test Quick Checks

1. 24-80 km/h (15-50 mph): With light acceleration, a moaning noise is heard, and possibly a vibration is felt in the floorpan. It is usually worse at a particular engine speed and at a particular throttle setting during acceleration at that speed. It may also produce a moaning sound, depending on what component is causing it.

   Refer to Tip-In Moan Diagnosis charts.

2. 40-72 km/h (25-45 mph): With steady to heavy acceleration or deceleration, a rumble-type noise is heard. It is very intense during heavy acceleration or deceleration and very light during cruise or neutral coast. The vibration is hard to duplicate with vehicle supported on a hoist, since wheels are coasting free.

   Refer to Driveshaft Vibration Diagnosis.

3. High Speed: With slow acceleration and deceleration or at constant speed, a shake is sometimes noticed in the steering wheel/column, seats, floorpan, trim panels, or front end sheet metal. It is a low frequency vibration (around 9-15 cycles per second). It may or may not be increased by applying brakes lightly.

   Refer to High Speed Shake Diagnosis charts.

4. High Speed: A vibration is felt in the floorpan or seats, with no visible shake, but with an accompanying sound or rumble, buzz, hum, drone, or booming noise.

   It will exist in all drive modes, but may vary somewhat in acceleration, deceleration, float, or coast modes. In some cases the driveline vibration is eliminated in the float mode.

   Refer to Driveshaft Vibration Diagnosis charts.

5. 0-High Speed: A vibration is felt whenever the engine reaches a particular rpm. The vibration can be duplicated by operating the engine at the problem rpm while the vehicle is sitting still. It can be caused by any component from the fan back to the clutch or torque converter and by anything that turns at engine speed when the vehicle is stopped.

   Refer to Engine Accessory Vibration Diagnosis charts.