Tech Tip: Noise, Vibration and Harshness Diagnosis

Tech Tip: Noise, Vibration and Harshness Diagnosis

Since the earliest days of the automotive service business, "squeak and rattle" diagnostics has become a euphemism for low-profit, nuisance repair work. Unfortunately for those of us in the auto repair business, even our best customers have the occasional need for noise, vibration and harshness (NVH) diagnostics. On the plus side, if NVH diagnostics are approached in a methodical manner, profitability and customer satisfaction can be greatly improved.

By Gary Goms
Import Specialist Contributor

Since the earliest days of the automotive service business, “squeak and rattle” diagnostics has become a euphemism for low-profit, nuisance repair work. Unfortunately for those of us in the auto repair business, even our best customers have the occasional need for noise, vibration and harshness (NVH) diagnostics. On the plus side, if NVH diagnostics are approached in a methodical manner, profitability and customer satisfaction can be greatly improved.

In the following text, I’ll summarize the many issues involved in NVH diagnosis. Let’s begin by defining NVH in automotive terms. A noise is an unpleasant or abnormal sound being conducted into the passenger compartment. A vibration, on the other hand, is a high-frequency, occasionally cyclical movement of a vehicle component that’s conducted into the passenger compartment via the steering wheel or body sheet metal. While harshness in an industrial application can be interpreted as the magnitude of a noise or vibration, the term “harshness” in the automotive service sector generally refers to the ride control qualities of a vehicle.

It’s important to get an accurate description of the complaint from the customer and verify the complaint by riding with the customer on a road test. Because vehicle owners tend to “tune in” on strange noises emanating from their vehicles, an otherwise inconsequential ticking noise might sound like a drum beat in the ear of the customer.

To help sort out NVH complaints, it’s helpful to devise a checklist that covers the specifics of “when” and “how” the noise occurs. Variables that produce intermittent NVH complaints like wind, temperature, humidity, vehicle speed, smooth road, rough road, tar strip contact, and drive, float or coast throttle openings should be included.

Modern NVH tolerances are very tight because import manufacturers spend thousands of hours of laboratory research time eliminating NVH from their vehicle platforms. Because NVH is a common complaint in new-car service departments, auto manufacturers often supply some very sophisticated NVH diagnostic equipment to their dealer shops. In most cases, when a field test locates an engineering fault, the manufacturer issues a TSB explaining how to define and resolve the complaint.

Before diagnosing any unusual NVH complaint, always review all available TSBs to ensure that you’re not trying to resolve a complaint that might require the services of an engineering laboratory to diagnose.
Next, remember that most NVH complaints develop after a collision repair, tire and wheel, or mechanical service has been performed. Starting with the vehicle’s service history and applicable TSBs can help locate “mysterious” NVH complaints in an efficient, profitable and expeditious manner.

Noise diagnostics requires sharp hearing and the possible aid of stethoscopes and wireless electronic microphones that can be attached at different points on the chassis or drive train. Noises can be difficult to locate because they can be “conducted” from the “originator” component and “resonate” at an entirely different location in the chassis.

When diagnosing a constant or intermittent noise, consider the physics involved. For example, a grinding noise from a worn wheel bearing would be constant, whereas a humidity-induced grinding noise from the brakes would be intermittent. While frequency will be more thoroughly discussed later in this article, it’s important to differentiate among the relative frequencies generated by the rotating speed of the engine, engine accessories, drivetrain and wheels.

Sometimes it’s possible to magnify the noise of an originator like a pitted wheel bearing or worn shock mount by driving at a slow speed in a parking lot and rocking the steering wheel to transfer the vehicle’s weight from side-to-side. If the vehicle has been driven through a thorough warm-up cycle, a non-contact pyrometer or “heat gun” can be used to measure relative wheel bearing and drivetrain component temperatures, with the higher temperatures usually being produced by the noisy part. Similarly, remember that a worn engine or transmission mount, or even a missing coil spring pad, can magnify normal operating noise by “grounding” the noise into the body sheet metal.

Custom exhaust systems and other custom modifications are also prime noise offenders. When installing an exhaust system, always provide a 1/2-inch or more air gap between exhaust and body components to allow for heat expansion in the pipe. Similarly, always secure any cables, wiring harnesses or hoses with a cable tie or other fastening hardware to prevent them from vibrating against body sheet metal.

Chucking noises that occur when driving over a tar strip are usually caused by loose strut or shock absorber mounts or by loose brake caliper mounts. If a washboard road is required to duplicate the noise, the noise is most likely caused by loose exhaust shielding, a loose substrate in a catalytic converter or loose body components.

Mechanical noises are very often heat and road condition-related. Many years ago working at a dealership, I fell heir to an intermittent “clanking” noise that occurred on a new vehicle precisely at 25 mph after the vehicle had been thoroughly warmed up and was being driven on an uneven road surface.

A thorough road test and mechanical inspection solved the problem. The fault proved to be a differential housing with an oversized bore that failed to support a differential side gear. The excess clearance allowed the axle to slap up and down on an uneven road surface, causing the mysterious intermittent clanking noise.

If an NVH diagnostic tool is being used to diagnose a vibration complaint, it’s important to know that the frequency of vibration is measured in occurrences per second expressed in Hertz (Hz) and the force of the vibration possibly expressed in gravity or “G” forces. Vibrations can generally be broken down into engine speed, and engine accessory, driveshaft and wheel speed frequencies. Noises related to engine speed usually vary according to transmission gear range and throttle opening. Engine accessory noises are generally three to four times the frequency of the engine speed. The relative frequency of an engine accessory can be determined by simply dividing its pulley diameter into the crankshaft pulley diameter. Driveshaft speed, where applicable, is generally three times the wheel speed.

Keep in mind that a “resonant” vibration, such as a rearview mirror vibrating as the vehicle reaches 60 mph, is usually symptomatic of another issue, such as a defective driveshaft universal joint. In this case, the universal joint would be the “originating” component and the mirror the “reacting” component.

Also remember that modern chassis design is much more sophisticated than in years past. Years ago, we assumed that a seat vibration indicated a vibration on the rear axle, while a steering wheel vibration indicated a vibration on the front axle. Because modern chassis design is “tuned” to dampen chassis vibrations, intuitive evaluations of vibration complaints are of less use in diagnosing NVH in today’s vehicles.

When diagnosing any vibration, it’s also important to think “out of the box.” To illustrate, water from a seldom-used air hose can be injected into a tire and will freeze overnight into an icy wheel weight that vibrates until the tire reaches operating temperature.

Drivetrain vibrations tend to be related to road speed and throttle opening. If the vibration can be eliminated by opening, floating or closing the throttle, the pinion gear bearings in the rear axle might not have enough preload, or it could be that the driveshaft support bushing on the transmission might be worn.

To help tune out mechanical vibrations, propeller angles generally shouldn’t exceed more than three degrees of deflection at either end and should have at least a 1/2-degree difference between the front and rear angles. Last, remember that weights attached to axle half-shafts, transmissions and axle housings are designed to “tune out” resonant or harmonic vibrations. Modifying driveshaft angles or leaving vibration-tuning devices off after a repair can create difficult-to-solve vibration issues later on.

Because ride control in modern imports is very finely engineered, replacing OE struts or shock absorbers with replacements designed for performance applications or off-road use can often cause a ride control complaint. While the topic of computer-controlled or adjustable ride control systems is too complex to cover in this space, remember that changing firmness according to driving inputs or the command of the driver should always be taken into consideration.

Because many chassis engineers have used tires to “tune out” undesirable harshness in their ride control systems, changing from the OE to a substitute tire design can create ride harshness complaints. Last, tire pressure should always conform to manufacturer’s specifications for optimum harshness control on modern vehicles. On many occasions, the key to resolving an NVH complaint is to restore the vehicle to its OE specifications and configurations.

Gary Goms owns Midland Engine Electronics & Diagnostics in Buena Vista, CO. He is an ASE-certified Master Auto Technician, ASE L-1 Advanced Engine Performance Technician, and specializes in performance-related diagnostic issues.

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