Section 00-04: Noise, Vibration and Harshness
1996 F-150, F-250, F-350, Bronco and F-Super Duty Workshop Manual
Consideration of noise, vibration and harshness (NVH) is becoming more and more important because vehicles are becoming more sensitive to these factors. This section is designed to give a working knowledge of the process of diagnosing noise and vibration. It is divided into several sections based on the description of the condition.
How to Use this Diagnostic Procedure Section
Begin with the customer interview. Use the Glossary of Terms in this section to find the descriptive name of conditions not encountered before. After naming it, proceed to identifying the condition and locate the proper diagnosis. Remember, just by beginning at that point, most other systems in the vehicle have been eliminated. When the proper method of diagnosis is identified, the job is partly done. Follow the steps within the diagnostic procedure. Quick checks are described within the step, while more involved tests and adjustments are found in the Adjustments and Service Procedures portions of this section. Always follow each step exactly, and make notes along the way to recall important findings later.
It is important to interview the customer. Customer feedback can supply information that could be helpful in diagnosing the concern. Ask questions like:
Identifying the Condition
Noise, vibration and harshness (NVH) usually occur in the following four areas:
It is important, therefore, that an NVH problem be isolated to its specific area as soon as possible. The easiest and quickest way to do this is to perform a road test. If necessary, refer to the Road Test procedure in the Diagnosis and Testing portion of this section.
Noise Diagnostic Procedure
There are some noise conditions that can sound just like axle noise and have to be considered while performing road test diagnosis. The five most common sources of noise are the transmission, exhaust system, tires, roof racks and trim mouldings.
Make sure none of these non-axle noise conditions are the cause of the noise concern before proceeding with an axle teardown and diagnosis.
If noise is produced as part of a vibration condition, correcting the vibration will eliminate the noise. If the noise is related to a specific component or system, refer to the appropriate service manual section for further diagnosis.
Vibration, technically, is a high-frequency trembling, shaking or grounding condition, felt or heard, that is constant or variable in level and occurs during a portion of the total operating speed range. The types of vibrations that can be felt in the vehicle can be divided into three groups:
For engine or accessory vibration, all speeds, refer to Pinpoint Test D in the Diagnosis and Testing portion of this section.
For high-speed shake/vibration, 80 km/h (50 mph) and up:
For tip-in moan, refer to Symptom Charts in the Diagnosis and Testing portion of this section. If the symptom still exists, refer to Pinpoint Test B in the Diagnosis and Testing portion of this section or the engine assembly in Section 03-01A, Section 03-01B, Section 03-01C or Section 03-01D.
For brake shudder, refer to Section 06-00.
Vehicle vibrations can also be subdivided into those that occur at low speeds and those that are most noticeable at higher speeds. Since the dividing line between low and higher speed vibrations is not clear, there will be vibrations that overlap the two ranges.
Typical Low-Speed Vibrations (Less Than 72 km/h [45 mph])
Typical High-Speed Vibrations (Above 72 km/h [45 mph])
Harshness is the term commonly used to describe the ride quality concern of the vehicle. A hard ride or harshness is usually caused by the tires or suspension system, namely:
Other harshness conditions that affect ride quality may be summarized as follows:
Leakage Conditions/Low Lube Level
Most rear axle lubricant leakage conditions may be corrected without a teardown. However, it is important to clean up the leaking area enough to identify the exact source.
NOTE: If vent cannot be cleared, it must be replaced.
A plugged axle vent will cause excessive seal lip wear due to internal pressure buildup. When a leak occurs, check the vent which is located near the top of the differential housing. Make sure the vent hose is not kinked. Remove the vent hose from the vent nipple and make sure the hose is clear of dirt or other foreign materials. While hose is removed, pass a length of mechanics' wire or small diameter Allen wrench in and out of the vent nipple to clean it of any dirt or foreign materials. Install vent hose.
Make sure axle lubricant is to specified level. Refer to appropriate section in Group 05.
NVH diagnosis should always start with the road test.
Noise, vibration and harshness (NVH) usually occurs in four areas: tires, engine accessories, suspension and powertrain. It is important, therefore, that an 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.
NVH Diagnostic Locator
A gear-driven unit will produce a certain amount of noise. Some noise is acceptable and may be audible at certain speeds or under various driving conditions. The slight noise is in no way detrimental and must be considered normal.
Make notes throughout diagnostic routine. It will help cover all bases when starting to add up clues. Use a road test form that includes space for comments. Write down even the smallest bit of information, as it may turn out to be most important.
Road test the vehicle, and define the condition by reproducing it several times during the road test.
Perform the road test quick checks as soon as the condition is reproduced. This will identify the appropriate diagnostic procedure. Run through the quick checks more than once to make sure they are getting a usable result. Remember, the road test quick checks may not tell where the concern is, but they will tell where it is not.
Make a visual inspection as part of the preliminary diagnostic routine, writing down anything that does not look right. Note tire pressures, leaking fluids, loose nuts and/or bolts, or bright spots where components may be rubbing against each other. Check cargo area for unusual loads.
Some hints on performing a road test for the diagnosis of NVH complaints follow.
Neutral Engine Run-Up Test (NERU)
A very useful first step in the road test is a neutral engine run-up (NERU) test. It identifies engine related vibrations and helps in sorting out vibrations that are found in the road test.
This test takes only a minute or two and represents time well spent in testing.
If the vehicle is equipped with a tachometer, it can be used. Otherwise, a tach should be connected. A tach is absolutely necessary in both the NERU test and the road test.
Locate the vehicle away from other vehicles and walls which may reflect sound differently than a road reflects sound.
Put the vehicle in N (neutral) or P (park) and do not set the parking brake or press on the service brake since these are not engaged in a road test. Pressing on the service brake puts forces into the brake and clutch pedal support and into the dash panel that are not present in driving. A similar situation arises with the parking brake.
Run the engine up from idle to approximately 4000 rpm and note any moans, vibrations, noises, etc., and the rpm at which they occur. Sometimes it is possible to "tune in" on these by running up and down in rpm to determine a precise rpm at which they occur; in other cases they will fall over a broad range of rpm. This establishes a baseline against which driving vibrations can be measured.
If you suspect that the exhaust system is vibrating, hang a ring of keys or something similar from the tailpipe and listen for the rattling of the keys as the engine is run up and down. Sometimes it is very clear with just this simple test that the exhaust system is responsible.
Some exhaust systems contain flexible couplings and may not require evaluation unless the exhaust system is damaged.
If you suspect the air conditioning system or drive belt, turn on the air conditioning system before performing the NERU test. If a noise occurs at a specific rpm, turn the air conditioning system off and on to see if the noise is connected to the air conditioning compressor or A/C system lines.
Results of the NERU test should be compared with the results of the road test to sort out vibrations and noises caused by the engine from those caused by the transmission, driveshaft or axle. If, during the road test, a vibration or noise occurs at a particular speed, try operating in another gear at the same speed. This changes the engine rpm and helps to sort out engine induced vibrations and noises. If it occurs at a particular rpm, use different gears to test at the same rpm at different road speeds. Turn the air conditioning system off and on at road speeds where noise occurs to isolate air conditioning system problems.
Drive Engine Run-Up Test (DERU)
CAUTION: Do not conduct this test for over 30 seconds, or without periodic driving or shifting to neutral to circulate transmission lubricant, otherwise, the transmission will overheat causing severe damage to the automatic transmission.
Set both the parking brake and service brake and take care to perform the test with enough space ahead of the vehicle to eliminate the possibility of an accident should the vehicle unintentionally lurch forward.
Put the transmission in D (drive) and run the engine up and down between idle and approximately 2000 rpm. Note the nature of any vibrations and noises and when these concerns occur in relation to engine rpm. This test is particularly good for examination of the exhaust system, and hanging keys on the tailpipe can serve as a "telltale" indicator.
The following worksheet is invaluable for recording customer comments as well as inspection and road test data.
Road Test Form
Road Test Form (Continued)
Road Test Quick Checks
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 Pinpoint Test B.
Slow acceleration and deceleration: A shake is sometimes noticed in the steering wheel (3600)/steering column tube, 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 Pinpoint Test A.
At road 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. Coast with transmission control selector lever in NEUTRAL and engine idling. If vibration is still evident, it may be related to wheels, tires, front disc brake rotors (1125), rear brake drums (1126) wheel hubs (1104), wheel bearings (4000), driveshaft balance, rear axle universal joint flange runout or transmission balance.
Refer to Pinpoint Test A.
0-High Speed: A vibration is felt whenever the engine reaches a particular rpm. It will disappear in NEUTRAL coast. The vibration can be duplicated by operating the engine at the problem rpm while the vehicle is parked. It can be caused by any component, from the accessory drive belt to the torque converter which turns at engine speed when the vehicle is stopped.
Refer to Pinpoint Test D.
The type of road and its surface condition are important factors in the road test. A smooth asphalt road that allows driving over a range of speeds is best. The brushed concrete road surface found on many expressways and the coarse aggregate sometimes found in concrete can mask many vehicle noises and make NVH diagnosis difficult.
If the customer complains of a noise or vibration on a particular road and only on a particular road, the source of the concern may be the condition of the road surface and not the vehicle. If possible, try to test on the same type of surface. In this case, a smooth asphalt road may not be the best.
Check and verify if vehicle is equipped with original equipment (OE), type, size and brand wheels and tires. If non-original equipment (OE) wheels and or tires are installed, it may be necessary to substitute original equipment (OE) level tire/wheel assemblies to eliminate this factor before proceeding with the NVH diagnosis. Check and verify that ride height of vehicle has not been modified.
Check and set tire pressure.
Unlock the front hubs and back up approximately 20 feet to make sure of hub disengagement if the vehicle is a 4x4.
Remove any externally mounted accessories such as luggage racks, or magnetic base or the antenna which may cause wind noise that interferes with NVH diagnosis.
Use a tachometer.
Note the fuel level. Some vehicles change in their response to various excitations when the fuel level changes. If the customer mentions that the concern does not occur all the time, this might be a factor.
Try to duplicate the conditions with the customer present, particularly the speed and throttle operation. It is not uncommon to find concerns which the customer didn't notice so it is important to identify the specific concern causing the complaint.
Find the speed where the concern is most severe.
Turn the air conditioning system off and on to isolate air conditioning system impact on the noise. Accelerate gently through this speed to a few mph above it and then coast back down a few mph below it and note if the concern changes character when driving through or coasting down.
Repeat this procedure if necessary to get a feeling for the behavior. Then drive about five mph above the speed, put the transmission in neutral and coast down. Note any change in behavior.
Try "floating" the driveline by backing off slightly on the throttle at the problem speed. The idea is to unload the axle gears and the universal joints as much as possible. This test is difficult to master and sometimes it helps to test on a slight downgrade so that gravity is counteracting the aerodynamic drag.
If the concern does not change in all these modes of operation, the cause may well be driveline imbalance since the imbalance is not changed by the throttle position.
If there is a distinct change in the character or intensity between the drive and coast modes of operation, the concern is not likely imbalance but is most likely due to the axle or the universal joints since they are sensitive to the load on the driveline. (The axle pinion gear teeth are machined on two separate machines. One machine does the drive side of the teeth, another does the coast side. Hence there can be a great difference in performance between driving and coasting in gear.) If the noise and/or vibration goes away when the driveline is "floated" or when coasting through the speed range in neutral, the diagnosis is strengthened. Driveline imbalance does not go away in a neutral coast, but axle and joint concerns may.
To become familiar with the symptoms of imbalance and to determine if driveline imbalance exists on the vehicle, intentionally imbalance the driveline. This can be done by installing three hose clamps with screw heads aligned on the rear of the driveshaft. Test drive the vehicle again. If the same vibration is still present but now intensified, the concern is driveline imbalance. If a new vibration is now present as a result of the intentional imbalance, the original condition would not be caused by driveline imbalance.
After a road test, but only after a road test, it is sometimes useful to do a similar test on a hoist. Use an axle hoist, not a frame hoist. An axle hoist will not change the driveline angles. If only a frame hoist is available, axle stands should be used.
Elevate the rear wheels slightly. Check to make sure that both are turning, and run the engine up with the vehicle in gear.
Explore the speed range of interest using the drive/cruise/coast/float tests as described under Vehicle Preparation.
A coast down in neutral should also be conducted. If the vehicle is free of vibration when operating at a steady indicated speed and behaves very differently in drive and coast, an axle concern is likely.
A test on the hoist may produce different vibrations and noises than a road test because of the effect of the hoist on the rear axle. It is not unusual to find vibrations on the hoist that were not found in the road test and in most instances they may be ignored. Similarly, some problems may be apparent only on smooth asphalt roads. If the condition that was found on the road can be duplicated on the hoist, a great deal of time can be saved by doing experiments on the hoist.