Did You Hear That? - Transmission Digest

Did You Hear That?

In my experience NVH, (engineering speak for noise, vibration, and harshness) is one of the more time consuming and difficult diagnostic situations. The manufacturers work extremely hard at ensuring that their automobiles have as quiet environment as possible inside the passenger compartment. They have to contend with road noise from the tires, air noises while the car is at speed, mechanical noises from the operation of the engine, driveline, and other components, noise due to weather, harmonic vibrations from various components, and increasingly sensitive consumers.

Up To Standards

  • Author: Mike Weinberg
  • Subject Matter: Noise, vibration and harshness
  • Issue: No detail is too small to overlook.

Noise: The Mother of All Frustrations!

In my experience NVH, (engineering speak for noise, vibration, and harshness) is one of the more time consuming and difficult diagnostic situations. The manufacturers work extremely hard at ensuring that their automobiles have as quiet environment as possible inside the passenger compartment. They have to contend with road noise from the tires, air noises while the car is at speed, mechanical noises from the operation of the engine, driveline, and other components, noise due to weather, harmonic vibrations from various components, and increasingly sensitive consumers.

The ideal cabin environment would be the absolute silence of a recording studio, with the proper acoustic architecture to make the driver feel that he was in a concert hall when listening to his favorite music while at speed. They do a great job in making a relatively peaceful interior cabin. This comes at a cost, as there are no mechanical components that are silent in operation. Engines are designed and mounted to eliminate harmonic vibrations from the operation of a four-stroke design from influencing the passenger.

This means tuning the exhaust correctly, mounting the motor so that none of the components are grounded so the NVH becomes audible. Clutch components are dampened with springs to prevent harmonics from rattling the gear train. Transmission gearing is designed to be as silent and noiseless as possible. In most cases noise complaints mean that something is worn, broken or out of place in the vehicle. You will get customers who have driven a car with bad bearings and no complaints, as they have just turned up the radio volume to drown out the noise of impending doom, and they will only come to you when there is not enough volume control left to cancel the failed bearing noise.

Noise travels

Noise is very subjective and as such requires a level of patience and structured investigation so that you spend the minimum amount of time locating the area that needs correction. Einstein defined insanity as repeating the same process continuously, while expecting different results. Noise cannot be seen, only heard and felt. The last thing you wish to do in an NVH complaint is remove components from the car before you have located the cause of the issue. You need to keep the vehicle operational as your test bed. There is always an urgency to “get the job” so countless transmissions, transfer cases, rear end are removed for inspection only to find nothing wrong with them. I have been doing this a long time and have yet to “see” a noise.

It is smart to remember that noise travels. It travels fairly slowly in air (approx. 700 mph) and more rapidly in liquid and more rapidly yet in solids. To test this for yourself, take two small rocks and bang them together, hearing the click in the air. Get the same rocks into a swimming pool and repeat, only that noise will hurt under water. Place one of the rocks to your ear and strike it with another and it will be less enjoyable yet. The point here is that when noises travel through chassis components they may be audible a long way from the source.

Origins, remedies

Drive shafts act as megaphones and will transfer a differential noise down the shaft where it may appear to be right under the driver, and be considered a transmission or transfer-case noise when it is really coming from one of the differentials. The most common manual-transmission noise comes not from the transmission, but from the clutch. Neutral gear rollover noise creates hours of lost time and wasted effort. The clutch disc and flywheel are designed to prevent the firing pulses from the engine from migrating into the transmission and causing the gears to rattle at idle. This does not hurt the transmission but is annoying to the driver. The clutch disc is specifically designed around the seismic vibrations produced by the engine, which determines the type of damper springs used in the hub of the disc. In some cases they will use a dual-mass flywheel that has further dampening to create a noise free power flow.

The problem starts when a shop does not have experience with neutral gear rollover noise.

If you have a unit that is rattling the gears at idle and somewhat beyond, there are several very simple tests to isolate the problem. First thing to do is at and idle with the clutch engaged, slowly use the throttle to raise the rpm of the engine to 2500-3000 rpm. If the noise goes away at the higher rpm it will be in the clutch set or engine and not the trans.

Another option is to take the shift lever at idle and without using the clutch just move it lightly to 2nd gear. Using the synchro ring to press lightly on the speed gear should stop the noise if it is neutral rollover noise. This is very common on performance vehicles and diesel-powered cars and trucks. You take a brand new transmission that was purchased because the customer had a noise, install it and wind up with the same noise, because the customer installed an aftermarket performance clutch that has a clutch hub not sprung for the vehicle. On Ford diesels with a ZF trans, to prove it to the customer, set the vehicle up on a lift, drain out the trans fluid, and remove a PTO cover. Start the vehicle and watch the gears vibrating until the idle is raised and the noise floats out. A bad clutch, dual-mass flywheel, or out-of-time injector pump can be the cause, but it ain’t in the trans.

Patience

To be good at locating noise, one needs patience and a comprehension of the system of operation for all the related components. Always work with a notebook and work from the outside in, making notes of all conditions on the vehicle for future reference. No detail is too small to overlook. In the old days it was common to have a speedometer cable lose a mounting clip and wind up on a frame rail, and the rotation of the cable at speed walked through the housing into the passenger compartment. Check all mounts, CV joints, drive shafts, wheel lug nuts, diffs and axles, out-of-place exhaust components, chassis and suspension bushings, etc. This inspection will save you endless hours for the short time it takes. Using a pipe, screwdriver or stethoscope to track down the source and position of the noise will be worth the effort.

Remember the last things to do are the big labor items like unit removal, until you are sure that is the source.

Bad vibes, noises

Listed below are the most common noise creators you will come across:

Bearing noise: Bearings usually produce a noise that is a constant pitch that increases with vehicle speed. Pinion bearings usually have a higher-pitched noise. The diff bearings will be at a lower pitch and can be growl or a whine. Transmission bearings will be similar but if you know the theory of operation of the trans can be easily diagnosed. For instance a bearing noise that goes away or changes lower in direct drive (3rd, 4th or 5th) can be attributed to countershaft bearings as there is no powerflow through the counter gear in direct drive. Wheel bearings should be tested by driving in a straight line and then initiating strong turns right and left. In the turns, you will be loading the bearings and the noise should change if the wheel bearings are defective.

Gear noise: Noises caused by axle gears can be created by incorrect backlash, out-of-spec pinion depth, damaged or worn gears, worn or bent rear-end housings, worn or bad carrier, loose pinion nut, or low lubricant levels. When road testing, try to cover all the throttle applications, constant load, accel, decel, and coast. Gears will usually be noisy at specific ranges of speed. Once you are satisfied that you have located the range, get to that point and coast to see what effect it has on the noise. Always check to make sure the unit has sufficient lube fill and the correct lube. Too little lube is no good, obviously, but too much is no good either, particularly in transmissions in which too much lube can cause notchy, dragging shifts. Diff side and pinion gears will show up in turns, as they are only supposed to function when cornering and not in a straight line. Mismatched tire sizes can create differential gear noise as they may make the diff operational in a straight line driving situation.

Knocking: Worn U joints, CV joints, damaged or worn side gear thrust washers will all cause a low-speed knock, as will a pinion-shaft bore that is out of round.

Vibration: Vibrations will usually show up at specific speeds and may stop or increase as higher speeds are reached. Damaged drive shafts, out-of-balance drive shafts (missing balance weights), worn U joints, CV joints, pinion CV joint flange with excessive runout, worn or out-of-balance wheels, bent or worn axle shafts, loose or broken springs, loose wheel lug nuts, damaged front-end parts, trans or engine mounts, engine accessories, drive belts, and brackets can contribute to what appears to be a rear-end vibration. Inspect very component of the driveline and suspension before coming to a conclusion.

Snap or shock load of the driveline: Excess idle speed, excess side gear to diff-case specs, incorrect ring-and-pinion backlash, loose pinion nut or CV-joint flange, excess take-up in the transmission, worn trans or engine mounts, bad CV joints, partially stripped axle splines, body and suspension bushings that are torn or frozen, stretched T-case chain will all cause the driveline to snap when engaged in gear or on takeoff.

You only get one chance to do it profitably. How come there is never enough time to do it right, but always enough time to do it over?

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