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Bad automotive vibes

Don't get shaken up over vibration complaints
Tuesday, June 5, 2012 - 11:56
Don't get shaken up over vibration complaints

A customer brings their vehicle into your shop with a vibration complaint; you hesitate to take the job because you know that the success rate of diagnosing and repairing vibration concerns is not very high. You hate to turn away repair work, so you accept the job and hope you can find the problem. Many shops will begin by balancing the existing tires or by selling new tires to the customer; many times this will correct the vibration problem, but what if it doesn’t? Now what do you do?


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Vibration diagnosis is one of those subject areas in automotive repair that most service technicians try to avoid. When a vibration diagnosis job comes into the shop, most technicians fight over who has to take the job rather than who gets to take the job. Many times the job ends up in the hands of the least experienced and least qualified technician because nobody else wants it.

Wheel balancer system manufacturers are very familiar with this situation. They receive phone calls from their balancer customers who claim they have balanced the wheels and the vehicle still has a vibration concern; their customers think there must be something wrong with the wheel balancer.

Many wheel balancers cost well over $10,000 and can do a wonderful job of correcting tire and wheel assembly related vibrations if they are used correctly. Unfortunately, it has been my experience that the expensive balancer system is rarely used for anything other than balancing. Why? Nobody knows how to use the other features (runout compensation, match mounting, road force variation compensation, vehicle tracking compensation) and it takes longer to use them.

You might be asking yourself, what is this guy talking about? Aren’t wheel balancers for balancing tire and wheel assemblies? Answer this question: Can a square tire be balanced? The answer is yes, but obviously a square tire will still cause a vibration while rolling down the road. Hopefully you are learning that there is a lot more to vibration diagnosis than tire balancing. 

Parts Groups
The real story is that there are many other parts on a vehicle that can cause a vibration, even if they are balanced. Anything on a vehicle that rotates can cause a vibration. I like to group the rotating parts into different groups that are related to their common rotational speeds. The groups are listed from most likely cause to least likely cause:

Group One - Tire Speed Related Vibrations. This group includes any parts that rotate at the same speed as the tire: tire, wheel, brake rotor, brake drum, wheel hub, solid axle shaft, constant velocity drive axle shaft, differential case, axle ring gear, differential side gears, differential side bearings, etc.

Group Two - Driveshaft Speed Related Vibrations (rear-wheel drive and four wheel drive vehicles). This group includes any parts that rotate at the same speed as the driveshaft: transmission output shaft, transfer-case input and output shafts, driveshaft slip yoke, universal joints, constant velocity joints, axle pinion flange, axle pinion gear, axle pinion bearings, etc.

Group Three - Engine Speed Related Vibrations. This group includes any parts that rotate at the same speed as the engine’s crankshaft or one of its belt-driven, chain-driven, or gear-driven accessories: crankshaft harmonic balancer, crankshaft, flex-plate, torque converter, flywheel, clutch pressure plate, clutch disk, clutch release bearing, alternator, air-conditioning compressor, water pump, idler pulleys, belt-tensioners, drive belts, cam shafts, balance shafts, etc.

Group Four – Electric Motor Speed Related Vibrations. This group includes any parts that rotate at the same speed as the electric motor that is driving them: electric motor armature, electric cooling fans, electric blower fans, electric water pumps, electric transmission oil pumps, electric air-conditioning compressors, electric drive traction motors in hybrid-electric and electric vehicles, etc.

As you have read, there can be hundreds of parts on a vehicle that can cause a vibration. Obviously tires are not the only possible cause. How are you going to accurately diagnose the source of the vibration? Almost any technician can fix a vibration problem that already has been diagnosed; it takes a technician with special training and tools to accurately diagnose a vibration concern correctly every time, otherwise you are just guessing. 

The good news is that every part rotates at a unique speed that can be identified with an Electronic Vibration Analyzer (EVA). Once the part has been identified, testing, repairing, or replacing the part is easy.

Tools
The original EVA was introduced in 1991 and is used by General Motors (GM) dealerships. Since then other vehicle manufacturers have used it including Ford and Chrysler. The EVA has been updated and is now called the EVA2. It still might be available through SPX Kent-Moore Tools (1-800-345-2233) and is part number J-38792A. The cost is usually in the neighborhood of $2,000 for the basic kit, and $2,700 for the kit that comes with a timing light kit that is used to strobe balance driveshafts. 

If you already own an inductive timing light, buy the basic kit; your timing light will work as the strobe light. Occasionally I see EVAs available on eBay for as low as $300. There are other vibration analyzers available in the industry, I have purchased some of the other analyzers and use them, but in my opinion, the EVA2 is the best deal for the money spent.

Terminology
To properly use the EVA2 and understand the techniques involved in diagnosing vibration concerns, you must first understand some basic vibration terminology.

Runout: I mentioned earlier that a balanced tire and wheel assembly could still cause a vibration. I think that because a tire and wheel assembly looks round, we assume that balancing the assembly is all we need to worry about. When a tire is not perfectly round, we say that the tire has runout. Many people do not believe that you can balance a square tire, but you can and obviously it will still cause a vibration while trying to roll down the road. Every other rotating part on the vehicle can also have runout and cause a vibration, even if it is balanced. How much runout is allowed? Each part has its own runout specification and most are easily found in your service information system.

Cycle: One complete disturbance or shake felt by the customer or you.

Frequency: The number of cycles that are heard or felt in a certain amount of time. The unit of measurement for vibration frequency is typically Hertz (Hz), indicating cycles per second; however, RPM, indicating cycles per minute, can be used also. The advantage of using RPM rather than Hz is the ease of matching the RPM of the vibration to the RPM of a rotating component. For example, if the vibration frequency measured on the EVA2 is 2,000 RPM and the engine RPM is also 2,000, it is easy to see that they match.  However, if Hz is used, a vibration frequency of 33.33 Hz is not an obvious match for the 2,000 engine RPM.  Some vibrations can be felt, but not heard (typically frequencies lower than 20Hz which correspond to the speed of most tires), and some vibrations can be heard but not felt, or felt and heard (typically frequencies in the 20Hz – 20,000Hz range).

Amplitude: The measurement of how hard or harsh a vibration is. Low amplitude vibrations usually cannot be felt or heard. High amplitude vibrations can be heard or felt or both depending on the frequency of the vibration. Decreasing the amplitude level of a vibration concern is the primary objective in the repair procedure. On the EVA2’s live data screen, the lower the G’s number, the milder the vibration. Typically there is not a specification for how low of an amplitude level is acceptable; you must take before and after amplitude readings to verify that your repairs were worthwhile.

Order: The number of disturbances or shakes in one revolution of a rotating component. Certain orders of vibrations can only be caused by certain failures. Anything that is out of balance will only cause a first order vibration.  In other words, balancing a tire with a second order vibration (two disturbances in one revolution of the tire) is a waste of time. Understanding this concept can be very helpful when diagnosing the source of the vibration.

  • One shake per revolution is referred to as a first order vibration, two shakes per revolution is referred to as a second order vibration, three shakes per revolution is referred to as a third order vibration, four shakes per revolution is...
  • There is no limit to the numerical value of order; however, the higher the order, the less likely you will ever see it.
  • Order values can also be real numbers having decimal values like 1.5 order, 0.5 order, 1.23 order, etc.  They do not have to be Integers like 1, 2, 3, etc.

Anything that is out of balance will only cause a first order vibration. Any higher order vibrations are not caused by an out-of-balance component.

EVA2 Usage
The EVA2 has an Auto Mode and a Normal Mode for vibration diagnosis. The Auto Mode was designed for unmodified GM vehicles from 1991 through 1998. The Auto Mode attempts to automatically calculate the source of the vibration on a GM vehicle. Using the Auto Mode on other vehicles, especially most crossover SUVs, other vehicle years, or on modified GM vehicles (larger or smaller diameter tires) can lead to misdiagnosis.  Instead of using Auto Mode, use Normal Mode.  Normal Mode works on all makes, models, and years of cars and trucks.

After selecting the Normal Mode from the Main Menu of the EVA2, the three highest amplitude vibration frequencies detected by the sensor are displayed.

Most technicians do not know what to do with this information, so the EVA2 sits on a shelf unused. To make sense of the information displayed, you will either need to mathematically calculate the rotational speeds of all the rotating parts on the vehicle and match it to the EVA2 screen, or purchase a software program to automatically do it for you. 

There is a software program called Vibrate 5.1 available from SPX Kent-Moore Tools (part number J-38792-VS). This software runs on a PC and graphically displays the rotational speeds of all the rotating components on your vehicle. The user of the software prints the graph and takes it with the EVA2 on the Road Test. The software is also available at www.vibratesoftware.com. (Editors note: Don’t have an EVA2 or other electronic vibration analyzer? Check out the tips on the website for optional methods. They won’t be as easy but may help you solve that problem child in your bay.)

Diagnosis
The vibration graph shows Engine rpm along the bottom and Vibration Frequency along the left side of the graph. The driver places the magnetic base vibration sensor on the seat frame under the driver’s seat and then brings the vehicle up to the speed where the vibration is most noticeable and records the Engine rpm and the highest amplitude Vibration Frequency from the EVA2 on the graph. 

If the EVA2 is not displaying a consistent steady reading as you maintain a steady driving speed, move the sensor to another location. There is a point on the graph where the two measurements intersect; this point represents the rotational speed of the part that is causing the vibration. The part can be identified by the diagonal line on the graph it is over or very near.

The part identified may be damaged, defective, or it may simply be misadjusted. You will need to follow the step-by-step diagnostic procedures in the vehicle manufacturer’s service information or the Vibrate 5.1 Help file to diagnose, adjust, or replace the part identified as the source of the vibration.

Summary
With a little practice, the proper tools, and some training, vibration diagnosis can be a precision diagnostic routing with little room for error. Even a tire speed related vibration can be narrowed down to the corner of the vehicle causing it by comparing the amplitude levels from the sensor of all four corners of the vehicle. The corner with the highest amplitude level is the corner with the problem. 

There is a lot more that can be written about this topic, but for now I suggest that you can improve your vibration diagnostic skills and reduce your comeback ratio by using the proper tools and incorporating the use of the EVA in your troubleshooting.

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