The world of NVH (noise, vibration, harshness) diagnostics can be rather interesting. Over the years, many different pieces of equipment have been used to point the technician in the right direction when tackling noise and vibration complaints.
Before I go too far on the subject, we need to understand the causes of NVH that cause our customers to bring their vehicles in for diagnosis. When it comes to a vibration, the only difference between the slamming of a door (which creates a noise and a vibration in the vehicle) and a vibration that will make the dashboard rattle, is the frequency of the vibration and the amplitude (harshness) of the vibration. Keeping these two things in mind can make the finding of the problem a lot easier.
Applying technology to NVH
Over the years, I have used several different types of tooling to find the causes of NVH; reed tachometers, the Chassis Ear®, stethoscopes and the Pico NVH kit, for example. Each of these tools has their place and should be used as the need arises. NVH problem analysis starts out much as any other problem analysis by gathering a lot of information over a wide area. This might start with a test drive and listening and observing the operation of the vehicle while it is being driven around corners, over bumps and on smooth roads, all while trying to duplicate the concerns of the customers. We can call this “getting a direction” so the vibration or noise can then be pinpointed with other tooling or testing.
I think most all of us have had our fair share of wheel bearings that would talk as the vehicle is driven. Many times it is easy to use a stethoscope or even grab hold of the coil spring and turn the wheel to both hear and feel the vibration that is caused by the rough wheel bearing, but what about a wheel bearing on a vehicle with torsion bar or leaf spring suspension? This adds to the difficulty of finding the problem. There is no one simple way to accomplish the task.
When it comes to vibrations in the drivetrain (engine, transmission, driveline and differential), these vibrations can all be synchronized with engine RPM or tire RPM. It all boils down to the frequency of the vibration.
Over the years, I have had vibrations that gave me a run for my money. I remember one on a Mercedes Benz E320 that came in for a rear wheel bearing noise. I found the problem with the right engine mount. The mount had deteriorated, which let the engine lean over to the right, which let the exhaust pipe bump on the frame, which transferred the noise down to the right rear wheel bearing. Stopping the wheel would make the noise go away. Vibrations can be transmitted throughout the vehicle easily, which can make the problem of finding the root cause of the NVH hard at times.
For those of us that have done NVH work for a while, I think I can safely say, “There is no one part that vibrates the same way, or makes the same noise when it fails.” Take for instance a wheel bearing; I have heard them squeak, growl, moan or not even make a noise when they fail. Having a tool to give a good direction to NVH is a way cool tool to have.
The tool I am currently using is the Pico NVH tool, which uses either 1 or 2 accelerometers which are hooked to interface boxes, which are then hooked to a Pico 4 channel labscope. The tool also needs to have an RPM signal, either from data from the vehicle Diagnostic Link Connector (DLC) or from a digital RPM pickup from the engine crankshaft. By far, the easiest is to use either the PICO cable that is designed for this purpose or you can also use a J2534 box to hook between your laptop and the vehicle DLC. By hooking to the DLC, you not only get RPM data, but you also get vehicle speed data. Both pieces of data can be very helpful when it comes to pinpointing vibrations.
The basics of vehicle vibrations
A few basics on the tool can be seen in Figure 1. Listed on the scope screen are plots T1, T2 and T3. These are vibrations caused by the tires. T1 is a first order vibration from a tire (one occurring every rotation of the tire — such as an out of round tire). T2 (second order) and T3 (third order) vibrations are a vibration that is two or three times the speed of the tire. Changing the vehicle speed will cause these vibrations to come and go. By using these variations, you can pinpoint issues with tire balance and out of round tires or anything that could be related, such as an out of balance brake drum.
To the right of T1, T2 and T3 is E1 and E2. These are engine vibrations. Since this is a front wheel drive vehicle (Ford Focus), there are no driveline vibrations listed. The E1 and E2 (first order and second order engine vibration) vibrations are normal for a four-cylinder engine, although, pay attention to the amplitude of the vibrations.
The E2 vibration on a 4 cylinder is a vibration two times engine RPM which is the frequency of the firing events of the pistons. A 6-cylinder engine would show an E3 vibration and a V8 engine would show an E4 vibration, so pay close attention to the amplitude of those vibrations. Most vehicles will not exhibit a vibration that can be felt in the steering wheel.
Before we leave the basics of the waveforms, I want to speak to the problem of ghost vibrations in the waveforms. Figure 2 is a classic example. Notice the vibration of 60 Hz and 120 Hz. These seeming vibrations in the waveform will not change with engine speed. The cause of the waveform is the laptop charger is plugged in and the screen is displaying the frequency of the AC current that the laptop is running on. The fix is to just unplug the laptop and run it on battery. Here again, learn what is good in a waveform.
The problem vehicle of the day
The vehicle is a 2006 Toyota Rav4. It has been in my shop a few times for some maintenance. The first time it was in, I noticed an engine vibration when I backed the vehicle out of the shop. The vehicle owner wasn’t much concerned and the vibration had been there for quite some time. Two month later, the vehicle was back with a complaint of, “the engine sounds like a chainsaw when it idles.” When I started the engine, sure enough, it sounded a lot like a chainsaw and the vibration I had noticed before was even worse.
My first step in this vibration analysis was to hook up my NVH tool to see if I could get a quick direction. Since this was an engine vibration at idle, there was no need to drive the vehicle. All the diagnostic work was done in the shop bay. I start all my NVH diagnostic routines by sticking the accelerometer to the inside driver’s seat rail. With the engine idling, I captured the waveform found in Figure 3; it shows an E2 vibration of 375 MG (Milli-G) which is a measurement of acceleration. This vibration is strong enough to be felt in the steering wheel and make things rattle in the dash. If you compare this vibration of 375 MG to the 20 MG vibration found in Figure 1, you can get the vibration into perspective. The Ford Focus engine is not all that smooth at idle.
The vibration in the Rav 4, along with the noise from the engine, is not normal at all. One of the benefits of using the NVH waveforms on a problem like this is to get a direction and a possible pointer on the next place to go and test. It is easier to spend a few minutes looking at a waveform than it is to start taking an engine or transmission apart in hopes you can find the problem.
Since this is an E2 vibration, what could be the cause? It is normal for a 4-cylinder to have an E2 vibration since this fits the frequency of the combustion events, but this vibration is a little strong. In fact, it more than a little strong — it is a lot strong.
Clues to the problem surface
So far I have two clues to the problem — the E2 vibration and the noise. Using a stethoscope on the engine, the noise is loudest down in the lower part of the center of the engine and in the center of the engine oil pan. Knowing this, we can rule out anything related to a crankshaft vibration or a piece of piston being broken off causing a vibration from one light piston. It narrows the problem down to something rotating two times for every crankshaft rotation. From my experience with 4-cylinder engines, many of them have a balance shaft that sits under the crankshaft. These balance shafts turn either two or three times the speed of the crankshaft. By removing the engine oil pan, the balance shafts can be accessed for inspection.
With the engine oil pan removed and the lower balance shaft housing removed, the balance shafts will drop out without any problems. With the first balance shaft out, I found the bearings on one of the balance shafts were badly worn as you can see in Figure 4. This badly worn bearing was causing both the noise and the bad vibration in the vehicle.
The repair needed was a set of new balance shafts and new bearings. With the new parts installed, the “after the repair” test shown in Figure 5 confirmed the problem was fixed.
By using the NVH tool, service information and experience, I was able to know where the problem was before I started taking the engine apart. The time spent finding out where the problem was before surgery started on the engine was a great investment, which made finding the problem quick and easy.
Many times the real truth about the problem comes out after the problem is fixed. I was told by the vehicle owner that this vibration problem was present after the vehicle had been taken in for warranty repairs for oil consumption. The engine had gotten a set of new pistons and when the vehicle was picked up after the repairs, the vibration was present. When I took the engine apart, I found the balance shafts were not in proper time with the crankshaft. This built-in vibration had taken its toll over time on the balance shaft bearings.
