Other Articles - January/February - 2019

Make It Stop Shaking! Isolating Vibration Issues

Vibrations can be one of the most frustrating things that you can have to repair. Finding the source of the vibration and then eliminating the cause can be a challenge for even the most seasoned technician.

Several years ago, I wrote a selection of GEARS articles explaining the theory, diagnosis, and repair for common vibrations faced by most shops. The nice thing about vibrations is that they’re generic, so you can diagnose and repair them using the same techniques no matter the year, vehicle type, or brand.

One of the frustrations for most technicians was that you needed to do some math. The equipment used to locate the issue (a reed tachometer or electronic vibration analyzer) only gave the values in raw numbers, so you needed to do some calculations to isolate the cause of the issue.

The good news is that, as times and technology change, some things do actually get easier; diagnosing vibrations is one of those things. Many shops have purchased a trick little tool to help them with all sorts of vehicle diagnostics: a Pico Scope.

We all think of the Pico Scope as a laptop-based tool designed to help you with electrical diagnosis, but it’s really much more. One of its features is the scope’s ability to diagnose vibrations. Software and hardware are available to allow you to diagnose the cause of the vibration and even perform some repairs, such as balancing a driveshaft or engine.

The tool and software are available in two versions: OEM and aftermarket.

So what do you need to know before you can get started? Let’s start with some vibration terms:

  • Vibration — A repetitive motion.
  • Frequency — The number of vibration cycles in a given time: seconds or RPM (How often you get your paycheck would be an example of frequency). Frequency is like the fingerprint of the vibration: If you know the frequency, you can identify its cause.
  • Amplitude — The strength of the vibration.
  • Order — How many times the disturbance occurs in one rotation.
  • Resonance — The point of maximum vibration.
  • Cycle — Repeatable over and over again.
  • Beat — Two vibrations with almost the same frequency. This appears to make the vibration come and go.
  • Hertz — Hz or cycles per second. Vibrations are referenced in Hz or in CPM (cycles per minute).

The vibration kit is an add-on for your laptop Pico Scope. The sensor, known as an accelerometer, sends the vibration frequencies and their amplitudes to your laptop. The free Pico Scope vibration software displays the values on your laptop.

The software even goes a step further: It isolates the causes of the vibrations that are present so you know if it’s an engine, accessory, driveshaft, TCC (yes TCC shudder!), or a wheel or tire issue that you’re facing.

The sensors are available in single-axis or three-axis designs. The three-axis design sensor is preferable as it monitors up-down, fore-aft, and lateral movement. Install the sensor in the correct direction (note the arrows on the sensor) if you want the laptop to display the correct information.

Mount the sensor with its cord vertical and the sensor as close to horizontal as possible. Typically, you’ll mount the sensor to your driver’s seat frame unless you’re using it to pinpoint a vibration, such as isolating which tire is causing the vibration.

With the sensor correctly mounted and the Pico Scope program active on your laptop, you need to enter the vehicle specifics: engine, transmission, gear ratios, and tire sizes. Next, operate the vehicle until the vibration is present. You can snapshot the vibration so you can review it after you get back to the shop.

When looking at the laptop display, notice the vertical bar graphs along the top. Each graph will be identified for what it represents: T1 (1st order tire), T2 (2nd order tire), T3 (3rd order tire), P1 (1st order propshaft), P2 (2nd order propshaft), E1 (1st order engine), E2 (2nd order engine), E3 (3rd order engine), E4 (4th order engine), and unknown vibrations.

So what do the values mean? The order refers to how many times the disturbance occurs each time the component rotates. The height of the bar graph and the digital value displayed in the chart indicate how bad the vibration is and its frequency. A chart shows the vibration amplitude, or how bad the vibration is at each of the frequencies displayed. You typically can’t feel values less than 10 MGs, so don’t worry about them.

Since the software does all of the calculations for you, simply knowing what each order represents and which component indicates the highest vibration amplitude is all you need to know to use the tool to repair a vehicle.

T1 — Tire balance or runout. T2, T3, or other T numbers are typically caused by radial force variation. The sidewalls of the tire act as springs for the tire. A difference in the stiffness will cause a vibration even though the tire is in perfect balance.

You can use tools such as the Hunter 9700 road force balancer to check your tires for force variation. Remember, just because the tires are in balance, look perfect, or are new doesn’t guarantee they aren’t causing the vibration.

Isolating which tire is causing the vibration is also quite simple with the Pico Scope. Simply move the sensor to different corners of the vehicle and check the amplitude. The corner with the highest amplitude indicates the tire that’s causing the issue (figure 1).

P1 — Propshaft runout excessive or propshaft balance.

P2 — Propshaft phasing or u-joint working angle. Phasing indicates the slip yoke/u-joints aren’t aligned properly to cancel shaft speed changes from the u-joints as the shaft rotates or the driveshaft twists (figure 2).

Engine vibrations are represented by E values on your laptop (E1, E1.5, E2, E3, and E4). Some engine vibrations are normal, such as the engine’s firing frequency. The engine, transmission, and exhaust mounts should manage normal engine firing frequency vibrations, so, if your normal firing frequency vibration is causing the complaint, that’s where you need to start your repair (figure 3).

Figure 4 shows a matrix of various vibration sources and possible causes.

Some engine firing frequency issues aren’t normal and indicate a problem that you’ll need to address. If you find the vibration is engine-related, remove the drive belt to see if the problem is still present.

Diagnosing accessory vibrations just got much easier with the advent of the tool. The software has an accessory vibration feature that asks you to measure the pulley diameter of each of the accessories and input those values into your laptop.

With the sensor mounted on top of the engine and the drive belt installed, operate the engine until the vibration occurs. The software will identify the vibration coming from each accessory, such as the alternator, power steering, air pump, or A/C system (figures 5 and 6).

DIAGNOSING TCC ISSUES

Isolating a TCC issue is always a challenge, but the Pico Scope is making it a little easier. You’re probably aware that the 8L45/8L90 applications have a lot of TCC-related complaints. The engineers at GM and now the dealer technicians are using the Pico Scope to identify TCC-related issues. To locate the issue, operate the vehicle like this:

  • Fluid temperature 120º-160ºF (50º-70ºC)
  • 8th gear
  • V8 mode
  • ISS 1050-1300 RPM
  • OSS 40-55 mph (64-89 km/h)
  • Steady throttle, torque values 200-375 Nm
  • TCC On

TCC-related issues will display values of about 22-26 Hz, with amplitudes of typically 16-24 Mg. GM currently suggests that you flush, drain, and refill the unit with Mobil 1 Synthetic LV ATF DEXRON HP (#19355656 U.S., #19355657 Canada).

BALANCING

The Pico Scope program is capable of balancing components, such as a propshaft. To balance the component, you’ll need to purchase an optical sensor attachment for your laptop (figure 7).

As you can see, the tools of today are making our jobs a little easier, even as the vehicles continue to get more and more complex. Without the correct tools and training, repairing today’s vehicles is truly a challenge.

Until we see each other at the ATRA 2019 seminars, remember, “Life is like riding a bicycle: To keep your balance you must keep pedaling.”