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Vibration Analysis, Part 2: Driveline

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Vibration Analysis, Part 2: Driveline
by GM

The sources of the vibration conditions that may be found on some 2014 Silverado 1500 and Sierra 1500 and 2015 Silverado, Tahoe, Suburban, Sierra, and Yukon models are most often the tires or driveline components, including axles and propeller shafts. These vibrations often occur at speeds of 35–45 mph (56–72 km/h) or 60–70 mph (96–120 km/h) and are felt in either the seat or steering wheel.

TIP: If the vibration can be duplicated on the rack, the test should be performed a second time with the wheel and tire assemblies removed from the vehicle and the wheel nuts installed to retain the brake discs and/or brake drums. If the vibration is eliminated, focus on the wheel and tire assemblies as the source of the vibration. If the vibration is still present, focus on the vehicle driveline as the source of the vibration.

Some vibrations may be difficult to diagnose even when the vibration can be duplicated. One example from a Technical Assistance Center (TAC) case was a concern about a vibration at 45 mph and higher on a 2014 Silverado 1500 4WD. The vibration was easily duplicated at these speeds. Initial diagnosis focused on a tire vibration.

A road test by a field service engineer using the CH-51450 Oscilloscope Diagnostic Kit with NVH showed a 1st order propshaft vibration with an amplitude of 7.83 mg at 49 mph. (Fig. 7)
fig07

The propshaft was balanced using the oscilloscope, but the condition did not improve.

TIP: For vehicles that are out of balance, perform a system balance. Using the two hose clamp method, the best driveline balance results are obtained under 10 g-cm. (Fig. 8)
fig08

Once the rear housing cover was removed, a 0.25–0.28 mm (0.010–0.011 in.) total variation of the backlash of the ring gear was found. The positions of the ring gear were swapped and side shims were installed to bring the backlash down to 0.1–0.12 mm (0.004–0.005 in.). However, the vibration was still present.

TIP: If the difference between all the measuring points is within specifications, the backlash at the minimum lash point measured should be 0.08–0.25?mm (0.003–0.010?in) with a preferred backlash of 0.13–0.18?mm (0.005–0.007?in).

The pinion and ring gear was replaced. A second road test showed a vibration amplitude of 0.722 mg at 45 mph. (Fig. 9)
fig09

Training and Tools
A recent TAC case on a 2013 Silverado 4WD illustrates the importance of proper training and the use of the correct tools during diagnosis.

In this case, after considerable time and multiple repairs, including a complete engine replacement, the vehicle was repurchased from the customer after the source of an engine idle vibration could not be found. The vehicle had a rough idle in gear during stops.

Using the CH-51450 Oscilloscope Diagnostic Kit with NVH showed a first order frequency with an amplitude of 5.66 mg at 525 RPM, which was significantly higher than a known good vehicle. (Fig. 10)
fig10

Once the baseline disturbance was measured, isolation of the first order engine disturbance and diagnosis could begin. Systematically, the serpentine belt was removed, and then the transmission torque converter was unbolted, which resulted in a first order engine frequency at an acceptable level. (Fig. 11)
fig11
The normal level of first order engine frequency was achieved by re-indexing the torque converter to the engine flywheel. No parts were required.