|Vehicle details:||Honda Civic|
NVH diagnostics requires a methodical approach and evaluation of test results to reach a conclusion that is satisfactory for all parties involved. The case study below describes the rapid identification of a customer’s complaint of vibration and how using multiple sensors (either microphones or accelerometers) become invaluable in locating the source of the problem.
The customer reported that a severe vibration was present throughout the vehicle while accelerating from 45 to 55 mph. (N.B. In this case, the customer was a trade person concerned about a vibration after an engine repair.)
Verifying the customer complaint is an essential step in the diagnostic process. However, it can quite often be a time-consuming task without much success. The initial road test did not reveal any major concerns for a vehicle of this age and mileage. However, after driving about 8 miles, we detected a severe vibration under acceleration only. When we accelerated, we felt a severe vibration. When we decelerated, the vibration cleared immediately. The vibration occurred around 45 mph, where items in the cabin (e.g., keys and coins etc.) would tremor in unison.
While we could describe the vibration as making the vehicle “borderline undrivable”, there were occasions where no vibration was present at 45 mph under acceleration, even after prolonged driving. (The vibration was intermittent.)
With the customer's complaint verified, we confirmed the Vehicle’s ID and Specification.
Confirmation of specification is of the utmost importance when it comes to diagnosis. Customers are often tempted to modify their vehicle with fashionable accessories that lack the fundamental quality control and engineering that was intended for the vehicle.
We noted that the vehicle had been fitted with three “budget" tyres, where two appeared to have covered very few miles indeed.
The customer interview follows the 4 targeted open-question principle below to help us establish facts from fiction:
1. How long has the problem been evident?
There was no previous experience of a vibration, even though the vehicle was originally scheduled for an engine running problem that developed into an engine failure (read more about this vehicle here).
2. When did you first notice the problem?
There were no reports of vehicle vibration (it was only discovered after the engine repair).
3. Has any work been carried out on the vehicle recently?
Yes, major engine overhaul due to timing chain wear.
4. When do you experience the problem?
Every drive after approximately 8 miles, when accelerating between 45 and 55 mph.
Quite often the customer interview leads to a probable diagnosis, and given the history of the major engine overhaul, it is difficult not to jump to conclusions. Especially since the vibration is only present under acceleration.
The basic inspection confirmed no fluid leakages, no visible signs of damage to hoses, connections, or wiring harnesses, or indeed that favourite discovery, accident repair.
A vehicle scan of all onboard control units revealed no fault codes, which suggests that our symptom is outside the detection condition of the Engine ECU.
• Engine misfire under acceleration
• Incorrect installation of engine mounts or ancillaries
• Worn or out-of-balance driveline components
• Worn suspension components
• Wheel and tyre runout or imbalance
The action plan was predominately governed by accessibility, probability and cost. Based on the symptom, we required objective measurement data to establish not only the level of vibration (amplitude) but also the offending frequency as well as the axis of the vibration.
What better tool for such a task than the Pico NVH kit?
• An intermittent severe vibration under acceleration (45 to 55 mph)
• The vibration was present after driving approximately 8 miles (not from cold).
• There was no vibration present under deceleration
• The vibration was load dependent
We started with a 3-axis accelerometer mounted on the driver’s seat mounting bolt and a microphone mounted inside the cabin. We now have all avenues covered.
The accelerometer identifies the level of cabin vibration along with the offending frequency and axis. A word to the wise, however. To identify the offending axis of vibration, you must make sure that the accelerometer is mounted in the correct orientation as seen in the image above.
Since the complaint is vibration only (0-20 Hz), the microphone was only used to record any narrations from the driver such as location, road surface texture, vehicle posture, gas pedal position and steering angle to name just a few snippets of useful information during playback.
In the image below, we have the first capture from our initial road test. We identified an intense vibration of 101 mg at 10.7 Hz in the Z-axis (a lateral vibration) under acceleration. Our engine speed was recorded as 2377 rpm at a road speed of 45 mph. The vibration also occurs at the same frequency as “T1”. In other words, a component rotating at the same frequency as the road wheel and tyre.
Note: The microphone signal has been hidden from view in all captures as we wanted to focus on vibration frequencies only (<20 Hz).
So, now we have our objective measurement data, and offending frequency and axis. What do we do next? This is where multiple accelerometers come into their own and can assist with the diagnosis and zoning of a vibration.
Since our vibration is lateral (Z-axis), we now disconnect PicoScope channels A and B from X and Y on the NVH interface, leaving only Channel C to capture the lateral vibrations (the Z-axis) from the connected accelerometer (See image below).
The following tests were conducted with three accelerometers (measuring the Z-axis only) and a microphone mounted inside the cabin. Note: For accurate detection, the accelerometers must be mounted vertically when measuring lateral vibrations.
It is advisable to keep an accelerometer mounted inside the cabin for all subsequent measurements. This will serve as a reference in which to compare vibration levels at strategic points about the vehicle.
Below we compare the lateral vibration levels at the rear subframe to the cabin.
Above, we concluded that the lateral vibration inside the cabin was greater than the level at the rear subframe. Therefore, our additional accelerometers (Channels B and C) would be better positioned at the front of the vehicle.
Below, accelerometers B and C were moved to the front shock absorbers, alongside the hub assemblies.
Above, we had a clear increase in lateral vibration at the front shock absorbers, compared to the cabin. Note: We also had the formation of T2 and T3 vibration orders (T3 is three disturbances for every one revolution of the road wheel and tyre).
What I found bizarre about this severe vibration, was that I could feel very little via the steering wheel. It all appeared to shake the cabin and floor pan (the chassis was the responder and not the steering).
Further road tests would periodically see a dramatic increase in the lateral vibration levels at both front shock absorbers. It was near impossible to qualify which side of the vehicle was the true offender. Where was the source of the vibration?
Based on the analysis above, we needed to return to our list of possible causes and eliminate accordingly.
• Engine misfire under acceleration
I believed we could move away from a misfire, based on our T1, T2 and T3 offending frequencies, since a misfire would return an E0.5 vibration order. The engine management system would also record such an intense and prolonged misfire if applicable.
• Incorrect fitment of engine mounts or ancillaries
Any anomalies here are highly unlikely to appear as a T1 vibration order. A visual inspection confirmed that all ancillary and mount installations were correct and secure.
• Worn or out of balance driveline components
This was most certainly a possible cause and required further investigation. A visual inspection revealed no concerns.
• Worn suspension components
A visual inspection confirmed that the suspension components were secure and with acceptable wear only.
• Wheel and tyre runout or imbalance
While this could be a possible cause of a T1, T2 or T3 vibration, wheel and tyre are unlikely to produce intermittent lateral vibrations under acceleration only.
A severe lateral tyre vibration (at the front of the vehicle) would also manifest itself in the steering wheel, which was not evident at 45 to 55 mph.
With that said, we used best practice to measure runout and balance and the results are recorded below.
As you can see above, we have what I can only describe as a pick & mix array of radial/lateral runout values accompanied by an assortment of tyres and rim damage. While these results require further attention, we balanced the wheels to 0 g and remounted them to the vehicle in their original orientation. Following the above, a brief road test confirmed that we had not achieved any improvements.
Note: We did not measure the radial tyre runout and perhaps we should have. However, excess radial runout is likely to cause vibrations in the X (fore/aft) and Y (vertical) axes rather than our offending Z-axis.
Given the intense level of vibration and the adverse effect it has on the cabin/chassis at 45 mph, it occurred to me that we could reveal the offending component by mounting a webcam under the vehicle.
See the image below:
By using the split-screen feature in Windows and the integrated Camera app of Windows 10, we were able to simultaneously monitor NVH data while capturing a response from numerous components within view of the web camera.
Watch the video below. Note how the engine physically moves in a repetitive lateral direction during acceleration and returns to rest during deceleration. Focus on both screens between 11 and 30 seconds.
The NVH file in the video highlights the lateral movement of the engine, which in turn is responsible for the response we feel in the chassis.
The keen-eyed amongst you would have spotted that Channel C in the video is returning a huge lateral vibration level, above 229 mg!
We relocated the accelerometer that was connected to the right-hand front shock absorber to the right-hand driveshaft support bearing, which was attached to the cylinder block. (Channel C below = 365 mg.)
Regarding the T1 amplitude recorded above, I would like you to think for a moment about the energy level required to repeatedly move an engine and transmission assembly side-to-side at a frequency of 10 cycles per second. If you were to repeat this process manually with a prybar, the effort required would be immense and no doubt physically challenging.
So, what component on our drivetrain could create enough energy to move the engine/transmission in a lateral direction?
• The engine/transmission assembly was forced in a lateral direction under acceleration.
• We detected T1, T2 and T3 vibration orders, but the main offender was T1.
• The accelerometer connected to the left-hand shock absorber indicated energy spikes
• The wheels/tyres required attention but were not responsible for the lateral engine movement.
• The offending component was rotating at road wheel frequency/speed.
When reading the recap above, the only components capable of developing such energy are the driveshafts. But which one was causing our problem?
We carried out radial, lateral and axial measurements on each driveshaft in an attempt to differentiate the levels of wear between the shafts.
The above measurements revealed the following:
The left-hand front driveshaft
The inner driveshaft joint lifted (within transmission) 1.20 mm.
The axial movement between the inner and outer driveshaft joints was 0.95 mm.
The radial runout of the shaft was 0.60 mm.
The right-hand front driveshaft
The inner driveshaft joint lifted (in the extension shaft) 0.55 mm.
The axial movement between the inner and outer driveshaft joints was 0.90 mm.
The radial runout of the shaft was 0.70 mm.
Based on the amount of lift in the inner drive shaft joint (the majority of which is within the differential carrier bearings), I leaned towards the removal and inspection of the left-hand driveshaft.
From a PicoScope measurement perspective, it was difficult to determine which was the offending shaft from the Frequency view alone. However, by using the Time Domain view, the energy spikes described earlier in this case study were used as further evidence to support the removal and dismantling of the left-hand driveshaft's inner tripod joint. The images below reveal a multitude of sins.
Tripod joints are typically responsible for three disturbances for every one revolution of the road wheel (T3). However, the images above reveal considerable damage to a single ball case and the associated guide in the inner tripod joint housing, which would create one disturbance per revolution (T1).
Looking at the joint guide, it would appear that the ball case may have been sticking, dragging or locking against the guide under the transfer of load when accelerating. This could explain why the fault was occasionally intermittent and temperature-related.
Such an event would have the potential for a repeated push-back against the final drive instead of the desired seamless axial movement of the spider inside the joint. The result here would be the repeated lateral movement of the engine/transmission that we captured on camera.
We replaced the left-hand front driveshaft and measured for radial, lateral and axial movement again.
The left-hand front driveshaft
The inner driveshaft joint lifted 0.90 mm (within transmission). A reduction of 0.30 mm.
The axial movement between the inner and outer driveshaft joints was 0.45 mm. A reduction of 0.50 mm.
The radial runout of the shaft was 0.60 mm.
Results / Confirmation of repair
Below, we captured the T1 lateral vibration level at the right-hand driveshaft support bearing (attached to the cylinder block) at 34 mg. Previously, this was recorded at 365 mg, which equates to an improvement of 331 mg or approximately 89%.
From a human perspective (subjective), the vehicle felt like a completely different car to drive.
Note the peak at E2. This is normal for a 4-cylinder engine under acceleration (2874 rpm) due to combustion and is not related to our customer's complaint. You can find more information on vibration orders here.
A left-hand front drive shaft assembly
There is never time to review why components fail, given the time constraints and pressures of a typical workshop environment. “If it’s fixed, great, get it out and get the next one in”.
I understand this wholeheartedly but let us pause for a moment. Why did the inner driveshaft joint fail in such a fashion? Without any valid vehicle history, we can only assume there had been a previous intrusion, leakage of joint grease, or driver abuse. Road wheel damage is often a clue of the latter. The differential carrier bearings did concern me, based on the amount of lift in the left-hand inner tripod joint (0.90 mm post-fix), but with that said, there were no differential noises and so only time can tell.
When reviewing this case study, it occurred to me just how vital it was to introduce multiple accelerometers and record the data for post-capture analysis in multiple views with included narration. These are all essential features that are not available with NVH mobile phone apps.
The ability to zone into the source of a vibration by using multiple sensors is key to success. Objective data cannot lie and provides measurements as evidence both pre and post-fix for our customers.
I must add that the recent improvements to our NVH software have made this tool a joy to use when road-testing. The M&S Hot-key functions we describe here provide a rapid means to Marking and Saving your data for later review when it is safe to do so.
One final word on post-fix captures. Our customer's complaint was a severe vibration at 45-55 mph, which is now resolved. Best practice should from now on also include a sweep of vibrations across the entire road/engine speed ranges. This is to make sure that there are no other non-typical vibrations present.
The capture below is one such example, where T1 raises its head again in a lateral direction at the left-hand front shock absorber.
Note: The accelerometer attached to the left-hand front shock absorber captured an increased lateral T1 vibration of 162 mg at approximately 95 mph. Channel C (connected to the right-hand driveshaft support bearing, which is attached to the cylinder block), however, reported only 8.66 mg at this same T1 frequency. (E.g. a minimal lateral movement of the engine.) Therefore, the stronger vibration on the front shock absorber is no longer related to our driveshaft as this vibration is now greater than the driveshaft vibration and most probably linked to our pick & mix array of wheel and tyre errors at high road speeds.
This is proof indeed why a logical approach to diagnosis is essential, as this vibration could be misinterpreted as the initial fault when the facts confirm another component is now responsible for a vibration at the same frequency of the road wheel and tyre. (I feel part 2 of this case study coming on.)
Many thanks to Kevin at Ives Garage for yet another diagnostic adventure.
June 28 2021
What a superb case study. And what an inspired idea with the dashcam and the split screen in Windows. I’ll never look at the abbreviation M&S in quite the same light again.