While preparing this month’s topic, it occurred to me that a short explanation of the process behind the scenes would be helpful. All the topics I have presented here over the years have been prepared from real issues we have been presented with in our workshop. This guarantees authenticity and technical credibility.  

The topic for this month is focused on a VW T5 van suffering severe vibration. I will begin by explaining that no repair authority was given at the conclusion of the diagnosis. The decision was based on a value versus repair cost and not through any disagreement. All cost was paid without objection.
     
The owner is a customer known to us. He often uses the vehicle for long journeys over extended distances between Lancashire and Cornwall. It was while down in Cornwall that the problem of vibration that brought the vehicle back to us began. The vibration was present with the engine running. In addition, it displayed a change in tone and reduction of intensity when full steering lock was applied.

While in Cornwall, we understand that a new alternator and power steering pump was fitted with no effect or reduction of vibration. Following this work taking place, with no change to the problem being seen, the decision was made to drive the vehicle to his regular trusted repairers. I.e, us!  This was brave to say the least, and potentially teeth-rattling for the duration of the drive back up to Preston.
    
In an odd sort of way, the diagnostic process had already begun as the van did in fact reach us, and did not display any additional problems. Power delivery was reported as normal, suggesting that the primary rotation engine components were working normally. Our initial checks were visual with a full serial evaluation showing no reported errors. The problem appears to be mechanical in nature with no collateral influence.

Before discussing the laws of physics when applied to a motor vehicle, why don’t we explain exactly what vibration is, and how it can escalate end cost if not accurately diagnosed.

Vibration is mass energy from a source, taken through the transfer path to a respondent. Not only is this wasted energy that could be converted into traction, it will also lead to premature component failure if left to continue.
Vibration is experienced in three ways; feel, sound and sight. How we experience it depends on the amplitude and frequency. High mass energy occurs at lower frequencies and is more likely felt and heard. Low mass energy occurs at higher frequency ranges often felt and seen.

Traditionally vibration has been diagnosed based on opinion rather than evidence. So, what’s the problem? Finish reading this article then you will understand the problem and risks. Vibration can also be affected by the transfer path and respondent. For example, a high mass vibration may be amplified by a light body panel or vehicle trim.

NVH
To succeed with NVH you must first forget you are working on a specific system and focus on frequency and amplitude. The motor vehicle is a series of mechanical systems in permanent conflict, a little like a modern marriage!

There are multiple components with mass differential (weight), vector conflict (direction), frequency (speed), and amplitude (volume). The Pico NVH kit uses a three-dimensional accelerometer and microphone, or multiples of each. They convert mass into a pictorial graph, bar chart or three-dimensional topography.

The primary requirements are engine speed via the serial port or optical input, transmission ratio data, and tyre size. With this information, the software will distinguish the area of responsibility along with any collateral transfer path and respondent frequencies. Further discovery is possible by entering individual component rotation ratio, for example power steering pump.

Physics lecture over. On to the T5. We did not need to enter tyre size as the vibration was present simply with the engine running. Crankshaft data came via a Mongoose serial interface with the accelerometer mounted directly on the engine.
Referring to fig.1, the left scaling is mass in milli gravity, the base scaling is frequency response.

E1 represents the crankshaft, E2 represents combustion mass. The cursors represent the number and ratio of events corresponding to E1.You can easily see that the vibration in the centre of the graph has no relationship with the crankshaft frequency or combustion events. That the frequency is higher (lighter mass) than E1/E2. Now we need to evaluate the engine mechanical ancillaries. These are driven by a complex gear train at the rear of the engine.

Looking at fig.2, note that the tension sprocket housing a counter rotation spring arrangement. Now for the maths based on the gear train ratios; The alternator ratio 2.62:1, power steering/air conditioning 1.59:1.

Now examine fig.3 to see the revised image. E1 frequency 13hz x 2.62= 34 hz, so vibration is caused by the alternator mechanical drive system. There is a drive shaft and cush drive coupling which transfers drive to the alternator. The secondary event at 66.6hz is a respondent event, probably body vibration.

Conclusion
Now for the knockout punch! The tension gear sprocket is not available separately; in fact, you must buy the complete short engine. I seem to recall David saying it is £5,500, notwithstanding the labour cost to build and fit into the chassis. Hence uneconomic repair diagnosed without any intrusion whatsoever. Diagnostic time 0.5 hours plus the ubiquitous coffee break.
Convinced? Join our NVH training programme. Or pay me and I will come and listen to your noises.