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Why do dual mass flywheels fail?

Evil plot to rip off vehicle owners or profit opportunity for garages?

With one in two vehicles built in Europe now being fitted with a Dual Mass Flywheel (DMF) it begs the question why are they so popular?

Modern petrol and diesel engines are expected to produce more power and torque while also being lighter. They are also expected to produce fewer emissions and be more fuel efficient whilst still providing a smooth and refined driving experience. DMFs are vital in meeting all of these factors that today’s buyers of the latest cars are now demanding. All vehicle manufacturers have recognised that without these essential components today’s vehicles would be very different and certainly would not be a desirable product to experience.

Why do they fail?

Like any component today they do wear out, whilst they are designed to last the design life of the vehicle (which will vary from one vehicle manufacturer to another) under normal operating conditions, they may wear out quicker under certain circumstances.  

Back to Basics

DMFs are there to absorb torsional vibration produced by the engine so therefore any abnormal increase in the amount of vibration will have a massive effect on the life of the DMF.

If a DMF does fail it’s important to identify the cause as just fitting a new DMF will not fix the underlying cause. Look for things like…

• Engine misfires – static and under load

• Uneven running – a cylinder to cylinder imbalance will have a major effect

• Compressions – low compression on one cylinder will create an engine imbalance

• Starting issues – does the vehicle start and stop cleanly, coughing and spluttering on start up will create extra vibration

• Cranking speed – slow cranking speeds will provide prolonged activity within the DMF

• Is the vehicle used for towing or carrying items? – towing excessive loads (particu- larly common with commercial vehicles) will push a DMF beyond its limits

• Chip tuning – each DMF’s damping rate is tuned to the engine its fitted to, therefore modifying the engine or ECU will drasti- cally shorten the life of the DMF

• Driving style – trying to save fuel by

driving in the wrong gear will labour the engine creating more vibration (common with Taxis)

Whilst some of these issues might be seen as a weakness of a DMF it should be considered that fitting a non tested conversion kit will not remove the original problem and will transfer even more of the abnormal vibration into the gearbox leading to more expensive repairs. In reality the now worn DMF will have protected the gearbox for longer than the conversion kit with limited damping.

How to identify a worn DMF

Before the customer disappears ask them how they use the vehicle and if they have had any recent work done. It’s always good to strike up a conversation and dialogue with the customer and depending on what they say, it may give you the opportunity to discuss the possibility of a DMF replacement before the job starts. Comments like ‘I tow my JCB with the car’ or ‘my six kids love coming away in the caravan’ should start alarm bells ringing.

Visual checks are important

• Check the friction face for excessive scoring, heat damage or cracks

• Excessive grease loss (as the springs are packed with grease) is a sign of overheating

• Visible damage to the bearing or ring gear

Functional tests

• Rotational Freeplay – as the arc springs wear they become shorter and therefore the gap between the springs will grow. Rotate the secondary mass until you can feel the springs then allow it to return. Mark the position relative to the ring gear then repeat in the opposite direction and count the ring gear teeth. LuK publishes maximum freeplay values for all of its DMFs on its website www.schaeffler-after market.com go to the ‘Online library’, click on LuK publications, select product cate- gory ‘DMF’ and it will display the data table or on www.RepXpert.com

Note: DMF with friction control plates (around 50 percent of LuK DMF) give the impression of a hard stop prior to feeling the spring, this will need to be overcome a little at a time until the spring can be felt.

• Rock – the bearing or bush between the two masses will need to be checked for rock NOT end float. Rocking and measur- ing (not guessing as it can be misleading) the side to side movement will give an indication of wear. Again the maximum values can be found on the websites.

Using the LuK Special tool Part No. 400 0083 10 will make this process a lot easier (especially on DMF with Friction control plates) and more accurate and it comes with instructions on how to use it.

 

 
Summary

With DMF increasingly being used by VMs today garages need to understand how they operate, how to test them and how to identify the root cause of early failure. They are an evolutionary product just like ECUs, DPFs and ABS modulators. We wouldn’t consider replacing any of these components with 20-year-old technology because it wouldn’t do the same job and neither does a conversion kit to replace an OE-tested DMF. Don’t shy away from them or be frightened to have the conversation with your customer, as professional garages our customers rely on you giving them good advice and explaining what work is required to maintain the car they are driving. Remember they bought a car that was technologically advanced with the refinement of a BMW, why would they want you to convert it back to a Ford Escort?

The LuK brand is backed by technical support and repair installation tips through the internet site RepXpert (www.RepXpert.com) and supported by a technical hotline; Ireland Tel: +44 (0)1432 264264.

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