Dual Mass Flywheels

September 17, 2009

What is a Dual Mass Flywheel?

The flywheel is effectively a weight which is fastened to the end of the crankshaft of the engine. The power from the pistons tends to be created in "pulses" and the weight of the flywheel smoothes out these pulses by providing inertia to the rotating engine. As well as providing a weight the flywheel has a gear around its circumference on which the starter motor operates and is a convenient means of attaching the clutch which provides a variable connection to the transmission.

Modern diesel engines generate high torque and as a result they need extra smoothing out or "damping". To help with this process a DMF (Dual Mass Flywheel) is fitted. This is effectively two flywheels that transmit the drive through a number of springs which cushion the drive to the transmission. Please look at the bottom of this post to see a description of what a flywheel does.

Is DMF failure inevitable?

No not necessarily. Some vehicles cover very high mileages and do not have any problems. Whether the DMF fails depends on what kind of duty the vehicle is subjected to and to some extent the way the vehicle is driven.

What happens when the DMF fails?

In practical terms, the first an owner will know is likely to be either a vibration and/or metallic jingling noise. The time these symptoms take to manifest themselves as a complete failure will vary dramatically. A complete failure will probably result in not being able to select any gears or in extreme cases a complete loss of drive. However, it is recommended that if any of the symptoms described are experienced that the vehicle is taken immediately to a suitably equipped workshop for further investigation. This may avoid the inconvenience of a roadside breakdown and the associated recovery costs.

The DMF on early models (up to those produced in August 2002) could under certain conditions come loose. This is the statement form Toyota GB regarding this matter;

"The issue regarding Dual Mass Flywheels relates to RAV4 CLA20 and CLA21 models (early diesel vehicles) and was found to be that under hard use (towing etc) the flywheel securing bolts were unable to provide sufficient tightness. This was remedied by an improved flywheel and revised fastening and tightening processes, which were introduced into RAV4 vehicle production from August 2002. The improvements were made from VIN numbers,

JT EYG20V400009863

JT EHG20V600026183

JT EHG20V606013132

With our commitment to customer satisfaction the warranty was extended for a period outside of the normal 3 year or 60,000 mile warranty. Because a production line fix was introduced from August 2002 claims would only be accepted on vehicles manufactured prior to this date. This extended period ran for 5 years or 100,000 miles (whichever comes first) and as such would therefore have expired in August 2007. I have to confirm that no extra time or mileage will be added to this warranty extension and all Toyota Centres are aware of the above information.

My recommendation would be that owners who experience a failure outside of this period should contact their Toyota Centre and if they feel it appropriate, contact would be made to our Customer Relations Department for their consideration."

Some Toyota Centres have insisted that a new that I have the ECU changed as well as the DMF and this costs more. Why is this?

If there is evidence of heat related damage the Service Department may recommend that a re-programmed ECU is fitted to reduce the possibility of damage due to clutch slip. The Toyota Centre will advise you if any of the cost of this work can be met under the terms of the warranty.

Why is it so expensive to change the DMF?

The DMF is quite a complex part of your vehicle and it is fitted between the engine and the gearbox. To change it requires all of the gearbox and transfer box oil to be drained, then the front to rear drive shaft, transfer box , gearbox and all of the clutch components need to be removed. The vehicle has to be elevated and there is quite a lot of labour involved.

What are Toyota doing about this and are any extended warranties available?

Toyota are committed to ensuring that their vehicles perform reliably throughout their life and have provided this statement;

"It is always concerning to learn of any product failure and if this does occur then we do look to the reasons to understand why this has occurred and take steps to prevent this from happening again in the future. This usually follows a remedy to the source on the production line, along with a modified part (normally identified by a superseded part number). As you have correctly advised the issue regarding the failure of the Dual Mass Flywheel on Toyota RAV4's has involved a revised tightening procedure from August 2002 production along with a modified part now supplied to the Toyota Centre Network.

Should an owner suspect their vehicle has a problem then our advice would always be to take this along to their Toyota Centre to be remedied. Toyota Centres are kept updated through our technical and warranty teams to ensure they are always aware of the very latest information in respect of our model range and be able to advise owners on a recommended course of action should this affect their vehicle.

With any failures outside of the Toyota warranty, which is 3 years or 60,000 miles whichever comes sooner, and no extended warranty has been purchased, then this would be dealt with on an individual basis between that owner and their Toyota Centre. This would also include any requests for goodwill outside of the warranty period."

Is this problem only applicable to Toyotas?

Absolutely not. A large number of vehicles from all manufacturers employ dual mass flywheels. To some extent their use is a necessary requirement with the evolution of modern high powered diesel engines to provide smooth operation and prevent any damage being caused to any other parts of the vehicle.

Do I have to take my RAV to a Toyota dealer for repair?

No. You can take your Toyota to any suitably equipped workshop. However, you should be sure that you have past experience of them or they come well recommended as the job is rather long and complicated. You can be sure that a Toyota workshop has all the necessary facilities to complete the work properly and a Toyota Centre will have access to guaranteed Genuine Parts and any information regarding modified or improved parts and processes that may not be available elsewhere.

I am thinking of buying a second hand RAV 4. Can I tell if the DMF is faulty?

You can test drive the vehicle and make sure it does not suffer from any excessive vibration. It is highly unlikely that you could detect any problems unless the failure was imminent. If in any doubt take the vehicle to a suitably equipped workshop for a professional opinion. Most reputable dealers will provide a suitable warranty and motoring organisations will test the vehicle for a fee.

Should the possibility of any problems stop me from buying a diesel engined RAV 4?

No. These are generally a very reliable and highly regarded vehicle that will provide many years of trouble free service.

This section gives more information on the role of the flywheel and the DMF

The flywheel has to be heavy as it maintains the inertia of the engine. When the four pistons come down on the power stroke it is like they are being shot down the barrel of a cannon and they all take it in turns - 1,3,4,2. The trouble is that the next one in sequence does not start until the previous one as right at the bottom of the stroke so the turning moment on the crankshaft is very "lumpy". The crankshaft is like the pedals on a bicycle (except instead of two there are four in a row), it turns the reciprocating (up and down) motion of the pistons into rotary motion that is eventually connected to the wheels. In order to make the engine rotate smoothly, a flywheel which is nothing more than a very heavy round weight is bolted onto the rear of the crankshaft. When the mass of this flywheel gets spinning it helps to remove the "lumpyness" of when one piston gets to the botttom of a cylinder and the next one being fired from the top. In very old single and double cylinder engines it had to be huge to keep the engine turning. On this traction engine it is up by the driving cab;

You can see it very clearly on this single cylinder steam engine;

You can imagine that without this flywheel to carry the piston around to the next power stroke the engine would stop. In really big engines you need a really big flywheel and in this mill engine at Wigan Pier it weighs 70 tons;

Its hard for you to get a feel for just how big that is but if you look to the lower right of the picture there is a large double doorway into the mill. The flywheel has to be massive because it drives thousands of machines in the factory and as it is a spinning mill it is important that they turn at a constant speed.

Now we relate the role of the flywheel to the D4D engine. In most cars the flywheel would be a relatively simple affair - just a weight as already stated but these small modern diesels are phenominally powerful for their size and the power strokes are effectively very "lumpy". You can imagine that if you fired the pedals down on your bike with a cannon instead of pushing them with your legs then the bike would be very jerky! Of course you could fit an even bigger flywheel on an engine to smooth out the lumps but there are limitations because;

They absorb more of the engines power, it would rev up slowly and also slow down slowly which effects performance and slows down the gear changing process by having to wait for the speeds of different gears to synchronise.
They use more fuel.
They are difficult to accommodate.

So the way that modern diesels are smoothed out is with a Dual Mass Flywheel similar to the one in this diagram;

You can see that the flywheel is in two pieces. One is connected to the pistons (and dont forget that there are four pistons) and then the other is connected to the transmission via a set of annular springs around the circumference of the flywheel. These springs absorb the lumpyness of the pistons and transmit smooth rotary motion to the transmission. It makes the car feel smoother to drive and almost eliminates any vibration that would cause knock on damage to the clutch and gearbox.

In this picture of a failed DMF belonging to one of our members, you can see that the bolts have become loose and the resultant damage around the eight fixing holes as the flywheel eventually worked loose.

This problem was addressed during August 2002 and should no longer occur. However, as the DMF is no longer a simple one piece design and has become more complex it is not impossible for it to fail in other ways. It should not be confused with clutch wear or failure which is considered a consumable wearing part. This short clip of a VW DMF shows the result of the drive springs failing;

and this is a good one but note there is still some play;

This is a very nice animation of how the DMF is assembled and how it turns big vibrations into small ones;