A look at the Dual Clutch Gearbox

The DSG, or Dual Clutch Gearbox, is a manual transmission using a Mechatronic system to shift the transmission automatically via a double wet or dry clutch with double input shaft arrangement (Figures 1 and 2).

TVS Engineering is a company in the Netherlands that specializes in the repair and tuning of Volkswagen/Audi DSG transmissions. TVS began seeing a need to modify both the clutch assembly and programming approximately 10 years ago (Figure 3). Premature clutch failure was on the rise due to a turbo being mounted to these vehicles (Figure 4). They currently repair about 1,500 of these a year through 18 dealers around the world selling clutches, Mechatronic, gearboxes and software (Figures 5 and 6). This article is the result of having and interview with Ronald Logmans, the software engineer for this company.

Currently, there are four generations of high production DSG transmissions on the road worldwide (others are on their way). They could be either longitudinally (AWD) or transversally (FWD) mounted. Of the four generations, one is a six-speed while the remaining three are seven-speed transmissions. The names of these different transmissions are as follows:

2003 - DQ250 – 6 speed wet – F/AWD

2008 - DQ200 – 7 speed dry – FWD

2008 - DL501 – 7 speed wet – AWD

2010 - DQ500 – 7 speed wet – F/AWD

The D represents Dual Shift Gearbox (DSG), the L is for longitudinal (RWD) and the Q is for transversely mounted (F/AWD).

Besides the various generations and designs, what makes this a bit more confusing is that there does not seem to be any logical pattern as to application. An Audi A3 could be fitted with a 1.8L engine and have either a DQ200 or a DQ250.

The first DSG was a six speed “wet” clutch design that started in 2003 for the Audi TT. This transmission is referred to as the DQ250 (02E) and had AWD. The second generation began in 2007. It was the DQ200, which is a seven-speed double “dry” clutch design that came as a front wheel drive only. There are no AWD with a dry clutch design. It is the only dry clutch unit in the DSG family, and of all the DSGs, this is the most problematic of them all. It also was the most sold transmission worldwide except here in the United States. Globally, it is in two Audi vehicles (A1 and A3) and 16 different Volkswagen vehicles (Beetle, Bora, Caddy Van, Golf, Jetta, Lavida, Passat, Polo, Sagitar, Scirocco, Sharan, Tiguan, Touran and UP). Skoda and Seat models use the DQ200 and the DQ250.

The main failure with this transmission is the clutch wearing out. Signs of failure begin with feeling a judder or shudder on mild acceleration after deceleration from 18 mph to 3 mph during a 2^nd gear hold, especially in turns. This DSG holds 2^nd gear even longer than the others during deceleration. The computer commands the clutch to slip as the vehicle is nearing a stop to prevent engine surge and stalls. The design of this dry clutch assembly, as well as it being a dry clutch, along with this long hold 2^nd gear slip strategy, the clutch assembly overheats. This is especially true during acceleration before coming to a complete stop.

There is an approximate two-by-one-inch gap in the clutch housing to allow air to enter for cooling. But it has proven to be insufficient as clutch temperatures can reach as high as 392°F, resulting in the upper layer of the clutch lining to glaze causing the judder. They usually see failures as low as 13,000 miles to a maximum of 62,000 miles with gas powered vehicles. In an attempt to improve the life, the dry clutch assembly has undergone six different design changes. These encompassed design changes from the clutch arm to the friction material.

Due to the assembly design of the dry clutch, this transmission is inherently noisy. When you are driving on a bad or bumpy road with the windows down, you can hear what sound like pieces of metal in an empty paint bucket echoing off the passing houses. Some of these noises can be attributed to loose internal mechanical end play clearances causing shift levers to hit the gearbox casing. Other issues causing this noise can be play in the flywheel and play in the dampening springs in the clutch (which are identical to what you would find in a manual gearbox clutch assembly).

Damage to these parts increases the noise especially when traveling in seventh gear; engine rpm can be as low as 1,150 rpm making it just right for it to rattle. A slight change in the programming that TVS offers alters the rpm by increasing it to 1,400, which is enough to eliminate this vibration spot. Curing this under rpm problem not only assists is lengthening the life of the clutch, it also improves the performance of the vehicle. Additionally, they changed clutch pressure control and eliminated the 2^nd gear hold bringing the transmission down into first when it should make the downshift. All of this is designed to alleviate premature failure of this easily overheated dry clutch assembly.   

This DQ200 also experiences bearing failure causing the gearbox to wear producing excessive metal (Figure 7). The magnets used as shift fork position sensors begin to attract this metal. If there seems to be about 15 mm of buildup, VW recommends a new transmission as changing these bearings may not produce the desired repair (Figures 8 and 9).

The DQ250 on the other hand does not seem to experience the same frequency of bearing failures. TVS builds about 1,000 of these transmissions in a year and they might see as many as five of them with bearing failures. The complaint they do experience frequently with this transmission is that the shifts get clunky due to synchromesh ring wear. This is a tough problem to deal with due to the fact that these synchros are not sold separately. In fact even rebuilding one of these transmissions can be challenging. More times than not you have clunk issues with Drive and Reverse engagements.

The double wet clutch drum assembly can be a troublesome area as well. It is known to develop a shudder on acceleration from as early as 2^nd gear on up through the gears. It comes in at around 2,000 rpms when the turbo kicks in at max torque. It is the type of shudder that can feel like a fuel or injector problem. Changing the complete clutch assembly in some cases resolve this problem while reprogramming the computer with a modified slip program has a higher ratio of success reducing future failure.

Ronald made a point to say that though one might have 20, 30 or 40 years experience working on manually shifted transmissions, working on these automatically shifted manual gear boxes is a whole new experience.

On to the Computer
The computer software used to shift these transmissions is very complicated. If it is not understood, diagnostics becomes difficult. Temic developed the software, which works in a closed loop system with adaptive values.

Ronald analyzes the software and makes modifications to it. Just with the six-speed only (DQ250-02E), there have been 40 generations (like windows ’95 ’98 ‘XP etc) and 1,000 versions of software. Modifications include changes in the structure of the program with things like changing, adding or subtracting variables. At the moment, there are about 2,000 variables written into the software. An example of one of these variables is, as a safety precaution, software is written to never let the engine exceed 7,300 rpms. Remember this: The DSG controls the engine, not the opposite.

An example of variables related to the function of the transmission is how the clutch is engaged. Engine idles around 700 rpm while in Park or Neutral. When Drive or Reverse is selected, engine rpm is increased to 1,000. As the clutch is engaging, engine rpm decreases slightly to 990. The DSG Mechatronic commands the ECM to provide 5Nm more torque, engine rpm increases back to 1,000 rpms launching the vehicle. As the brake pedal is being released, the DSG Mechatronic command clutch pressure to be around 34.8 psi, which Ronald calls the kiss point, or when the clutch is fully applied to drive away. These examples of controlled variables also pointed out an aspect of the operating system that is important to understand: the DSG Mechatronic is the master and the ECM is the slave.

When writing software, OEMs need to be concerned about emissions and fuel economy. Sometimes this can cause problems with the livability of the gear box. An 

example of programming issues that has come up is when you select D and lift your foot from the brake. The instrument cluster may show 1^st or 2^nd gear or will blink for just a fraction of a second. Engine rpm suddenly ramps down to about 400 rpm. This is done to prevent engine stall, as the clutch is being applied. It then ramps up before the clutch is fully applied causing a bump on the engagement. This problem can only be resolved with programming corrective TVS software.

Another interesting programming issue that comes up with this transmission is the way clutch clamping pressure works in proportion to output transmission speed and wheel speed (ABS); speed meaning just a small degree of rotation, not full revolutions. If clutch slip and wheel speed is not what the DSG Mechtronic unit is expecting to see, a command is given to increase or decrease pressure. The increase and decrease in pressure goes back and forth like a swing. This swing, or surging sensation, can be caused by the breakdown of the viscosity in the fluid and can be temperature related.

Let’s dig in to this a little deeper and explain on of the multiple “regulations or functions” that take place inside the software. With the DSG Mechatronic unit being controlled by a tricore processor, it can make thousands upon thousands of calculations per second.  When the brake is released, it commands 34.8 psi to apply the clutch and also commands the ECM to supply 50Nm of torque at 1,200 rpm. The car should then accelerate. Acceleration is measured by looking at the speed of the drive shafts and ABS signal, +/- 30 times per 1 rotation.

With this accurate signal, it is possible to measure the exact acceleration of the shaft, and even the quadratic function of that called A2. If the shafts do not accelerate exactly according to the to the pre set variables stored in so-called maps, then a 10^th of a second later it increases clutch pressure from to 36.2 to 40.6 psi until it sees the specified quadratic acceleration. With a sudden increase in pressure, the clutch will grab and the vehicle will accelerate faster than wanted.

Clutch pressure is then commanded low (37.7 psi) at which time the clutch begins to slip causing the vehicle to slow down. A command is then given to increase pressure and the cycle repeats itself. All of this taking place within seconds resulting in a swinging motion of the vehicle. Besides degraded fluid, lazy solenoids or compromised valve trains can produce the same surge or swing complaint. There has been occasion when the solenoids, valve body and fluid are all good yet it still has the surging problem. Nothing can be seen with the clutch assembly but change the assembly and the surges are eliminated. TVS assumes that since the friction is made up of some organic material, a break down takes place in the co-efficient properties of the friction.

This DQ250 is being used in many different models of cars, all using their own unique software versions, with specific maps for acceleration cruise control and other. Things like the weight of the car curb weight value, AWD losses are all integrated variables in the software. Therefore you will see hundreds of different software versions. If you have a gearbox code LTE it is used in over 10 different models of Audi/VW and all using there own specific software. So be aware!

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