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A look at the Dual Clutch Gearbox

This manual transmission will send even the most experienced techs looking for a refresher course.
Monday, June 30, 2014 - 07:00
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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.

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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.

FIGURES 7-9

 

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 1st or 2nd 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 10th 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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