COMMON DUAL-CLUTCH PROBLEMS discusses repairs for dual-clutch transmissions

While it is important for technicians to know as much about as many aspects of car repair as possible, the days when every technician knew everything about everything are long gone. Modern cars have become too complex for any single person to know everything about everything that makes a particular car run, and this is especially true of transmissions and their control systems.
If you are an experienced technician, but not a specialist in transmissions, has a guide for dual-clutch transmissions with reference to what they are, how they work, what their advantages/disadvantages are, and why they fail. The following is a truncated version of the guide.
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While limited space precludes a full exposition on all the possible problems and issues that affect dual-clutch transmissions on all applications, there are nevertheless some issues that are more or less common to most designs. These could include one or more of the following:

Mechatronic failure
The term “mechatronic” is a generic term that is often applied to the control module of all dual-clutch transmissions.
Essentially, it is an electronic control module that incorporates a mechanical valve body that strongly resembles the valve body on a conventional automatic transmission. Failure of, or defects in this hybrid control system is by far the most common cause of dual-clutch transmission issues. Note that high-end diagnostic equipment is required to extract fault codes from the mechatronic unit. 
It should be noted that almost any failure or defect in the mechatronic unit will illuminate a warning light, and force the transmission into a failsafe, or limp mode that will persist until the fault is corrected. However, since the unit will also store fault codes that relate to the issue, these codes are invaluable diagnostic aids. For instance, if any codes are present that relate to any transmission related sensors, gear ratios, or unexpected/uncommanded mechanical engagements/disengagements, the root cause of the problem is almost invariably related to a failure of the mechatronic unit itself, as opposed to failures of the components that are identified in the code definitions.
Depending on the application, when some codes mention “Clutch Limits”, “Clutch Limits Reached”, or other wording that relates to the clutches, it is almost certain that the clutch friction linings have worn down to below a minimum allowable thickness.
Note that while some manufacturers of dual clutches do supply repair kits, the only reliable remedy for worn clutches is to replace the entire clutch kit as an assembly. Be aware however, that installing a dual clutch requires both special tools and very specific skills. In fact, many, if not most manufacturers and suppliers of dual clutches will not honour a warranty claim if the installer had not successfully completed a training course on installing these clutches. 
Special note should also be taken of the fact that while there are many self-proclaimed experts that advertise their ability to repair mechatronic units, the fact is that all of these specialists are limited in what they can diagnose and/or repair, since large chunks of the mechatronic’s programming is proprietary to the manufacturer of the affected application.
Be aware therefore that if a repairer says the unit is “OK”, or that “No faults were found”, there is a very high likelihood that the repairer could not diagnose the root cause(s) of the failure because he was unable to access all of the unit’s programming. Replacement and proper integration of the replacement mechatronic is the only reliable, long-term solution to mechatronic failures.
Note that on some dual-clutch transmission variants, and especially the DQ250 variant used in many VAG-group applications, the partial or complete loss of reverse gear is usually caused by a failure of the mechatronic unit.

Adaptation failures
One of the functions of a mechatronic unit is to monitor the amount of wear on the clutch friction material, and to make suitable adaptations to the travel of the clutch actuator to compensate for this wear. However, one of the biggest causes of gearshift issues on dual-clutch transmissions is the failure of the mechatronic unit to adapt the clutch actuator’s travel to compensate for clutch wear, which can cause problems with gearshifts that are largely similar to the problems that occur when a conventional clutch fails. 
In many cases though, performing a simple clutch adaptation with a high-end scan tool will resolve the issue, unless there are trouble codes present that refer specifically to the clutches being worn.

Wet clutch failure
On some variants of dual-clutch transmissions, the clutches are housed in a separate enclosure where they are bathed in oil with a very specific formulation. While this arrangement has several advantages, the biggest problem with it is that when cross contamination of the transmission lubricant and the oil bathing the clutches occurs, both the clutches and the transmission can be damaged. In fact, this particular problem was so severe (and common) on some wet-clutch designs that several manufacturers abandoned the wet-clutch principle altogether.

Noisy bearings
Although noisy bearings are a relatively common feature of most dual-clutch transmissions, the transmission is unlikely to fail catastrophically because of it. In fact, most designs will work fine for several thousand kilometers after one or more bearings have become noisy; however, as with all mechanical failures, the problem should be rectified as soon as possible. 

Excessive vibration
Blaming excessive vibration on the dual-clutch assembly is arguably the most common mistake many technicians make when they diagnose issues with vibration on dual-clutch transmissions, since excessive vibration is usually caused by issues with the dual mass flywheel. If the clutches were out of balance, they are likely in the process of disintegrating, which would manifest as mechanical noises, failure to shift gears, failure to engage one or more gears, or in some cases, the transmission being locked into a limp mode.

Non-starting issues
Like conventional automatic transmissions, dual-clutch transmissions are fitted with safety cutout switches that prevent the engine from being started unless the transmission is in neutral. Failure of, or defects in this switch and/or its associated wiring will cause a no-start condition, which may or may not be accompanied by a warning light and a trouble code, depending on the exact nature of the problem.

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