John does many other things, but meanwhile he earns his sandwiches fixing automatic transmissions.
The powershift transmission is one of a growing number of dual-clutch transmissions that are expected to be fitted in more than half of cars in Europe by 2020. These transmissions are favoured for their improved efficiency and quicker, smoother shifting than more traditional automatic transmissions. It’s because of this popularity that it’s increasingly useful to know what kind of faults you’re likely to see in these transmissions, and the symptoms they produce. After all, there’s a growing chance you might have one in your car.
But before we get into that, let’s briefly go over how the Powershift automatic transmission works.
How The Powershift Transmission Works
Because it's hard to explain what's wrong with something if you don't know how it works.
Let's quickly touch on the naming of this transmission. It is commonly referred to as powershift. However, the proper name for the transmission I'm referring to in this article is 6DCT450. There are also variants called 6DCT250 and 6DCT470. These variants have differences, however, most of what is detailed below can be applied to all variants.
The main difference lies in the clutch assembly. The information below regarding repairing the dual clutch assembly applies only to the DCT450 and DCT470. There are no repairable components for the DCT250, which uses a dry clutch assembly, and can only be replaced.
How it Works
A typical automatic transmission has a series of “wet clutches” that are activated in particular combinations to achieve the desired ratios. Conversely, a manual transmission has one dry clutch and a bunch of actual metal gears. Dual clutch transmissions like the powershift are a combination of the two.
A pair of clutches are used together, with one clutch being responsible for the odd-numbered gears and the other one for the even-numbered gears. These clutches are controlled by the transmission control unit, rather than a foot-powered clutch pedal.
This hybrid of transmission designs allows for the increased efficiency of a manual transmission, the ease of use and driving comfort of an automatic transmission, as well as quicker, smoother shifting over a regular automatic transmission.
When they’re working correctly, they give the driver the best of both worlds. But when they’re not, they can be a nightmare. And the first stop on the road to ending that nightmare is working out what the problem is.
Common Powershift Faults
The most common faults I've seen in powershift transmissions.
Sensor Problems, Part I
If you find your powershift transmission going into failsafe mode (also known as “default,” and “limp mode”), and a diagnostic scan reveals trouble codes relating to sensors or shift positions, your first port of call should be to check the service history of your vehicle.
Powershift transmissions—more so than regular automatic transmissions—need regularly servicing (roughly every 30,000 miles). If your powershift transmission has not been serviced regularly enough, or if it has spent a lot of its time in shift-heavy driving (such as around town, in traffic, etc.), you may simply have too much contamination in your oil.
Unfortunately, if you’re getting faults, it’s already too late to simply change the oil and filter.
Read More from AxleAddict
Inside the transmission is a number of sensors for reading the position of various internal components. They do so by picking up readings from little magnets, but those magnets can get covered in tiny bits of metal that gets into the oil.
Now, don’t panic, the tiny metal filings are not a problem—that’s just regular wear and tear—but the build-up of this debris interferes with the sensors and confuses the control module.
In short, the sensors and magnets will need cleaning.
It’s sometimes possible to clean them off without removing the transmission, by removing the plastic sump and valve body, but if there is a lot of debris in the transmission, it may need stripping down completely.
Keeping on top of oil/filter changes should prevent this problem from occurring in the first place, however.
Servicing a Powershift Transmission
Juddering/Shuddering on Take Off
For clarification, when I say “on take off,” I mean when you are transitioning from stationary to moving. This particular judder can vary in severity from a light shaking that is barely noticeable, to a violent shaking that can rattle the whole vehicle. You will generally find that the lighter you are on the throttle, the less severe the juddering. Crawling away from a standstill on light throttle might not produce a judder at all while putting the pedal to the metal and trying to spin the wheels might cause an almighty bang. This is to do with pressure and is why you might still find you get a juddering when trying to set off on a hill, even if under light throttle.
This fault is caused by a faulty clutch assembly. The actual fault will determine how expensive a problem this is.
Firstly, the transmission will have to be removed to get at the clutch assembly. If the fault is simply worn clutch plates or broken damper springs, those individual components can be replaced without swapping out the whole assembly, which cuts down the cost of parts considerably. There are also other parts that can be replaced, such as the clutch-apply pistons.
The clutch assembly itself can often wear down, however, and when this happens there is nothing for it but to replace the whole assembly.
Sensor Problems Part II
Let’s say you’ve looked at the symptoms of your misbehaving powershift transmission and you’ve concluded that it matches up with what I described in the above Sensor Problems Part I section. But you’ve had the sensors cleaned and the oil changed, and it hasn’t helped in the slightest.
Unfortunately, this is a control module fault. And it’s rarely cheap.
The control module is essentially a little computer that sits inside the Powershift transmission, listening to all those sensors and taking information from the engine, and ultimately deciding what gear to be in and when to change. It’s the brain of your automatic transmission.
So, as you can imagine, it’s not good when it goes wrong.
There’s really nothing for it but to replace it. At the time of writing this article, I am not aware of anybody who can repair these units, so you’ll need a new one. And it is a new one, as these modules are coded to the vehicle they are fitted in, so you can’t buy a second-hand module, stick it in your vehicle, and hope for the best. Furthermore, they come completely blank (like a Windows computer without Windows installed), so you’ll need to get the software installed, which requires special manufacturer diagnostic tools.
This is not a job for the home enthusiast.
The control modules also come with the hydraulic component (whether you want it or not), which brings me onto the next fault.
If your transmission is shifting poorly, typically resulting in harsh, thudding gear changes, there’s a good chance you have a faulty valve body on your hands. The valve body is the hydraulic, physical aspect of the control module.
Picture the transmission as a big machine with a lot of controls. The control module mentioned above is a manager with a megaphone, bellowing orders out. The valve body is a small army of people at the controls to that machine, flicking switches and pulling levers under the manager’s orders.
The valve body will typically need replacing, especially if the problem is caused by wear and tear. However, in Powershifts where the debris mentioned above has been allowed to build up excessively, it may be possible to dismantle the valve body and clean it out. A much cheaper prospect, because you can’t buy a valve body without buying a control module!
Other Useful Information
Not exactly common faults, but if your vehicle has a powershift transmission, it won't hurt to know the following...
ATSG Learning: Powershift DCT450 and 470
If you’ve noticed transmission oil leaking from your vehicle, be sure to get it fixed as soon as possible. The oil in an automatic, such as a powershift, is not just lubricant, it is also the hydraulic fluid used to operate the internal components. If it is allowed to drop too low, the transmission will start to misbehave, and will likely damage the clutches if driven enough with this fault.
Crunching and Whining
Noises when driving, such as crunching sounds when changing gear, or whining noises when under load, indicate a problem with the mechanical gears themselves. These may be deceptive as the transmission could operate perfectly well despite these noises, but they should be dealt with as soon as possible. As mentioned at the top of this article, these gears are hefty lumps of metal. If they break, they could cause all sorts of damage to everything inside the transmission, not just themselves.
Gear Selection Issues
If your gear selection doesn’t seem to register correctly, it’s probably just the cable that runs from the selector to the transmission that needs looking at. In those cases it can often be simply adjusted or, at worst, a replacement cable is needed.
If you’ve had work on your Powershift, however, and the selector is not reading right after said work, there is a good chance that whoever performed the work did not get the transmission range switch properly aligned before putting your Powershift back together. For that, refer to a Powershift manual on how to properly locate the switch.
And that concludes my list of the most common faults I’ve seen working with these transmissions. Remember, this isn’t a comprehensive encyclopedia of every fault you might come across in a powershift transmission, but it is, for now, the most frequently seen faults for this technician.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2017 John Bullock