Table of Contents:
- Introduction to the ZF 8-Speed Transmission
- In this Drivetrain 101 we will be addressing
- ZF 8-Speed Transmission Product Resources
- Popular Transmission Information Resources
- A Brief History of the ZF 8-Speed Transmission
- Problem #1 - The Valve Body
- Problem #2 - The Solenoid Set
- Problem #3 - The Internal TCM/Mechatronic System
- Problem #4 - The Torque Converter
- Problem #5 - The “E” Clutch Assembly
- Problem #6 - The “A” Clutch Assembly
- Problem #7 - The “D” Clutch Assembly
- Problem #8 - The Bushings
- Conclusion to the ZF 8-Speed transmission
Introduction to the ZF 8-Speed Transmission:
As the executive choice in high quality and proprietary ZF 8-Speed transmissions, rebuild kits and parts, we’re often pelted with questions about it’s numerous problems and how to prevent them. As a result, it became evident that this popular and often problematic transmission needed it’s own installment in our educational Drivetrain 101 section.
Among the most complicated passenger vehicle gearboxes ever designed, the ZF 8-Speed transmission is inundated with moving parts and electronics. Although this German transmission is somewhat reliable compared to some other options on the market, it is still prone to a variety of well documented problems; especially in trucks, SUV’s and performance vehicles.
In this publication, we will be sharing our wealth of data and information on this unit accrued from years of manufacturing up-armored ZF 8-Speed transmissions for a cornucopia of different applications. Further, we will be explaining how these issues can be addressed to endorse a more reliable, better performing transmission.
In this Drivetrain 101, we will be addressing:
- The most popular failures we witness on the ZF 8-Speed transmission
- Its capabilities in reference to TCM tuning and shift/lockup protocol modification
- The strengths (believe it or not, there are a few!) of this transmission
- Resources for relevant updates and upgrades
And much, much more!
Popular Transmission Information Resources:
- Top 5 Benefits of an Upgraded or Built Transmission
- Calculating Torque Multiplication Factor, Why is it So Important?
- Top 5 Ways to Extend the Life of a Factory Transmission
A Brief History of the ZF 8-Speed Transmission:
Debuting in 2008 as a replacement to the aged ZF 6-Speed automatic, the ZF 8-Speed transmission was first featured in the BMW F01 7 Series, available exclusively in upper level models. By 2013, the equipment had become standard across the 7 Series production range. The advent of the 8-Speed unit also allowed the car to decrease tailpipe emissions and increase fuel economy, while shifting in as little as 200 milliseconds, critical to domestic and global markets.
Seen as proof of concept, the transmission was quickly adapted by other manufacturers like Mopar and Volkswagen. Popular for it’s general reliability, fuel economy and difficulty to repair by a third party, OEM’s were excited to offer the ZF 8-Speed transmission in an array of different vehicles.
Despite a few revisions, the ZF 8-Speed transmission is actively in production today. These transmissions are manufactured exclusively in Germany, and are licensed to OEM’s directly by ZF. At this time, there are no immediate plans for a successor to the ZF 8-Speed transmission, although rumors of a 10 or 11-Speed replacement are becoming more credible. Without further ado, we will now analyze all of the faults and complications of this transmission and explore how to resolve them.
Problem #1 - The Valve Body
As with most transmissions, the ZF 8-Speed transmission’s valve body assembly is a fertile area for problems. This valve body is exceptionally complicated, largely because it has to control 8 different forward gears. Even the activation of the park rod system is hydraulically activated, an uncommon design feature on the transmission world.
This valve body is prone to a variety of crossleaks that cause the transmission to misbehave. Most common, crossleakage will present at the pressure relief valve, end plugs and priming valve, although other valves produce leaks as well, just not as frequently.
The pressure regulator valve is responsible for controlling when line pressure is released. When it begins to leak, the transmission will lose line pressure when exceptionally high figures are commanded. This results in a functional ZF 8-Speed transmission during normal driving that displays symptoms of malfunction under hard throttle or heavy load. It is important to upgrade this valve during the remanufacturing process or when installing an upgraded valve body.
The priming valve helps the transmission decide which gear to start in. When it holds oil pressure adequately, it works just fine. Once it begins to leak, the unit can start in an inappropriately high gear or shake during takeoff. This crossleak is also associated with pressure and gear ratio codes. A physically upsized and anodized valve resolves this concern permanently.
Valve body end plugs hold oil pressure in certain circuits and are not designed to move. Poor fluid maintenance can cause material to wear these bores aggressively, resulting in oil pressure venting out of the respective oil circuits through the end plugs. This can cause slippage, flare shifts and line pressure codes as the problem advances. O-Ringing the end plugs is a permanent resolution to this concern.
Lastly, the line pressure produced by this transmission is not adequate to be reliable in demanding applications. At Next Gen, we’ve developed a high pressure ZF 8-Speed transmission valve body featuring all of these aforementioned updates for almost every vehicle featuring this transmission. But, from the factory, it is simply not enough. Increasing line pressure safely is a critical reliability and drivability update.
Problem #2 - The Solenoid Set
Although the ZF 8-Speed transmission solenoids themselves are usually reliable, external factors are known to cause them to fail. What does this mean? Most manufacturers tell you not to service this transmission for upwards of 100,000 miles, or sometimes ever. This is absurd, and causes them to fail shortly out of warranty. OEM’s plan this so you’re roped into expensive repairs with them shortly outside of warranty. Proper fluid maintenance is extremely important.
Because this form of maintenance is frequently neglected, dirty oil and high temperatures frequently aggress upon the factory solenoids, negatively effecting their longevity. The O-Rings that seal these solenoids to the valve body are notorious for beginning to leak as well as tearing.
Further, the ZF 8-Speed transmission solenoids often activate slowly and improperly at high age when proper maintenance intervals are not maintained. Because there is no way to trust with accuracy the integrity of used solenoids, we replace them with a new OEM set in every build. This is pricy, but simply the correct way to do things.
Problem #3 - The Internal TCM/Mechatronic System
A good and bad idea at the same time, the ZF 8-Speed transmission’s TCM is manufactured internally to the transmission. This adds a massive amount of complexity to the gearbox itself, as well as exposing it to sustained high temperatures. Fortunately, it’s not as unreliable as it might sound, but it is still known to fail.
Much like the OEM solenoid set, poor fluid maintenance adds undue burden to the factory mechatronic system trying to operate inside the factory gearbox. The mechatronic system has a very important role, serving as more than just a transmission control system; it is married to the vehicle’s antitheft devices and houses all of the software necessary to make your transmission operate.
As a general practice, we replace the mechatronic assembly on all ZF 8-Speed transmissions. This is uncommon in the transmission rebuilding community because it’s the most expensive single component in most of these units. However, much like OEM solenoids, there is no true way to know that it is healthy and ready to go several hundred thousand miles. Reusing the mechatronic system in a rebuild is like reusing a condom. It might work… but that doesn’t make it a good idea.
Problem #4 - The Torque Converter
Known agreeably as the ZF 8-Transmissions biggest drivability issue short of the valve body, the OEM torque converter assembly leaves much to be desired. Much of the issue here is that everything from a Toyota Supra to a Bentley Bentayga have almost the same design of torque converter, simply varying in size and stall to correspond with the application.
The issue is that what shines in some applications, will not in others. This torque converter features a clutch pack at the front that synchronizes to the motor under a variety of conditions, like most modern torque converters. Unfortunately, the clutch pack is not very high quality. The friction materials used are your generic OEM friction material, optimizing for cost before quality.
Further, the steels are stamped and tumbled. another cost cutting design feature. Although this is reliable out the box, it does not offer good longevity or lockup quality, resulting in a very soft lockup that is known to flare during application. Heavier vehicles as well as vehicles with lots of torque are likely to experience ZF 8-Speed torque converter issues.
Another pleasing piece of information is that the impeller hub, the snout that splines into the pump, is known to break. This is most common on larger trucks but has been documented in sports cars as well. This is caused by the more cost effective metal used to manufacture this critical part of the ZF 8-Speed torque converter.
Fortunately, we have designed an upgraded torque converter for nearly every ZF 8-Speed transmission. Upgraded lockup clutches and billet CNC Machined steel impeller hubs are standard on all of them, a bare minimum for success and favorable driving dynamics. Stall speed can be adjusted, but OEM stall speed is usually exactly where it should be on vehicles equipped with the ZF 8-Speed transmission.
Problem #5 - The “E” Clutch Assembly
In most transmissions, the clutch pack responsible for sustaining overdriven gears is the most prolific failure point. In the ZF 8-Speed transmission, the “E” clutch assembly, utilized for gears 2, 3, 4, 6 and 8 is most popular to fail. We’ve attempted to understand why this is, breaking rank with typical transmission orthodoxy and have concluded that it is a compound issue stemming from 2 different problems.
Firstly, the “E” Clutch is easily deprived of oil. Oil must travel exceptionally far to reach the “E” clutch which is hydraulically far from the pump and valve body assembly. This makes it a challenge for this clutch pack to stay cool and synchronize properly.
Secondarily, the clutch hub that these frictions spline onto is very flimsy. As a result, clutches will quickly wear into the component. This is especially common on heavier vehicles, and is secondarily common on higher performance vehicles.
For all appropriate applications, we at Next Gen Drivetrain utilize a CNC Machined Billet Aluminum or Billet Steel hub, a permanent resolution to this well documented problem. Further, for the most aggressive applications, we can add additional friction elements to this clutch pack, increasing load capacity immensely.
Problem #6 - The “A” Clutch Assembly
As seen in many modern longitudinal torque converter style transmissions, the ZF 8-Speed transmission has clutches attached to the pump assembly. Specifically the “A” and “B” clutch are on the back of the pump. A design defect in the ZF 8-Speed transmission is that the “A” Clutch backing plate flexes under high line pressure, destroying load capacity.
This results in 7th and 8th gear flutters as well as shaky or slow takeoffs. Although we only use upgraded frictions in any ZF 8-Speed transmissions, the most demanding applications will require more. We offer a CNC machined and coated steel backing plate for this clutch pack that enables additional friction elements to ensure extraordinary rigor in high stress conditions.
Problem #7 - The “D” Clutch Assembly
The “D” Clutch pack of the ZF 8-Speed transmission is used in gears 4 through 8, as well as reverse. This makes it the closest thing to an “Overdrive” clutch pack available to the ZF 8-Speed transmission. As such, it is a common failure point.
A damaged “D” clutch pack can result in a shuttering reverse condition, but most frequently causes issues in upper gears. Upper gears have the highest torque multiplication factor due to their longer gear ratios making them the first to malfunction in most transmissions.
To our favor, the clutch hub and shell are not often damaged, the clutches themselves simply lose their friction material due to the demand of the application and can be found blackened and unusable. It is a standard practice to utilize a stronger friction material in this place to support better longevity and trustworthiness in upper gears.
Problem #8 - The Bushings
A babbitt material from the factory, the ZF 8-Speed transmission bushings are likely to degrade over time. This, coupled with OEM’s manipulating consumers to not perform proper fluid maintenance make the factory bushings a common wear point. Roughly 2 out of 3 ZF 8-Speed transmissions we remanufacture have at least one bad bushing.
When bushings wear, the transmission can shuffle or vibrate under certain conditions. Further, many bushings are “Oil Dams”, or areas that hold oil pressure in a certain fluid circuit. As they wear, the become a crossleak of their own. Knowing of this to problem, a new set of upgraded bushings is a standard feature on all Next Gen Drivetrain commissioned ZF 8-Speed transmissions.
Conclusion to the ZF 8-Speed transmission:
As one of the most common transmissions on the market today, there is no shortage ZF 8-Speed transmissions needing repair. Unfortunately, it’s extraordinary complication and intricacy discouraged the majority of transmission experts from feeling comfortable providing internal repair services for them.
At Next Gen, we have taken a very strong interest in the ZF 8-Speed transmission, striving to perfect them. Have questions about your ZF 8-Speed transmission? Call in and speak to one of our knowledgeable experts today!