When I was a kid growing up in a small, east coast city, there were other kids who moved in and out of our neighborhood regularly. Then there was that one kid that arrived and we knew he wasn’t going anywhere anytime soon. He was the kid that seemed to fit in without any problems, and he was easy to get along with. This is the role that the ZF 8HP transmission has found in the transmission industry worldwide.
The ZF8HP series transmissions blend shift quality, gear ratios, unit foot print, efficiency and durability into a package that produces a stellar performance under every conceivable situation. OEM engineers pair diesel, gas (petrol) and hybrid power plants to these units with stunning results. In return, the ZF8HP series units are some of the most popular 8-speed, rear-wheel drive applications on the road today. Currently, there are well over 3 million units produced to date, and counting.
In this article, we will explore how to identify these units, the similarities and differences between the variants as well as common issues you may see when one of these transmissions come to your shop.
Vehicle Applications and Unit Identification
BMW is the manufacturer that uses the ZF8HP the most (figure 1). From the most powerful engines to the most economical, these units are programmed intimately into the powertrain management system. FCA (Fiat, Chrysler, Alfa Romeo) purchased rights from ZF and produces their own version of this unit (figure 2). Also, note that there are FCA models that use the true ZF unit as well.
Unit identification is extremely important. Externally, there are 2 different size units in general. A smaller case unit for the lower torque applications (8HP45/845RE through 8HP50/850RE), and a larger case unit for medium to high torque applications (8HP55 through 8HP95). Of course, with different OEM applications, the bell housing configurations will be different as well.
While these units look almost the same inside, there are enough subtle differences that could result in getting the wrong parts in your unit. It is best to assume that these units are all different unless you can cross reference a part number through ZF or FCA. All of these units will have tags on the case with information that must be used when ordering parts (figure 3). FCA uses the part number or sales code for parts ordering purposes. Note, the sales code information is only available by using the vehicle VIN and WiTech2 or the FCA service department information look up.
As of this date, hard parts may be difficult to obtain, since these units are rather new in the aftermarket. In the US, core units are available through several suppliers. Contact ZF for the most current availability of parts.
Why Do They Fail?
These units are tough. They are built to handle everything that the engine is programmed from the factory to put out. Even when they get high mileage, adaptive programming keeps them shifting as close to the original shift quality as possible. As a result, there are no specific service intervals listed for most OEM applications. BMW recommends ‘Case Based Servicing’ only. The service technician will determine if the fluid needs to be serviced by analyzing a sample taken during a regular, scheduled vehicle service inspection.
As durable as these units are, they do eventually need to be repaired. There are some factors that come into play that could cause an 8HP unit to fail seemingly before it’s time. Here are a few things to look out for.
Incorrect Fluid Level
One of the most common causes of unit failure is incorrect fluid level! It seems simple, but don’t be fooled; it’s real easy to get burned (figuratively speaking) if you don’t pay attention to proper procedures. Complaints from intermittent harsh or flaring shifts to gear ratio codes and/or solenoid performance codes are associated with transmission fluid levels set either too high or too low. Torque converter clutch apply and release issues as well as overheating can also be a result.
Using incorrect fluid will cause problems similar to having the fluid level incorrect. Always use the OEM recommended fluid while these units are under factory OEM warranty. The ZF Lifeguard 8 fluid is the manufacturer approved fluid for these units.
Each OEM application has their own version of how to achieve the proper operating level, however, in the end, they reach the same goal: the correct level at the correct temperature. Note that the fluid used in these units responds drastically to temperature changes. The hotter the fluid gets, the more it expands and the higher the transmission fluid level is.
To ensure that your 8HP has the correct fluid level, follow this procedure:
- Ensure the vehicle is level on a lift. Turn off the Traction Control (ESC). Follow all safety procedures to ensure the vehicle is properly lifted and can be safely operated while in the air on the lift.
- With the engine off, make sure the transmission fluid temperature is below 86°F (30°C).
- For service and dry fill, remove the fill plug. Add transmission fluid until it begins to run out.
- Start the engine and immediately continue adding fluid until it begins to run out again, then install the fill plug.
- With the brakes applied, shift the vehicle into reverse for 5 seconds. Make sure the driveshaft does not rotate!
- Shift the vehicle to drive and hold for 5 seconds. Manually shift and hold 1st gear for 5 seconds, the shift and hold 2nd gear for 5 seconds.
- Shift the vehicle to Neutral and bring the engine speed to 2,000 rpm. Hold it there for 5 seconds.
- Allow the vehicle to idle and then shift to Park.
- Verify the fluid temperature is between 86°F and 122°F (30°C-50°C). Remove the fill plug.
- When fluid slows to a trickle, reinstall the fill plug. Clear any DTC’s and allow fluid to reach operating temperature.
- Use an infrared temperature gun to verify fluid is circulating through all transmission cooling components.
- With traction control disabled, manually shift through all ranges, allowing the transmission to remain in each range for 5 seconds.
- Bring the wheels to a slow stop, place the transmission in Park and turn the engine off.
- Allow the transmission fluid to cool to 86°F-122°F (30°C-50°C). Restart the vehicle and recheck the level.
Note, vehicles using the all-wheel drive version of the 8HP unit will have additional instructions. Refer to OEM procedures for special instructions. Failure to follow them may lead to unit failure!
Since all of these applications have a thermostatic bypass valve in the transmission cooler system, it is necessary to ensure transmission fluid is flowing to the cooler, and that all cooler lines are full before allowing the vehicle to cool for the final level check. If the cooler lines were empty, up to 1 liter of transmission fluid may be needed. The final step may require the vehicle staying in your shop overnight.
D Clutch Failure
The D Clutch drum is a common component that fails inside this unit. The inner seal leaks and causes the clutch to fail. Flared 3-4 shift and/or gear ratio errors from 4th through 8th gears can be a complaint. It must be addressed anytime the unit is removed for an overhaul or repair. Failure to do so would cause a comeback or a possible no-go.
Since the inner seal is riveted to the P4 planetary, the factory level solution involves replacing the entire drum/ planetary assembly. The D Clutch drum assembly retails around $500USD.
A tool kit to service this drum and replace the internal seal assembly is finally available. There are 2 different service tools available; one for the 8HP45, 8HP50, 845RE units and one for the 8HP55 through 8HP90 units. Contact your soft parts supplier for availability.
Since these units are found in muscle cars like the Hellcat, Charger and Challenger, there is an aftermarket focused on making these amazing cars perform just a little better. Aftermarket programming is available through speed and performance shops as well as Do-it-yourself packages on the internet.
As I mentioned earlier, these units are designed to handle everything the engine is programmed from the factory to put out. The torque output curve of the engine is managed to provide the best possible shifts relative to the driver’s demand, even at wide open throttle. As soon as you start to change the factory settings, the durability factor is at risk. Keep in mind, the programming is not just for transmission shift points and quality, but it’s about engine management as well.
Programmers attempting to create more shift ‘feel’ and better launch capability place these units at greatest risk. Shock loading the transmission and driveline components is the most common failure as a result. Thanks to social media, there are several examples on YouTube of this! Also, there are numerous components inside the transmission that can be damaged as well. Several hubs and splines are made of aluminum and were not designed to endure repeated shock loading (figure 4).
Many transmission types come and go, however, over-all design, versatility and durability means the ZF 8-speed unit will most likely be in production for a long time. As more manufacturers use this unit in their choice of powertrain solutions, the more of a chance we will see these units coming into our shops. So, now is the time to become more familiar with the new kid on the block, ‘cause it doesn’t look like he’s going anywhere any time soon!