Like other transmissions, the 8L applications have undergone numerous updates and changes through the years. Many of those updates/changes were due to shift-related concerns, while others occurred due to vehicle system changes.
As discussed in the ATRA seminars, GM has implemented an update to the active fuel management and displacement on demand systems (AFM/DOD) it has used on many of its applications since 2004/05. It is now known as DFM (dynamic fuel management) and was introduced in the 2019/2020 models. The system controls all of the vehicle’s cylinders rather than just four cylinders, as with the AFM/DOD systems.
The system uses solenoids to control each cylinder’s lifters, so the number of solenoids in the lifter manifold increased dramatically. We are all aware of the issues with the previous AFM/DOD system, which could lead to a costly repair if it failed. The DFM system is like an AFM system on steroids which means changing your oil and using the correct oil and filter is critical to system operation and component life expectancy.
The DFM system can control any cylinder or combination of cylinders necessary to achieve the best fuel economy. As a result, the system can cancel anywhere from 1 to 7 cylinders during system operation in any combination. Remember that during normal engine operation, the crankshaft slows down between cylinder firings. As the next cylinder is fired, the crankshaft speed dramatically increases. Now you should not notice the change in speed that is occurring as the speed the engine is running tends to hide the changes in crankshaft speed. The changes in crankshaft speed between cylinder firings are what we have used for years to determine if a misfire is present.
One of the more common issues we see with the DFM system is an “intermittent jerk” when the system cancels a specific cylinder. This symptom is very hard to diagnose unless you understand what is happening. Proper DFM operation requires that the cylinder should only be canceled or reinitiated when the lifter is located on the “camshaft base circle.” If the solenoid is acting up, the lifter may try to activate/deactivate when the camshaft is not in the correct position resulting in an “intermittent jerk” and possible lifter damage over time. This issue is typically due to a problem with a DFM solenoid. Note that the DFM solenoids can be replaced individually on DFM applications.
The slow down between firings is accentuated when DFM is active. For example, let’s say the system canceled one cylinder. The crankshaft speed would drop slightly between firings, but if it canceled two cylinders, the drop in speed would be even more dramatic. When the crankshaft speed varies due to DFM operation, we can develop what appears to be a vibration. Engineering needed to come up with a solution for DFM operation to be acceptable to the customer.
Starting in 2019/2020, some models utilize a new design torque converter on the 8L and 10L applications with DFM. GM has used the same design in the 6L50, dating back several years on small truck diesel applications. GM refers to the design as a CPA (centrifugal pendulum absorber) torque converter, more commonly known as a “pendulum dampener” torque converter. GM is not the only company to use this design. Mercedes, ZF, and others also use the “LUK” technology on all sorts of transmissions, from conventional automatics to CVTs, as well as some DCT applications.
The CPA design torque converter design utilizes a series of counterweights typically attached to the intermediate spring dampener within the torque converter. The counterweights are designed to counteract the change in engine speed to smooth out the “engine firing frequency,” which can lead to vibration during DFM operation and low-speed TCC application. The dampening level and “natural frequency” are based on the length of the dampener weights and the centrifugal force applied to the weights.
So, what does this technology mean when you are repairing an 8L90? In simple terms, you must install the correct torque converter. In addition, a CPA design converter is required to avoid vibration issues with the late model 8L90 DFM applications (figure 1), or for that matter, anything with DFM.
8L90 DFM applications have an additional change affecting the front support and bushing. The CPA design converter applications DO NOT use a bushing in the front support (figure 2).
You must pay attention to the parts you are getting as it can cost you a lot of money if you install the wrong component on your application and you need to pull the unit back out. Just what you wanted to hear, some more things that you need to pay attention to!
Until next time remember, “Don’t let yesterday take too much of today.”
