Other Articles - July - 2019

Sipping Fuel

A Look at the New Dynamic Fuel Management System and How it Affects Your Transmission

Product changes; some technicians love them, some technicians hate them, but we all have to deal with them. As I’ve preached for decades, a transmission isn’t just a transmission, it’s part of a complete vehicle propulsion system. Today we’re seeing more and more operational characteristics from different vehicle systems being integrated into the software for the transmission. This means that you need to understand more and more about systems other than just the transmission if you want to succeed in this business.

As long as we have internal combustion engines, fuel economy and emissions output will remain the two primary players in the design changes we continue to see. So, how does one effectively impact those moving targets? Depending on the manufacturer, the engineering philosophy may vary a little but the premise remains the same. You can make a smaller engine, add a turbo or super charger to an engine, control some of the engine components such as valve lift and/or cam timing, control the engine compression via variable compression systems or control/cutoff the air/fuel into specific cylinders to change the effective displacement of the engine. Different manufacturers use various combinations of the above systems to help improve fuel economy while reducing emissions.

Back in the 2005 ATRA seminar, we introduced you to a system used by GM and Dodge that seamlessly changed the engine from 8-cylinder mode to 4-cylinder mode as you drove the vehicle. That system was typically referred to as Displacement on Demand (DOD), and later known as Active Fuel Management (AFM). In the last 14 years, most technicians have experienced numerous transmission-related symptoms that were traced back to issues with the AFM system.

Starting in 2019 a new system has been introduced. As with DOD/ AFM, the new system has a couple of different names depending on the application. This system is known as either Dynamic Fuel Management (DFM) or Dynamic Skip Fire (DSF). The principle is still the same, most drivers have more power available than they’ll typically use during normal driving. This means the engine is operating at a fraction of its capability/ engine efficiency and fuel consumption is significantly reduced. DFM is designed to enhance the vehicles fuel economy during most typical driving conditions. This system utilizes the same philosophy as AFM but it is a quite a bit more sophisticated.

DFM was implemented on the 2019 GM direct injected, variable valve timing 5.3L (RPO L84) and 6.2L (RPO L87) engines. In 2019, these engines are in the Silverado and Sierra but will move into the 2020 Yukon, Tahoe and Escalade vehicle families. Other 2019 and 2020 engines such as the 4.3L (RPO LV3) V-6 and 5.3L (RPO L82) applications will utilize the current AFM system rather than the new DFM system.

The AFM/DOD system controls 2 cylinders (V-6 applications) or 4 cylinders (V-8 applications) which were either turned on/off by controlling the fuel injectors electronically and the engine’s valve lifters through an engine valve lifter oil manifold (VLOM) via solenoids. The DFM system now utilizes solenoids for each cylinder and can control any number of cylinders an in any cylinder combination. The system software is capable of choosing between 17 different cylinder firing pattern software maps and 64 different available fractions depending on vehicle operation. The system updates cylinder firing sequences 80 times per second. Doing this allows the system to stay in some form of DFM mode more than 60% of the time. The DFM system can choose any combination of cylinders all the way down to 1-cylinder mode depending on engine parameters. Now, 1-cylinder mode is unlikely unless you are going down hill with a tail wind, but it is still one of the possible combinations.

The DFM lifters operate like the old AFM/DOD lifter. Oil feeding the lifter is controlled by a solenoid. A latching pin is used to control the lifter operation, depending if oil is allowed to flow to the lifer or not. The lifter can either transmit force to the push rod or collapse inside itself preventing the camshaft lobe motion from being transferred to the pushrod and valve train (figure 1).

The new engines were fitted in front of either the 8L90 (RPO MQE) or 10L80 (RPO MQB) series transmissions (Note: the transmission RPO codes for the DFM applications are different than non DFM applications as they are different transmissions). A new design torque converter is used on DFM applications. The converter has a new design that varies its dampening effect based on TCC slip rates and converter RPM through the use of a centrifugal pendulum counter weight (figure 2). This allows the TCC to be applied at very low speeds even when cylinder deactivation is active without vibration issues occurring.

Coupled with the new design torque converter, the transmissions are also equipped with a “start/ stop” feature which shuts the engine off when the vehicle comes to a complete stop. Yes, I know, it’s the most annoying feature ever invented, but it’s all about fuel economy. Unlike many other GM applications this start/stop system may be disabled on these vehicles by pressing a “disable switch” located in the center of the dash. The system will be enabled again if the key is cycled.

Like the AFM/DOD system, you will hear me preaching about the importance of using the correct engine oil and filter along with the need to change it often. A tremendous amount of the issues with the AFM/ DOD system on both the GM and Dodge applications are due to poor maintenance on the part of the owner, it’s an expensive lesson to learn. While all these vehicles are equipped with engine oil life monitors, it’s been shown to be prudent to change your oil and filter well prior to the system indicating that the oil should be changed.

Diagnosis of the system will be a challenge as this system adds complexity to your diagnostic thought process. Since the system can select any cylinder or combination of cylinders to disable, the diagnostics uses a combination of sensors. The Crankshaft Position Sensor (CKP) and the manifold absolute pressure sensor (MAP) are the main players. For decades we’ve used the CKP sensor to diagnose cylinder misfire issues by monitoring the acceleration rate of the crankshaft during cylinder firing.

This system compares the crankshaft speed changes to the pressure changes measured by the MAP sensor when the system is active. Since the ECM knows which cylinders are disabled, it compares that information from the CKP speed change to the change in MAP pressure fluctuations due to the disabled cylinders. In other words, when a cylinder is disabled, a fluctuation in intake pressure occurs as the intake valve is no longer open. With a dead cylinder, the acceleration rate of the crankshaft during cylinder firings is also affected. The ECM compares the two values and determines if the cylinder is disabled and if so, which cylinder is disabled. If the ECM determines the cylinder isn’t operating correctly, a DTC will be set. In addition, the ECM will monitor the electrical operation of the solenoids and DTCs will be set if an electrical issue exists.

Now as far as the transmission is concerned, DFM dramatically increases what you need to know to diagnose transmission related issues. Since 1997, GM has used torque modeling to control the transmission pressure and shift points. That is why mass air flow (MAF) issues can lead to transmission damage. This system really elevates the use of torque modeling for transmission pressure and shift control. This means that issues with just about any sensor on the engine can cause pressure and shift problems. The need to have a quality scan tool and some quality training on systems other than just the transmission has never been greater.

As you can see, our industry isn’t standing still, it’s changing at an alarming rate. Your success is based on your ability to adapt to these changes, so get involved and learn as much as you possibly can. Until next time remember, “It always seems impossible until the job is done.”