The concept of “Displacement on Demand,” “Active Fuel Management,” “Dynamic Fuel Management,” or any other system that varies engine displacement isn’t new. But in the past, AFM has failed dismally. Of specific note is the Cadillac V8-6-4 L62 engine that debuted for the 1981 model year. And vanished that same year. The concept might have looked good on paper, but the electronic systems needed for trouble-free operation had not yet evolved.
Much Improved But Still Needs Work
GM’s current iteration of variable displacement, introduced in 2005 and updated in 2018, has fared much better. For the most part, these systems operate problem free, but their operation does appear to cause some mechanical issues. Specifically, premature failure of valve lifters and reduced torque converter clutch life have been linked to the effects of operating an eight-cylinder engine on four cylinders.
Whether lifter failures are a consequence of the system deactivating cylinders or manufacturing issues is a matter of debate. However, there is widespread agreement among GM vehicle owners that deactivating Active Fuel Management (AFM) reduces the potential for premature lifter failure.
Identifying The Culprit
That’s also the preferred method of dealing with torque converter clutch failure. At first glance, it would seem that there’s no relationship between cylinder deactivation and converter clutch wear. But there is a direct link. When a torque converter clutch is fully locked and a V8 engine is operating on four cylinders, the drivetrain will frequently vibrate or shudder. To eliminate V8-running-as-a-V4 vibration, the Transmission Control Module (TCM) is programmed to allow the converter clutch to slip.
The technique used to control slippage is pulse width modulation, which continually locks and unlocks the clutch. The amount of slippage programmed into the TCM varies but typically ranges from 10 rpm to 40 rpm. There are a number of means to deal with the effects of Active Fuel Management, but we’ve found the EFILive FlashScan and Autocal systems to be the best.
EFILive Options For AFM Control
If you do your own tuning, the FlashScan system is what you want. It provides the hardware and software needed to connect to a vehicle through the diagnostic port, read the existing tune file, and store it on a laptop computer. After that, you can examine the tables in the calibration file, make changes and flash the new calibration back into the Engine Control Module or Transmission Control Module. Bear in mind some modifications may be illegal for vehicles driven on public roads, so be sure to check emissions regulations in your area.
Autocal systems are designed for vehicle owners who don’t do their own tuning and want to purchase a “plug-and-play” tuner. Specialists who provide tuning services can create a custom calibration file, load it into an Autocal and send it to a vehicle owner. The vehicle owner then connects the Autocal to a vehicle and reflashes either the ECM or TCM (or both) with the custom calibration. Autocals and FlashScan systems can also read existing calibration data, read and reset trouble codes, and monitor and log data in stand-alone mode with no computer connected. Additionally, both systems provide excellent remote tuning and troubleshooting solutions, as logged data can be emailed to a specialist.
Working Through The Data
When addressing potential torque converter clutch failure, the most common first step is to use a FlashScan system to switch off Active Fuel Management. That’s quickly done by clicking on the “Active Fuel Management” tab in the navigation window and then the “Parameters” tab. After the parameters tab opens, the data settings are shown at the right of the FlashScan software. The data entry listed as “B0921” at the top of the page allows a “Yes” or “No” option. To turn off Active Fuel Management, select “No.” (The number for each table or entry may vary depending on the type of controller and operating system. The accompanying photos show data as displayed for an E92 ECM and T43 TCM.
Although turning off AFM will certainly improve converter clutch life, additional steps will provide further improvement. Several tables in the T43 transmission controller show that stock settings lock and unlock the converter clutch with the frequency of a turn signal operating when the lighting system is low on blinker fluid.
As the accompanying table (D2302) shows, converter clutch slippage is programmed to occur even when the engine runs on all eight cylinders. Clutch slippage isn’t set to zero until the engine produces at least 184 lb-ft of torque. Although this is not as extreme as programmed clutch slippage in AFM mode, where clutch slip is called for at all RPM and load levels, it still accelerates converter clutch wear.
Listen To The Pros
Some transmission specialists also advise eliminating some lock/unlock cycles as an additional means of extending converter clutch life. Typically, stock calibrations lock the converter clutch in each gear, with the clutch being unlocked during each gear change. Transmission fluid flow through the cooler increases when the converter is locked. Hence, a typical custom calibration eliminates clutch lock-up only in first, second, and third gears and retains it in fourth, fifth, and sixth, when loads and heat generation are higher.
In addition to the inconvenience and expense caused by a converter clutch failure, time is also an issue, as replacement parts are often not immediately available. Supply chain problems combined with high demand have depleted inventories of the parts needed for repair. In many instances, the debris resulting from a converter clutch failure also damages internal transmission components, resulting in the need for a complete transmission rebuild or replacement.
Real World Results
In our testing, we saw surprisingly little change in fuel economy after several ECMs were reflashed to eliminate Active Fuel Management. Obviously, fuel economy varies according to vehicle and driving style, so your mileage may vary. That is most likely because our test vehicles were used primarily in stop-and-go driving or on interstate highways and freeways, where an engine spends relatively little time in four-cylinder mode.
Another consideration- regardless of the change in fuel economy, the additional cost will likely be less than the expense and inconvenience incurred when a transmission fails prematurely.