How to Deactivate AFM, DFM, MDS, and Start/Stop To Prevent Wear on Your RAM, GM, or Ford Truck

10 min read

How to Deactivate AFM, DFM, MDS, and Start/Stop To Prevent Wear on Your RAM, GM, or Ford Truck

10 min read

Americans have always appreciated big, torquey engines in cars and trucks, and not just for bragging rights or dragstrip win lights - serious work like towing or hauling around a lot of people and stuff requires equally serious power. US manufacturers want to fulfill that desire, but over the past few decades, volatility in fuel prices and increasingly strict emissions and economy standards have put them under a microscope. Anything that can be done to make engines cleaner and less thirsty is fair game, and even compared to the cars and trucks available in the '90s, modern vehicles have managed to offer more power than ever while significantly reducing fuel consumption.

That hasn’t been easy, and every possible method to squeeze additional tenths of a mile out of a gallon of gasoline has been tried, with varying degrees of success. The fact that a big-displacement V8 spends only a fraction of its life at wide open throttle led to engineers asking themselves, “How can we make a V8 run like a four cylinder when you’re just cruising down the highway and don’t need all that power?” The answer to that question, while simple in concept, has proven to be somewhat trouble-prone in practice, whether you call it Active Fuel Management, Dynamic Fuel Management, Multi-Displacement System, Displacement On Demand, or simply cylinder deactivation.

In the quest for improved mileage and reduced emissions, many modern V8s incorporate some system of cylinder deactivation. While the intention was good, the systems are far from foolproof and have demonstrated significant capacities for engine harm.

The idea isn’t new, either. As early as 1981, GM introduced their infamous V-8-6-4 engine, which was a 362 cubic inch version of the Cadillac 500 capable of turning off one or two pairs of cylinders via a solenoid that moved the rocker arms out of contact with the rest of the valvetrain. Unfortunately, the relatively primitive engine management of the time wasn’t able to achieve this with any sort of smooth transition, and buyers absolutely hated it, despite the fact that it did offer significant fuel savings over the very thirsty engine it replaced. This failed experiment poisoned the well so thoroughly that it wouldn’t be until the early 2000s that GM and Chrysler would take another shot at it.

Faster processors and innovations like sequential port injection made the concept more practical, but the motivation remained the same. “The reason the manufacturers tell you they came up with these technologies is for fuel economy,” explains Range Technology’s Jacob White. “The idea behind cylinder deactivation is that when you’re cruising down the freeway and there’s low load, the engine can turn off half the cylinders and in theory burn half as much gas. When you don’t need the full power of the engine, it provides better fuel economy.”

It’s not as simple as just shutting off the injectors to pairs of cylinders, though - if the intake and exhaust valves continue to operate, those cylinders essentially become an air compressor, still putting parasitic pumping loads on the engine. To get the maximum possible benefit, the valves have to be disconnected as well, and as GM learned with the V-8-6-4, that adds mechanical complexity to the system. Per White, “In GM vehicles, there’s a two-part lifter, and when a cylinder is deactivated oil pressure to that lifter drops and the lifter collapses so the valve stays closed.” Using specialized hydraulic lifters for deactivation is certainly better than previous systems, but any time you add more parts, you add more potential sources of trouble, and even when things work exactly as designed there are still negative consequences that can shorten engine life.

“The problem with that is you are adding more small pieces [to the valvetrain] that are cycling at several thousand RPM,” White explains. “Plus when you aren’t firing that cylinder and there is no heat going into that combustion chamber, then you run into other problems with ring seal that leads to oil consumption and carbon build-up on the cylinder wall. Over time, you have problems with oil consumption and valve train failure. It’s pretty common to hear someone say they have a Tahoe that’s only a few years old with 85,000 miles on it, and they have to put two quarts of oil in it every 3,500 miles. If one of those lifters collapses and stays collapsed, you can even wipe the cam out.”

With the added expense and potential warranty issues posed by these cylinder deactivation systems, there has to be a compelling up-side for manufacturers in terms of fuel economy improvement, and there is - but with a major caveat. In order to provide consistent results, government fuel economy testing is performed under a set of rigidly-defined circumstances. Though this ensures buyers can make apples-to-apples comparisons between vehicles, it also means that the OEMs are incentivized to come up with ways to maximize fuel economy under those specific test conditions.

This cutaway of a Gen 3 hemi shows oil gallery (yellow) that controls the MDS operation. Note the solenoid at the front of the engine and the specialized MDS lifter.

It’s not fair to call it cheating, any more than it’s cheating to take an SAT prep course to get a better score on that standardized test. But as they say in the ads, ‘your mileage may vary.’ “Unfortunately, the EPA tests don’t correlate with real-world driving, so you don’t see the same benefit,” White adds. “It lets them claim the vehicles get better mileage and helps meet emissions standards.” Depending on your driving style, you may not experience any benefit, but you’ll definitely notice the system in operation.

Range Technology AFM/MDS disablers simply plug into the OBII port under the dash and effortlessly disable both of those systems.

Switching from light-load four or six cylinder operation back to running normally on all cylinders isn’t an instantaneous process even with the most modern vehicles that employ cylinder deactivation; “In terms of drivability, there’s a delay reactivating those cylinders so you have issues with throttle response,” White explains.

Another fuel saving strategy being employed on more and more vehicles is ‘start/stop’ technology. Rather than idling when stationary at a stoplight, when the vehicle is motionless for more than a few seconds and other key parameters are met, the ECU is programmed to stop the engine, then restart it when the driver releases the brake and presses on the accelerator.

“Auto start/stop is done for the same reasons [as cylinder deactivation], but people will see more of the benefits from it,” White opines. “There have been concerns about it putting a lot more wear and tear on the starter, but that’s really not a big deal for the way they’ve designed the systems. What it really is more than anything else is an annoyance.” In very hot climates, though the vehicle’s ventilation system will continue to circulate air, without the refrigerant compressor being turned by the engine the air conditioning can get ‘behind the curve’ and have to run longer once the engine starts to catch up, and in cold weather no heat is being produced for cabin climate control, not to mention the extra demands placed on the battery when its capacity is at its lowest. Of course, there’s also the momentary hesitation when it’s time to get moving once again, and that’s the main reason many people dislike the start/stop feature.

Fortunately, there’s a simple, inexpensive, and easy way to restore full function to your late-model GM V8, Chrysler Hemi, or Ford vehicle with Range Technology’s plug and play devices. Their AFM/DSM Disabler for GM, MDS Manager for Hemi, and Start/Stop Disabler modules for both GM and Ford applications allow owners to quickly and reversibly gain complete control over these features via a small electronic module that plugs in under the dash to the vehicle’s factory OBDII connector.

Per White, “The parts are just about universal - we just have two part numbers for GM. One is for start/stop and the other is for AFM.” Another covers Hemi cylinder deactivation, and a fourth is designed to eliminate the start/stop function in a wide range of Ford applications. “It’s just a plug that goes into your OBDII port, and all it does is interrupt the signal from the PCM to the engine to activate that system,” White continues. “The vehicle never knows that it was ever engaged. You basically just plug it in, and the LED will light up to show it’s receiving power. You plug it in, turn your car or truck on, and forget about it.”

While there are other methods to achieve the same results, they often involve reprogramming the factory computer, which can lead to issues should you ever need to take your vehicle in to the dealer for service. Not the Range Technology devices, though: White says, “It won’t affect your warranty; if you take your vehicle in for service, just unplug it, and when they’re done, plug it back in. If you ever want to return your vehicle to stock, you just unplug it. There’s no trace, and it doesn’t interact with the computer - it’s just interrupting that signal. And it will work in any of the vehicles it covers, so you can move it from one to another.”

It’s literally so simple that if you can plug in a USB cable the right way once in every three tries, you’ll have no issues whatsoever with installation. “People think that they’re going to have to follow a lot of instructions or disconnect things under the hood, but it’s literally a five-second process. They think there has to be more to it,” White says with a chuckle.

“A lot of owners don’t even know what this technology is, and they think they just have to live with drivability or reliability issues,” laments White. “Once they understand how simple and inexpensive it is to fix, that’s all it takes to convince them. If they go to the dealer and say it’s acting funny, they aren’t going to find anything wrong because it’s doing what it is intended to do. But down the road a few years, they do a Google search on ‘why is my Suburban burning oil like crazy?’ and then they start to suspect this is what caused it.”


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