MDS Explanation

[1978 NYB]

Since we have the best mechanics and engineers (in my opinion) on the planet here on FCBO....

I have a non C-body question I'm wondering if anyone can explain.

I'm trying to figure out how MDS works in my 2018 Ram. It's a CC 5.7L, 4x4 w/3.92 axles. I know the MDS shuts off 4 cylinders when it is on. I thought the ECO light when on is the indicator that the MDS was on. I'm told that isn't true. Some people are telling me you can feel the difference in the engine when MDS turning on and off. I can not tell a bit of difference other than the ECO light turning on. I've been told I can use the buttons and down shift one gear and then push the button up until it in 8th gear and it will turn off the MDS. I would think that the MDS mode would just cause the other 4 cylinders to increase in fuel consumption to make up for the other 4 cylinders not firing. So what is the advantage of MDS other than saving the life of a couple of spark plugs??? MDS isn't an option so every 5.7L engine since this feature has been added has MDS. And I'm sure other manufacturers have a similar MDS feature.

Like I said in the beginning.....we have the brightest people in the world here and can maybe explain how this MDS works and what are the advantages.

Thanks in advance for your thoughts.

 

[Yatzee]

It may already be disabled. By conservative estimate, it takes only about 20 hp to drive a car like the Chrysler 300 sedan at 100 km – h – on smooth, level highway, with an average-size adult driving and no passengers or cargo on board. That figure drops to around 12 hp at 80 km – h, and to just 5 hp at 50 km – h. Why then do we need a 340 hp Hemi V8 in the line's hot 300C version? For acceleration, of course. And for climbing hills, carrying passengers and cargo, and towing a trailer. The greater the reserve of extra power, the faster we can accelerate, or the steeper the hills we can climb, or the more passengers or cargo we can carry, or the bigger the trailer we can tow. Or some combination of all those things. When we are not using all that available power or a substantial portion of it, however, the engine is operating at far less than its peak efficiency. We really don't need a V8 in those conditions – a four-cylinder engine would be more than adequate, and considerably more economical. That is the line of thinking that led Chrysler's engineers to develop the 5.7-litre Hemi's Multi-Displacement System (MDS). Both the Hemi and MDS are standard equipment on the 2005 Chrysler 300C and Dodge Magnum RT. (The 300's base engine is a 3.5 L V6, while Magnum starts with a 2.7 L V6 and offers the 3.5 L six.) Under light-load conditions, such as highway cruising at moderate speeds, the Hemi runs on only four cylinders, explains Jack Broomall, who directed the vehicles' engineering development. He challenged us to notice when the system switched between one mode and another. I couldn't tell in two long days of driving.And, so far as I heard, neither could any of my colleagues. The system's operation is truly invisible. The concept itself is relatively simple. When all the planets align – low engine load, low throttle opening, suitable engine and vehicle speeds, and other parameters determined by sensors already feeding information to the engine management, ABS and stability-control system computers – half the engine's cylinders are instantly deactivated. In just 40 milliseconds, their valves are shut and left closed and their fuel supply is cut off. No air and no fuel means no combustion and no fuel used by those cylinders. At the same time, the engine's electronic throttle is automatically adjusted to compensate, so the driver feels no interruption in power flow at all. Depending on the type of driving, and the driver, the fuel saving could be as much as 10 to 20 per cent, according to Broomall. When more power is needed, everything is reactivated, again in just 40 milliseconds. Technically inclined buyers may be amazed to learn how mechanically simple the MDS system actually is. An electromechanical solenoid at each of the four affected cylinders shuts off the flow of oil to the cylinder's hydraulic valve lifters when activated by a signal from the engine computer. With that loss of oil pressure, a pin connecting the inner and outer sleeves of the lifter retracts, disconnecting the two halves and letting the lifter collapse. The outer keeps moving up and down, driven by the camshaft, but the disconnected inner just sits there, not moving the rocker arms or, correspondingly, the valves. When oil pressure is reinstated, the pin locks into place again and everything returns to normal. It is that simple. The simplicity of the system is aided by the Hemi's overhead-valve – pushrod configuration, with its hydraulic lifters closely spaced down in the block. Where the Hemi's system gets complicated is in the computer programming, and based on the way the cars drive, it appears that Chrysler's programmers have done an exceptional job in that respect. Enabling big V8 performance without the usual fuel-consumption penalty, cylinder deactivation is clearly a technology whose time has come, and it is almost certainly destined to find broader application in the future. To turn it off take a look at this video that was posted on YouTube, it does work for deactivation.

 

[1978 NYB]

Thanks

 

[commando1]

Bob, when you get your Diablo (or whoever) tuner, you can custom program the mds parameters. On. OFF. WHEN. Etc.

 

[1978 NYB]

Already have tbe Pedal Commander and eyeing that Diablo tuner.

 

[73Coupe]

Cadillac tried this in the early 80s but it didn't go so well.

 

[CBODY67]

This is a completely different system than Cadillac had back then, Just as the current GM systems are completely different. Getting that out of the way.

When I rented a '2005 Charger R/T, the owner's manual gave some general parameters for when the MDS operated. Seems like I MIGHT BARELY hear an exhaust change at about 60mph sometimes, but nothing else. That car didn't have the Driver's Info Screen like more current models do, and with that option, you could see when it's operating.

In another place, the poster noted that it's run off of a solenoid and you could wire an indicator light for that circuit.

On more recent models, the ones with a Fuel Economy Screen in the Driver's Info Ctr, it could well also have the "Cyls" number indicated along with the bar graphs of Instant Fuel Economy and Average Fuel Economy. Scroll through the various DIC screens to see if you vehicle has that. I know the cars do, as do the current GM full-size pickups.

In short, it's all run off of vehicle engine load. The "eco" light might be an indicator, but I'm not sure about that. The last two 2017 Charger R/Ts I've rented for 300 mile road trips have averaged 29-30mpg, using the cruise, 70mph speed limits, and not really trying for fuel economy. So I know those were working!

Look in the Driver's Info Center screens and see if there isn't something in there.

As for engine sizes that work best, sometimes the smaller engines will do well, but the bigger engines might not be THAT far behind, if at all. The larger engines will generally stay in MDS mode longer when some small hills or overpasses are encountered. Whereas the smaller engine will kick out of MDS immediately upon any added vehicle road load, then back into MDS when that load is decreased. Hence, the great actual road mpg of the 2017 Charger R//Ts, when compared to their EPA highway mileage. At least in generally flat terrain.

Whether on those Charger R/Ts or on the GM pickup I drive at work, even when watching the DIC screens, I heard NO indication that the variable-displacement system was working or not. No perceived power "surge" or whatever. IF it stops working, I suspect it'll set a "Check Engine" code.

A point on "tunes" . . . I'm not sure if the Chrysler ECM (or whatever term they use for it) is as sophisticated as GMs have been for a while, but ANY activity as to "re-flashes" of the ECM are recorded in the ECM's memory. If you pull out the factory tune and add in another one, it's recorded. If you put the factory tune back in, removing the added one, that's recorded too. Not the specific specs of the tune, just the fact it was changed. So, should any failure result from a "power adder tune", then you pull that tune out, the footprints are still in the computer's memory. As long as there is some sort of warranty coverage on the powertrain/engine, this can be something to be cognizant of. No way around it, at least on the GM side of things. As I said, not sure of the situation with Chrysler, but it could be similar.

Enjoy the truck!
CBODY67

 

[Mike66Chryslers]

Cadillac tried this in the early 80s but it didn't go so well.

It suffered from several problems. First, the computer controls were primitive so there was a lag and you could feel it change between modes. Second, they had 3 modes, 8-cylinder, 6-cylinder and 4-cylinder. A V8 is also balanced running on 4 cylinders, but not running on 6 cylinders, so in that mode the engine ran rough. (That is why the new engines with cylinder deactivation only switch between 8 and 4 cylinders, not 6.) Third, they only put it in heavy Cadillacs, so just about the only time it entered 4-cylinder mode was going downhill. The good thing was that it was easy to deactivate by disconnecting one connector to the engine, and then was in V8 mode always.

 

[1978 NYB]

The Diablo tuner comes with its own ECM. So i could always put the factory ECM back in. Moe's Performance has a custom tune for the Diablo.

I haven't checked this out in detail but I was told I can get a custom tune from the Ram Dealer too.

 

[Mike66Chryslers]

The modern MDS systems do have their problems. It is always the same 4 cylinders which deactivate. Over time, the rings on those cylinders are more likely to get gummed-up. Also, my brother has first-hand experience repairing GM V8s with their MDS system. The special lifters eventually fail so that the pins don't re-engage, and the lifter gets stuck in the deactivated state. Then the top-end has to come off the engine and all of the special lifters have to be replaced. On the GMs he said this takes about 100,000 miles.

I had a RAM1500 with the 5.7L Hemi with the 8-speed and MDS as a rental for about a month. One day I did a sedate take off from a stop sign, then shortly after clearing the intersection I hammered the throttle. Just having a little spirited fun. ;) Anyhow, when I got off the throttle I thought I heard a faint ticking sound in the exhaust note. Over the next couple weeks it developed into a much more noticeable popping sound. Thankfully I returned the truck before the valvetrain actually failed.

I suspect that the cylinder deactivation was engaged when I hit the gas. Even though the ECU has control of MDS and throttle position because of drive-by-wire, it applied power while the MDS system was still engaged. Then the valvetrain on the deactivated cylinders started to float, one of the roller lifters floated off the cam lobe and bounced hard, damaging it. That's just my theory; I don't know for certain.

 

[Dobalovr]

Bob welcome to the world of MDS. At the end of the day in certain circumstances if conditions are right the MDS will kick in without any noticable change in sound or feel. This system allows us to continue putting real engines in our trucks rather than turbo’d V6 ecobees. (Apologies to the converted here). The ECO light is the indicator of if the system is active. As explained above the system is well engineered and is very reliable. Tuners can overide this feature but as also explained above FCA knows what you have done so warranty will become an issue. The only time I can tell when MDS kicks in is in a cat back exhaust setup and the exhaust note changes. Its about the smiles per gallon not the miles per gallon for me.

 

[1978 NYB]

Thanks Graham!

So if the ECO light is on that means the MDS is activated?

BTW, are you coming to Carlisle this year?

 

[commando1]

Is the Chrysler mds 4,6,8 or 4,5,6,7,8.

 

[Davea Lux]

Bob welcome to the world of MDS. At the end of the day in certain circumstances if conditions are right the MDS will kick in without any noticable change in sound or feel. This system allows us to continue putting real engines in our trucks rather than turbo’d V6 ecobees. (Apologies to the converted here). The ECO light is the indicator of if the system is active. As explained above the system is well engineered and is very reliable. Tuners can overide this feature but as also explained above FCA knows what you have done so warranty will become an issue. The only time I can tell when MDS kicks in is in a cat back exhaust setup and the exhaust note changes. Its about the smiles per gallon not the miles per gallon for me.

I very much like the fuel economy that is possible with the modern MDS. I still wonder if any of this stuff will still work after a major solar storm or an EMP burst.

Dave

 

[Dobalovr]

Thanks Graham!

So if the ECO light is on that means the MDS is activated?

BTW, are you coming to Carlisle this year?

Yes light on 4 cyl mode active. You also have the instant fuel economy guage you can watch in the cluster.

The wife and I will be on a boat in the Baltics during Carlisle unfortunately...

 

[Dobalovr]

Is the Chrysler mds 4,6,8 or 4,5,6,7,8.

1-4-6-7

 

[Carmine]

Is the Chrysler mds 4,6,8 or 4,5,6,7,8.

4 or 8, nothing between.

 

[1978 NYB]

Yes light on 4 cyl mode active. You also have the instant fuel economy guage you can watch in the cluster.

The wife and I will be on a boat in the Baltics during Carlisle unfortunately...

Too bad Graham.

I was hoping to meet you.

Maybe my trip across Canada maybe next year and we can cross paths.

 

[1978 NYB]

1-4-6-7

Wow! That is a weird sequence.

 

[1978 NYB]

Graham, do you have any MDS related engine failures at your dealership? Like lifters or ring failures?