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  #13  
Old 09-19-2011, 01:55 PM
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Re: MDS Functioning Properly?

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Originally Posted by dmsfun View Post
Soething ain't right if you only get 15 mpg. I get over 20 mpg now at 65 mph with '06 Overland 5.7 and just a CAI. Part off this is probably due to the engine loosening up; New it was under 18.
I was thinking my lift and tires are preventing me from getting better mileage.

I have a 2" Terraflex lift as well as 265/70-17 BFG All Terrains.

But as some others have said, even with this small lift, I should see higher numbers. I will have to take it into the dealer and see what they say.

I better not get the "It's a HEMI" answer.
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Old 09-19-2011, 04:38 PM
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Re: MDS Functioning Properly?

First you need to calculate your true mileage, as those large tires throw the speedometer and odometer off meaning the mpg readout is off. If it runs good and doesnt throw a code, the dealer will say they can't do or check anything anyway. The lift should slighly have a negative effect on mpg but it should still get 19.
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Old 09-19-2011, 06:30 PM
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Re: MDS Functioning Properly?

Among the many parameters that make the MDS function, one of the biggest is engine load. The load activation %age varies depending on sensor inputs, but mine on the highway kicks in at ~29% and lower and kicks out at ~31% and above. If you have anything that is causing the engine to require increased throttle opening (load), you'll not get much active MDS time. I know that wind drag (aerodynamics), elevation, wind speed and direction, and parasitic drivetrain (full time 4WD), all affect engine load. I've found that even road surface can affect engine load by as much as 2%. Sometimes just adjusting your speed by ~1-2 mph will be the difference between MDS on or MDS off. Like Walt said, any active, historical, or pending DTC will cause the MDS to not work at all, or at best be intermittant, depending on MIL/DTC status.

Hazards280, you may want to consider adding an MDS "ON" light to your 05. Back in 05 I came up with this mod. for the Hemi LX vehicles when I was active in the LX Forum community. Here's the reference to my install thread under my screen name, "magnuman". The wiring and pinouts are the same and it is actually easier to do this mod. on the WK than it was on the LX. If you have any questions, feel free to post up.
http://www.lxforums.com/board/f88/in...575/#post55813
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Old 09-19-2011, 06:43 PM
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Re: MDS Functioning Properly?

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Originally Posted by jeepgcoman View Post
Among the many parameters that make the MDS function, one of the biggest is engine load. The load activation %age varies depending on sensor inputs, but mine on the highway kicks in at ~29% and lower and kicks out at ~31% and above. If you have anything that is causing the engine to require increased throttle opening (load), you'll not get much active MDS time. I know that wind drag (aerodynamics), elevation, wind speed and direction, and parasitic drivetrain (full time 4WD), all affect engine load. I've found that even road surface can affect engine load by as much as 2%. Sometimes just adjusting your speed by ~1-2 mph will be the difference between MDS on or MDS off. Like Walt said, any active, historical, or pending DTC will cause the MDS to not work at all, or at best be intermittant, depending on MIL/DTC status.

Hazards280, you may want to consider adding an MDS "ON" light to your 05. Back in 05 I came up with this mod. for the Hemi LX vehicles when I was active in the LX Forum community. Here's the reference to my install thread under my screen name, "magnuman". The wiring and pinouts are the same and it is actually easier to do this mod. on the WK than it was on the LX. If you have any questions, feel free to post up.
http://www.lxforums.com/board/f88/in...575/#post55813
Jeepgcoman,

I actually have the MDS light installed already, the previous owner wired one in.

The roads that I am driving on are fairly flat, but there are some small inclines here and there. I drive about 60-65mph and leave the A/C off. I still have the braces going across the roof rack ON, I do recall reading that they may have a negative impact on MPG. I have at time even drove 58-60 mph and it still doesnt kick on. The only time I can make it kick on is when I disengage the cruise or am slowing down at an exit. In town it kick in quite often around 35-45mph.

The only TRUE thing that is "wrong" with my car at the moment is that the airbag control module has to be replaced. I have the new one next to me, however, I have not had time to take it to the dealer and swapped.

I can't imagine that would make it kick in less though. If anything I would expect that to completely disengage the whole system.

My previous car, a 2006 Grand Prix GXP had a LS4 V8 with what they called DOD, (Displacement on Demand) which worked on the same principle and that thing would ALWAYS kick down into 4 cylinder mode. (I KNOW, they are totally different cars and there is a lot more load on this v8 than there would be on that v8) but I figured I would mention it.
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  #17  
Old 09-19-2011, 07:09 PM
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Re: MDS Functioning Properly?

I did a little reading and I am meeting all these parameters.

http://www.jeepforum.com/forum/f309/...works-1176294/
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  #18  
Old 09-19-2011, 07:14 PM
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Re: MDS Functioning Properly?

Is the airbag problem throwing a code? See if it resolves itself after changing the control module and any indicated codes are cleared. I am not sure if every code will shut the MDS off, or not. Also some codes reset themselves after 3 good trips and then reset if the problem is intermittant. Walt may be able to chime in and clarify this. I think he's studied the MIL/DTC...MDS activation problem more than anyone else. You're up, Walt. Get your nose out of those physics books and help us out here.

What happens to your speed if you're cruising at about 65 mph, on level ground, and you let completely off the throttle? How long does it take for the speed to bleed off? I realize these things are like pushing a box down the street, but I would think you should be able to tell the difference between normal speed bleed off and some resistance (aerodynamic, brake drag, or ??) problems. I've done it on mine at interstate off ramps and the speed does bleed off much quicker than it does on my rear wheel drive Hemi LX. I think a lot of it is aerodynamics and parasitic drivetrain loss. Other things that may affect this are the larger tires, the lift and, mayble slightly the roof rack cross rails. I don't think any of these, independently or collectively, would cause the symptoms you are describing though. Oh, I just noticed you have fog lights. Where are they mounted?
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  #19  
Old 09-19-2011, 07:19 PM
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Re: MDS Functioning Properly?

hazards280:
Tire size calculator: http://www.miata.net/garage/tirecalc.html to help you correct for different sizes.

My EVIC mpg is within 1-1.5 mpg of the calculated value using miles driven and gallons used. The refill amount is somewhat subjective, so don't calculate it to 8 decimal places!
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Old 09-19-2011, 07:38 PM
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Re: MDS Functioning Properly?

Hazards, here's a little more on the MDS operation. This is from a guy (magnumrt) that worked on the development of the system. This is from his post over on LXForums.com when a bunch of us were trying to figure out how/if we could manually control the MDS operation. We knew that if it were possible it would have to be by tricking the PCM and having it turn the MDS on or off at our command. After his post, we gave up, and you can see why.

"The MDS system is much more complicated than most people realize. The design of the system took MANY more years than you may think, I actualy started working on this system back in 01 in some DC concept vehicles. Trust me, there were many issues to try and get it right. One of the requirements for this system was the electronic throttle controll. When you deactivate 4 of the 8 cyls, you get about a 50% reduction in air requirements meaning that when you hold your foot on the gas at 25%, in V8 mode, the throttle blades are at about 25% open, but when you go into 4cyl mode, we need to immidiatly adjust the throttle to about 15% throttle without YOU fealing it. This is an extreamly complex algorithm and is QUITE heavy in closed loop feedback with the MAP sensor, Crank/Cam position sensors, Input/output shaft speed sensors, etc. Also when you move from 8-4cyl there is obviously less vacuum being drawn by the motor. Lower vacuum to the MAP sensor is translated normaly as a higher load on the motor, but you need the PCM to realize that this is not the case at this particular moment. Now, as far as your question of why the motor doesnt stay in MDS longer to give you better fuel mileage? That is another compicated story that has been tuned quite extensively to give the best overall gas mileage without issues. When the lifters cut out and trap exhaust gasses in the cyl, that gas acts as a spring to keep the motor from wasting energy while the inactive pistons are going up and down. This gas begins to loose it's energy because the cyls will tend to leak a bit (through the rings and through the valves). The motor needs to re-initialize the cyl to allow the spent gases to escape. Now, the question is, why do you not re-initialize right away to save more gas? Simple, you gotta keep the engine's temperature equal so that you dont have 4 cyls working at a different opperating temperature than the others. The 4 cyls that are not firing will get cold as well as the valves, intake runners and specificaly the exaust pipes. Cold exhaust pipes dont flow as well as hot pipes and therefore you get an imbalance in those 4 cyls when running in V8 mode. The timing has been optomized for all these conditions to occur as transparently as possible. Extending the MDS time or frequency would end up creating more problems than are apparent to most.

So yeah, in conclusion.... It aint easy to do, there wont be any way for quite some time for you to increase the amount of time the MDS turns on for the simple fact that the design aint simple ;-) Not to mention the strategy has been developed to be as optimal as it could be. Trust me, the goal was fuel economy and transparency, TONS of time and effort went into making it as good as it can be."
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  #21  
Old 09-19-2011, 07:39 PM
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Re: MDS Functioning Properly?

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Originally Posted by jeepgcoman View Post
..... I am not sure if every code will shut the MDS off, or not. Also some codes reset themselves after 3 good trips and then reset if the problem is intermittant. Walt may be able to chime in and clarify this. I think he's studied the MIL/DTC...MDS activation problem more than anyone else. You're up, Walt. Get your nose out of those physics books and help us out here.

What happens to your speed if you're cruising at about 65 mph, on level ground, and you let completely off the throttle? How long does it take for the speed to bleed off? .... I don't think any of these, independently or collectively, would cause the symptoms you are describing though. Oh, I just noticed you have fog lights. Where are they mounted?
Mine (and its recurrent MDS issues) are maybe not a good barometer. MDS does cancel when a P-series DTC is set at the beginning of a trip. If it stays pending, MDS will function at the next startup. Not necessarily true for the other types of DTC's.

I get mine to kick in by getting to MDS conditions and then backing off the throttle slightly. MDS should work on relatively flat conditions up to 75mph. However, there are times when MDS fails and the engine goes into "engine braking" when I let off the accelerator and it does not coast down; rather it decelerates like it is 4th gear. I see a difference in Long Term Fuel Trim when MDS is functional. In my case, it is related to the passnger side exhaust leak at the manifold-to-pipe joint.

The xmission does not necessarily lock up in MDS mode, so you may some different rpm's for the same speed. The ECM and TCM work together to keep the load below 90% theoretical at that speed (or so I deduce). Some trips it is 1800rpm at 60mph, sometimes at 1925rpm, and sometimes at 2000rpm without apparent reason.

Have to work on Physics Test #1 tonight: Light, Mirrors, Lenses, Refraction, Reflection, Optical instruments, and Polarization. What does this have to do with MDS, except I know the Heep will set a MIL on the way home from school tomorrow night. I guess that counts as light.........

Hope this helps.
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  #22  
Old 09-20-2011, 08:33 PM
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Re: MDS Functioning Properly?

Video of MDS Problem
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