NEW-B! It is in a nut shell when 4 of the 8 cylinders in your hemi "shut off" when not under heavy load for increased gas mileage. I believe above 20 mph, your Jeep is always thinking when it should shut 4 off for fuel economy...that help?
Right on, 08Hemi. Here's a little more info.....MDS stands for Multi-Displacement System, and there are a host of items that must be within certain tolerances for the computer (PCM) to activate the system. A few of the biggies are, engine coolant temperature, oil pressure, engine load, etc. The MDS cylinders (1, 4, 6, 7) have special valve lifters which are "disengaged" when in the MDS mode. For the system to work properly, one MUST run 5W-20 weight oil....a good synthetic oil appears to work better than dino. oils. I can get mine to come on at ~17 mph and have had it on as high as 85 mph. 05armywk, since you have an 05, you may want to consider adding the MDS "ON" light, so you'll know exactly when it is on. Once you learn some of the parameters, you can adjust your driving habits to maximize MDS "on" time. Also, a louder than stock exhaust will let you know when it is active.
"Fuel economy has also been improved, but not at the expense of HEMI performance. Chrysler Group has developed the Multi-Displacement System (MDS) that deactivates four cylinders when the V-8 is not needed. The 2005 Grand Cherokee is the first SUV to offer fuel-conserving MDS.
The Chrysler Group MDS seamlessly alternates between smooth and economic four-cylinder mode when less power is needed and V-8 mode when more power from the 5.7-liter HEMI engine is in demand. This optimizes fuel economy when V-8 power is not needed without sacrificing vehicle performance or towing capability.
“The MDS was part of the engine’s original design,” said Bob Lee, Vice President—Powertrain Product Team. “This results in a cylinder deactivation system that is elegantly simple and completely integrated into the engine design. The benefits are fewer parts, maximum reliability and lower cost.”
Some of the significant technologies enabling the MDS are the speed of electronic controls, the sophistication of the algorithms controlling the systems and the use of Electronic Throttle Control. The HEMI is able to transition from eight cylinders to four in 40 milliseconds. This engine and MDS has completed over 6.5 million customer-equivalent miles through Chrysler Group’s development and durability testing. It is covered by the seven-year/70,000-mile Limited Powertrain warranty."
I came up with this mod. back in 05 for the LX's, and when I got my WK I found it to be wired the same. Here's the link to it under my screen name, "magnuman" over at LXForums.com: http://www.lxforums.com/board/f88/installation-mds-indicator-light-11575/#post55813 There is an updated version somewhere over there too that shows a different place to tap in the the MDS solenoid wiring. I found the WK to easier than the LX when I did mine. There is a pass through in the firewall just behind the PCM that makes it real easy to pull wires through behind the glovebox. I got the green LED from Radio Shack. Here's a pix showing where I mounted mine. If you have any questions, feel free to ask.