Hemi MDS is only designed to engage when cruising under light (low) load conditions. It does not engage at idle. It also doesn't engage at speeds where there is too much aerodynamic drag. Here's a copy of the operation description that has circulated for a long time. The info is about the 5.7L Hemi, specifically. There might be slight differences with the 6.4L, such as oil viscosity requirements.
From AutoInc.online:
How Chrysler's Multiple Displacement System Works
Posted 10/12/2009
By Greg Montero
In 2005, the Multiple Displacement System (MDS) version of the 5.7L HEMI engine was introduced on the Chrysler 300C and Dodge Magnum. This engine can deactivate four of the eight cylinders, and operate as a 4-cylinder engine during periods of low load, improving fuel economy and emissions. The MDS provides the 5.7L engine the fuel economy of a V6 while maintaining the performance of a V8. The fuel economy improvement is estimated to be up to 20 percent, depending on driving conditions. The MDS operation, invisible to the driver, is fully integrated into the fuel injection system.
"In 2005, the Multiple Displacement System (MDS) version of the 5.7L HEMI engine was introduced on the Chrysler 300C and Dodge Magnum. This engine can deactivate four of the eight cylinders, and operate as a 4-cylinder engine during periods of low load, improving fuel economy and emissions.”
When the MDS engine switches to 4-cylinder mode, the powertrain control module (PCM) controls solenoids that manage special "dual mode" lifters, allowing the intake and exhaust valves to remain closed on cylinders 1, 4, 6 and 7. There is one solenoid for each cylinder (two lifters). The PCM also deactivates the fuel injectors for these cylinders when in the 4-cylinder mode.
When the MDS solenoids are in the "off" state (8-cylinder mode), they supply oil pressure of approximately 3 psi to the lifter bores. This lifter oil keeps the bores lubricated and prevents air from entering the system. When the solenoids are activated, oil pressure of 16 psi or greater is supplied to the lifters and the engine enters the 4-cylinder mode.
When the special lifters are supplied high oil pressure by the solenoid, pins inside the lifter move inward. This unlocks the outer portion of the lifter and allows it to move independently from the center portion of the lifter. The camshaft can move the outer portion of the lifter without transferring motion to the inner portion of the lifter.
8-cylinder mode: Solenoids are in the "off" state (de-energized). Minimal oil pressure is applied to the special lifter. They are working normally in this mode.
4-cylinder mode: Solenoids are in the "on" state (energized). High oil pressure is applied to the special lifters and the lifters become inactive and essentially the four pistons are along for the ride. Both valves are closed.
The PCM will activate the 4-cylinder mode when the following enabling conditions are met:
Low engine load
1200 to 3000 rpm
Vehicle speed between 12 mph and 90 mph
Battery voltage from 9 volts to 15 volts
Oil pressure from 15 psi to 147 psi
Oil temperature greater than 120 F
Engine coolant temperature from 158 to 248 F
Ambient temperature from 14 F to 248 F
No throttle position sensor or MDS faults
The PCM deactivates solenoids one cylinder at a time in the firing order. If the system is "on" for long periods of time, such as a long flat road with no stopping, the system will return to the 8-cylinder mode approximately every six minutes to maintain engine temperature. The PCM will also unlock the torque converter before the transition, to minimize the effect on driveability.
The oil temperature sensor used for this system is a two-wire sensor, negative temperature coefficient, like the sensors for engine coolant temperature or intake air temperature (IAT). The PCM uses this input - along with oil pressure, load and rpm - to calculate oil viscosity and flow rate. This information is critical to MDS operation.
It's important to know this engine uses 5W-20 engine oil. If the wrong oil is used, fault code P1521 can be set, and the vehicle may show symptoms of a shudder - similar to a torque converter shudder - when the MDS system is activated and deactivated. Other symptoms may include MDS performance codes or engine misfire codes.
