Tach goofy, MDS fooling around, or is tranny slipping?

[07Sport Quadcab]

Vehicle: 2007 5.7 Hemi with MDS, 545RFE transmission.
My tach indicates quite a variety of RPMs at a steady 60 mph (5th gear as far as I can tell). I will notice it showing about 2100 at times, and at other times it may be 1900 or even 1750. I've never felt or heard any change in engine speed , but with road noise it would be difficult to detect anyway. There doesn't seem to be any correlation to hills; I'm talking level highway conditions.
Is it possible this is the MDS kicking on and off? Some posters talk as if they know when it's active or not active; how can you tell? I'm fairly new to this truck, but I've not noticed a change in sound or by seat of the pants that would tell me it's on or off.
I assume the 545 has variable line pressure- can that cause the clutch to slip a little at times?

 

[Karlsweg]

Mds probably but to find out put it in tow/haul and see if rpm changes. At 60 mph, my 07 seems to search for overdrive easily.

 

[07Sport Quadcab]

Thx-- will try that and report back

 

[07Sport Quadcab]

Can you detect the 4-> 5 shift on yours? I can't unless I move in/out of tow/haul

 

[hemihustlin]

Once you reach top gear the first rpm reduction is torque converter lock up then after that its mds engaging.

If you roll down all your windows and go cruising around town you are able to hear the mds engage. With a stock exhaust it will sort of sound like a soft helicopter sound pop-pop-pop-pop

Once you remove the stock mufflers I say it sounds like an underwater subaru, it warbles and drones. Thats when most people look to turn it off.

 

[07Sport Quadcab]

I tried locking it into tow/haul.
Strangely, the rpm at 60 is similar to what it often shows in drive-- about 1950. I guess this means that a lot of times when I glance at the tach the beastie is running in 4th. On the occasional times it is lower, it must be in 5th.
After running it for awhile in tow/haul I put it back into Drive and it was running a constant 1650. But I can't get it to consistently go to 5th gear; even speeding up past 70 then dropping back doesn't get me below 1950 rpm at 60 mph. I just can't figure out what it takes to get this thing to go into and stay in 5th so I can get the lower rpm.
At least I think I've assured myself that I don't have a slipping transmission; just a weird one.
Here's another strange occurrence-- the check engine light wants to come on after being in tow/haul for awhile. Eventually it goes off after several restarts. I will have to catch it when it's on and get to Autozone and have them get a readout.

 

[Karlsweg]

65 to 70 is where I notice my 07 drop rpms.

 

[07Sport Quadcab]

OK, y'all have been a big help. I haven't had this vehicle very long and haven't put much higher speed freeway driving under its belt. Most of it has been at 65 or lower. Come to find out, the reason I'm seeing the higher RPM at 60 mph most often is that the truck doesn't go into 5th all that easy, so a lot of what I've seen has been due to cruising in 4th. Today I learned, after reading what Karlsweg wrote, that the vehicle doesn't hit 5th gear until about 70 mph, at least it doesn't get to 5th before 70 very often. Hence the 1950 rpm at 60 is normal for 4th gear, and the 1650-1700 rpm at 60 is happening when I get over 70 for a shift to 5th, then drop back to 60 mph.

 

[Snake15eyes1998]

My tach doesn't work. I use an OBDII scan tool to see my RPMs( while towing) for the most part, It sits at 0. You're mds is more then likely working. I have an aftermarket system. So I know when mine kicks in. It's like others have said. Its an awful drone. But I drive mine in O/D off, so I never hear it anymore. The tach problem is really common. Not worth fixing in my opinion. Mine will sometimes do a full 360 degree back flip haha. At one point my tach kinda worked. My RPMs where way off though. Now it's done.

 

[Bheezy27403]

I tried locking it into tow/haul.
Strangely, the rpm at 60 is similar to what it often shows in drive-- about 1950. I guess this means that a lot of times when I glance at the tach the beastie is running in 4th. On the occasional times it is lower, it must be in 5th.
After running it for awhile in tow/haul I put it back into Drive and it was running a constant 1650. But I can't get it to consistently go to 5th gear; even speeding up past 70 then dropping back doesn't get me below 1950 rpm at 60 mph. I just can't figure out what it takes to get this thing to go into and stay in 5th so I can get the lower rpm.
At least I think I've assured myself that I don't have a slipping transmission; just a weird one.
Here's another strange occurrence-- the check engine light wants to come on after being in tow/haul for awhile. Eventually it goes off after several restarts. I will have to catch it when it's on and get to Autozone and have them get a readout.

I'm betting EGR valve. Was I right?