Hello.
I have a 04.5 2500 4x4 with the NV 5600 6 speed manual that I use to pull our 32 foot fifth wheel. I've only pulled the trailer 3 or 4 times yet, but from the beginning, it seemed like as soon as it got hot, the transmission was almost impossible to shift. It shifts excellent all the time when I'm not towing, only has trouble when pulling heavy.
After reading up on it, I changed out the trans oil and used Royal Purple synchromax oil. Still sticky. After reading some more, I dumped in an extra quart through the top of the trans, and this made it a little better, but still sticky. After that, I found some knockoff pod-type trans coolers from ebay. I was impressed when I got them at what good quality they seemed to be, so I installed them and filled it up, once again removing the shift tower and adding an extra quart. All tolled, I think it now has 8 quarts in there.
Took it out for a hundred mile spin pulling the trailer on a cool day the other day, and it's definitely WAY better, but still fairly sticky feeling when it's been pulling hard, up hills and such.
One of the pod coolers has threaded holes with plugs in them, so I was considering plumbing in a oil cooler with a little pump to circulate the oil.
If anybody has any advice about this transmission, I'd like to hear it.

Cheers
Bert

 

Good Morning, Bert. How many miles on that beast? My '03 with the 6 speed manual started to get hard to shift at one point in its life so I changed the clutch master/slave cylinders and the issue went away. I hope your issue is similarly easy.
Back to morning brew . . .

 

Hi FMB.
The truck only has 176K on the clock.
The trans shifts buttery smooth all the time, til it's pulling heavy for an extended period, then it starts getting sticky in the crashbox. After it sits at rest, even for only half an hour, it's back to being all easy to get along with.
Was your hydraulic system problem related to heat? I really think my clutch and hydraulic engagement system is okay, but I'm always open to suggestion.

 

The mileage on my Master/Slave R/R was ~145K. The original clutch lasted till ~185K.

OK, open to suggestions, huh? Try this one on, but it will take two people. When the truck is cold and you KNOW the truck shifts well, go under the truck and as your #2 person steps on the clutch, measure the travel of the Release Fork from the Slave Cylinder. Do the same measurement when you KNOW the truck is hard to shift, and compare measurements. The third test is to have #2 step on the clutch and hold it for a minute and see if the Slave holds the Release Fork at the same spot for the whole minute. Do this two times, once when shifting well and another when not shifting so good.

Your issue may be the Master/slave wearing out or just ready for new a new clutch.

Good Luck