Looking at a d39e that has a slipping trans. (I am a sucker for things that need work) It is showing 6000 hours. Does any one have experience on these? If slipping does it need a trans rebuild or should I consider multiple problems?
There are gauge connects inside the left side door that you can plumb into. Those machines have a hydraulic shifter that can leak and get you stuck between gears or jump out of gear as well.
Is there a chance it just has low pressure from an external pump and not a bad trans?
Is it slipping in one gear or direction or in all of them? One gear usually means a leaking clutch pack. All the gears might indicate low oil pressure. You should check pressures in any case if for no other reason than just a reference after it is fixed.
You own a bank?