The new Dp sensors from Deere sub to a non-adjustable sensor.
If that's the case then it looks like someone replaced it at some point with a genuine part. I like that!
Maybe the unit worked on a hammer before you owned it or was made weak in one location at the factory
Very interesting Simon, before I went through all this I would never have thought that a big lump of cast iron could have these issues. I knew that lots of heating and cooling could affect cast iron, like trying to weld it with the wrong technique.
It makes sense that vibrations and shock loading from a hammer drill would do some damage, however I am leaning towards the factory defect theory. When that one wrecker told me that they were "known" to crack that got my attention. When a guy who sells parts for a living knows about a problem, it must be fairly common.
Did a little test today. I checked the pump pressure with controls in neutral.
Spec in the book is 440 - 580 psi (P mode)
Old control valve:
Idle 260 psi
P mode 270 psi falling to 240 psi
New control valve:
Some strange things happened, I might have to re-test, something might be up with my gauge. After un hooking the gauge the needle remained at 380 psi. I am pretty sure it normally falls back to zero.
Idle (cold) 280-300 psi
Idle (20deg C oil temp) 360-380 psi
P mode 390 psi (stable)
I know these numbers are lower than the book spec but in other parts of the manual I found this...
The engine does idle better with the new control valve so the dp sensor must be getting the correct differential pressure to signal the pump to run at minimum output. With the old control valve the differential pressure would have been lower causing the computer to think that there was load on the system. It would then signal the pump to put out more flow which put an unexpected load on the engine while it was idling causing the engine to run rough.