What controls the High/Low shifting on the Grand L HST's?

[Grateful11]

Our L3940 that has almost 2900 hours on it is suddenly shifting from high to low and vice versa with no load on flat ground by itself, you can feel a definite shift. It seems to be doing it more in Mid range than any other. The only way to stop it is to either flick the Hi/Lo paddle to Lo and leave it or turn off the Auto Throttle lever on the steering column and just set the throttle control to say 2000 rpms and go about your business. The Turtle light and the Auto HDS light no longer works, I assume burned out, the Rabbit and Stall Guard light still works most of the time. This tractor is normally used to feed 4x4' round bales on a feedlot 365 days a year. No hills just a slight grade on the dirt road to and from the hay shed to concrete lot, it used to be a dairy so there's about 6000-8000 sq.ft. of concrete and feeders with stanchions.

I will say that we think it's been 1300 hours since the hydraulic fluid and filters have been changed according to our records. Our first thought is to change the filters and fluid but would like to understand a little more about what controls the shifting. Is it computer control, hydraulic pressure sensors, relays, solenoids or what? We really can't complain as this tractor has never had a repair except a couple batteries, not sure how it went that many hours without a hydraulic fluid and filter change but it did, it's possible that it didn't get written down. We usually write it on the filters, need to roll under and see. We've always used Kubota hydraulic filters and SUDT2 fluid.

As you can tell it's no pasture princess but this is about as dirty as it gets.

 

[RjCorazza]

Are you certain the Auto H-DS setting is off? If it's just that the dash indicator light is off, perhaps that function is enabled ?
I don't know why it would be shifting on flat ground under no load.

I think you're on the right track to change out the filters and fluid. I stretched the interval on my previous L3410 and the hst started acting up (mostly getting noisy), which went away with a fluid change.
As far as the valve itself, I can't offer anything beyond what you already know. It's a solenoid valve controlled either manually, or presumably through the ECU based on load on the hst when in Auto H-DS mode.