Creston... you've got several symptoms, and they all share a common circumstance...
Study the circuits... I'll bet that you'll find FIELD and ROAD mode to be an equivalent of HIGH and LOW ratio...
I don't know about your machine, but FREQUENTLY, a system that does this, is done by virtue of a hydraulic valve that performs a parallel/series switch.
In FIELD mode, you want a low ground speed, but plenty of motive force. In this mode, the hydrostatic pump unit's output goes to a TEE, where the branches split fluid flow to right and left wheel motors. The wheel motors effectively run independant of eachother, and will allow differential action between the two (left and right can go different speeds in a turn).
In ROAD mode, you want high ground speed. In this mode, the hydrostatic pump flows fluid into ONE of the wheel motors, then out and into the IN of the other wheel motor, then back to tank. In doing so, the hydraulic VOLUME to each wheel motor's supply is effectively DOUBLED, rather than divided between the two motors. This results in higher speed on each motor, but with less wheel force available.
The way this is handled hydraulically, is with a specially-plumbed spool valve with two positions- parallel (field) and series (road).
When the hydraulic valve shifts from one mode to the other, there's a bit of a hydraulic 'shock' that occurs during the shift, and there's usually a pressure relief valve either in the valve, or built into the supply plumbing going TO that valve, because the hydrostatic shock can either break, or hydraulically 'jam' the valve spool.
Now, while it makes sense to do so, that check valve doesn't necessarily need to bypass BOTH valves... it really only needs to bypass ONE, in order to prevent damage and valve jamming.
In the case of your machine, it very well may be that the check valve has crud in it, and in the process of normal operation, has managed to become slightly unseated, thus bypassing a little volume, even though the pressure isn't excessive.
I think Dr. Allis's suggestion of taking pressure readings is a good idea... the OTHER thing I'd do, is go looking at that Field/Road valve, and associated plumbing, pop it apart for an internal look-see. If there's crud in that check valve, or in the valve spool or it's actuator (electric? mechanical?) isn't fully seating the valve in one or the other positions (like, the detent isn't holding, or crud is preventing the spool from making it to the detent), then a simple scrubbing out of gak, and reassembly, and a serious hydraulic flushing would be my best suggestion.
FWIW... crud inside a hydrostatic system is a bad thing... and machines tend to accrue crud... with compounded interest...
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JohnColo wrote:
