A key purpose of the charge pump is also to cool the VDP. On larger tractors, like my 1845, the wheel motors also have a separate case drain for bypass and to help with cooling. Consequently, these circuits are not "pure" closed loop.
A BIG downside of closed loop is the lack of any filtering. Therefore, when the loop is opened for any reason, the loop should be filtered (e.g., with a 3 micron filter cart). I built my own filter with parts from Surplus Center and posted pictures on here somewhere of the design (it uses check valves so it can be used in forward or reverse without reversing the direction of the filtration...i.e., you can't connect it incorrectly).
I have three Power-Tracs and the VDP has required a rebuild on all three before 1000 hours. One of the rebuilds was discretionary but Terry said it was needed when I sent it in for inspection. This is VERY low hours for the service life of a VDP. I suspect this is caused by the lack of filtration in the motor circuit. Power-Trac considers the VDP as a wear item...i.e., it will require a rebuild from time to time. An **** retentive person would use a filter cart to filter the circuit on a routine basis to remove normal wear particles in the circuit. PT even recommended this procedure years ago and offered a kit for the filter. Lately, they don't mention it much...probably because one can easily do more damage than good if you're not careful.
I'm not "dissing" PT's engineering here. As mentioned, I have three...so I must like them. But I also have a couple Takeuchi machines and there's filters all over the place on those machines. Even my Kubota L3901 has several filters. In my opinion, PT could have engineered much better filtration. Or at least offered it as a profit making option...like they should offer larger displacement wheel motors as an option for those who want power over speed.
