Twelve years after selling my equipment (D8,D7,955), I reopened a quarry and purchased a CAT 973 - 90L. With 5000 hours, great u/c, a strong engine it looked great. But, the machine was not properly maintained, so.............. the hydrostatic transmission suffered. So many years of sludge/dirt to remove, several oil & filter changes etc. But after a few months the machine couldn't keep on working. The hesitant drive response became more pronounced, then steering went on one side. Dozing power worsened until i could not get forward drive.Even though the pressure readings were high, the Mechainc advised to pull out the transmission. The port plates (2 I think) were only slightly scored, the servo cylinder hoses had ruptured, he found no other damages, that's what I thought. Two mechanic changes later I am practically back where I started!!. Next post has the details. Really going to need some help on this.
No next post? Sent from my SAMSUNG-SGH-I317 using Tapatalk
With the transmission "rebuilt" the first test looked good. This CAT could spin, pivot, finnaly a quick response to forward selection. But in a few days the forward hesitation returned. No problem with reverse. In the island it is a long wait to get an experienced transmission mechanic who is even willing to touch a hydrostatic one. After 2 months I have a really good guy with me going over the workshop manual, testing, adjusting linkages, until he said the cover is coming off for a look. Found the relief valve without seals and rings, replaced but no improvement. In neutral he notices both hoses on Left pump building pressure intermmittently. Toggling directional lever gets back a steady pressure, but when forward selected his guages show steep drop in pressure. In reverse, pressure is at spec and steady, and will stall engine if overspeed valve is modulated!! Only in forward the action of underspeed valve is intermittent. Could it be an Internal leak? Charge pump pressure is on low side of range. Servo pressure high in forward but not as high as reverse. Why is L>H pump able to build sufficent pressure on its own but cannot maintain it when forward is selected. STUMPED!!
There is a thread somewhere on here by CMark of this parish about rebuilding a hydrostatic, I think it was a 963 from memory. It might be worth a look.