John Deere 4620(S/C syncro) – Loss of hyd. when clutch is depressed

john deere 4620 loss of hyd when clutch is depressed Problems

Thought I’d post the conclusion to the hydraulic pressure loss issue every time the clutch was depressed on our ’72 4620 syncro.

This problem had been apparent for quite some time, but since the tractor was not used for loader service it was swept under the carpet. Several years ago we decided to spend a little time researching it and found that the PTO valve became very hot after a short period of operation, while all other valves remained cold. Plugging the supply line to the PTO valve removed the heat and somewhat improved the loss of hydraulic pressure when the clutch was depressed, but it was by no means cured. So we replaced the PTO valve anyway (it was obviously washed out) and we continued to live with the problem.

Last fall we had a hydraulic cooler return circuit hose failure so we decided to replace all the straight and formed rubber hoses. It was then that we discovered our problem …….. my self-induced problem actually.

Many years ago Dad had a cooler hose failure that I repaired for him in the field. Apparently I didn’t pay enough attention to the orientation of the “T” connection connecting the hydraulic oil cooler and reservoir to the main return line. Well that “T” has an integral check valve on a branch that, if installed correctly, does a number of things. When closed, it forces the flow of oil from the main hydraulic pump to pass through the hydraulic cooler and back into the crankcase. When the clutch is depressed and no more oil comes out of the transmission pump, the check valve opens allowing oil in the cooler reservoir to gravity flow to the main hydraulic pump.

Installing the check valve backwards did 2 things. When the clutch was depressed and the main hydraulic pump attempted to draw oil from the cooler reservoir, the check valve closed immediately preventing any oil from escaping from the reservoir. The only oil the main pump was getting was from the cooler core, which is next to nothing. In addition, the check valve installed backwards also allowed most of the oil to not reach the cooler core. The transmission temperature gauge would normally read in the 1/3 to 1/2 range depending on the ambient temperature.

After installing the “T” correctly, the hydraulic pressure is now maintained as long as the clutch is depressed and the transmission temperature gauge just barely comes out of the minimum position.

My friend, who is one of the best mechanics in JD, just fixed the 4620 syngro with the same problem, not even 5 seconds without steering or hydraulics (engage PTO and no problem). Replaced PTO brake which was wore out and problem solved.

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