I have a John Deere 4020 with syncro range, the transmission, when you hold the clutch pedal to change gears you lose power steering, when you release the clutch pedal you get power steering again which could cause this , where the pressure is lost in. Thanks for any help.
It is a common problem in syncros. Ours did until we modified it. It has to do with the resivoir tank by the radiator which is designed to maintain pressure while keeping the clutch on. Some kind of spring mechanism in there to help hold pressure or something I’m just not sure what I heard. I’m not an expert, someone here can tell you more.
The service information on my website is a quick solution. I know it works. I made it to my ’68 John Deere 4020 and have let it set for several months, pushed in the clutch started it up and lifted the magazine and steered the lock to the lock before letting the transmission pump spin. I know that many others have used that information to make successful arrangements. Convert an elbow to a check valve to replace the worn check valve deep in the transmission that is hard to reach. The 4020 side console moved that check valve up so if it has a leak it doesn’t drain the front cooler / reservoir. I have found some bulletins on the side console but have not added them to my web page yet. I plan to do it, I already have them scanned, I just have to work on the page.
It shouldn’t do that in the time it takes to change gears, all syncros do it, the transmission pump stops when the clutch disengages. If it stops so early, you have:
1-The transmission pump is worn.
2-A line or an O-ring is blown from the pump, or another leak in the low pressure circuit.
3-A high pressure leak to the crankcase at the steering, brakes, rock axle, or SCV. If it is a side console tractor it may be on the PTO valve.
4-Plate at the bottom of the rock axle housing is cracked or loose.
5-Possibly an internal leak in the main pump, but it is not probable.
The easiest way to find a high pressure leak is to start cold and feel the hydraulic components and the return lines from them. A leaking valve will start to heat up in a few minutes. Sometimes a squeak can be heard in the valve, most noticeable when it is turned off. If the main pump has a destruction screw, turn it and listen for the motor to discharge. A high pressure leak will cause the pump to stay on race and load the motor.
The differential lock valve may also be the problem … if you suspect it is a component you can plug the pressure line to it and see if there is a difference.
For ordinary operations the hydraulic cylinder should run for hours with the clutch pedal depressed. All the front reservoir needs to supply is the difference in cylinder volume on the two sides, so when it extends it takes oil from the reservoir, but when it retracts it puts oil back into the reservoir UNLESS the valve returns. is connected to the transmission crankcase rather than the return to the SCV or transmission oil filter.
The front reservoir has several gallons of oil stored at about 250 psi. In early new generation tractors the oil return to the gearbox means that the available flow is limited by the charge pump, which is smaller than the main pump, not by the flow of the main pump. That is not the correct way to use the tractor hydraulics.
As I understand it there is a check valve between the drive pump and the front reservoir, but it is located near and above the reservoir (cooler) so when the check valve wears out it does not drain the reservoir I don’t have that manualshop or parts catalog to check.They are clearly different between the two main versions of the 4020.
On tractors before the side console, the check valve was on the charge pump at the bottom of the transmission, so when it has a leak it drains the reservoir.The remedy was to turn an elbow into a check valve using Deere parts from some other machine and it works great.