2940 John Deere

[CaptainQ]

I have a 1982 John Deere 2940 that I have a few Questions about. 1. The hydraulics work fine when the tractor first starts, but when it warms up and you idle the engine down for 10-15 seconds it sounds like the engine unloads and you loose most of your hydraulics but when you rev. it back up in about 10-15 seconds you hear the engine load up again and the hydraulics are back. The only things that I have done are check the fluid and changed the filter with no change. 2. Also my remotes and one steering cylinder are leaking are these hard to repair?

Thanks,

Carl

 

[J_J]

The hyd pump is always pumping, and there should be very little pressure until you activate a valve. Have you checked the hyd levels. Have you checked the QD's. When you say the engine loads up, are you doing anything. Some valves are repairable, and most cyl are repairable.

 

[CaptainQ]

I will not be doing anything when the engine sounds like it loads and unloads, it almost sounds like the pump looses prime, and when you rev. back up it sounds like it starts pumping again after 5-10 seconds, until then you don't have power steering and the three point is slow to lift.

 

[mmurphy]

TractorData.com John Deere 2940 tractor information
Shows you for a closed loop system. For some reason pump is not stroking right. Or, possibly charge pump is not working properly.

 

[AKKAMAAN]

"Closed loop" must be meant to be "Closed center"...Right mmurphy??!!

I just "fishing" here...hope to trigger someone with more "knowledge" and "understanding" to provide details about this specific, JD 2940, system, but...a Closed Center system with a fixed displacement pump, usually means there is an unload valve (dump valve diverts entire flow to tank when now hydraulics is used) between the pump and the directional Control Valve....the dump valve de-activates by a pressure signal from the CV, when CV is activated....there might be something wrong with this "pressure signal system", that makes pump load-unload.....would be great to see a symbol schematic over the entire hydraulic system....do you have a manual and a scanner or a digital camera???

 

[mmurphy]

YEP!!!!! Akkaamaan. I have been working on my car, thinking of closed loop after certain conditions are met.
After 47 years you would think I could at least speak much better english, or know what I am trying to say!
The other suggestions you made, may give him more to look in to.

 

[mmurphy]

John Deere Hydraulic Pump, R94658
Here is a reference of pump that is or should be on it.

 

[Aka jack]

I have a 2940 John Deere and steering is stiff and if you let go of steering wheel the wheels keep moving. Also the front end loader eases up by itself and is jerky when you hit the leaver. All help would be greatly appreciated.

 

[Tx Jim]

The hyd pump is always pumping, and there should be very little pressure until you activate a valve.

Sorry J_J but I must disagree.
Closed center main hyd pumps are always turning when engine is running BUT not always pumping oil. When the hyd pressure builds up to the "set standby pressure" the stroke control valve causes pressure oil to enter the pump crank case which forces the 8 pistons(on larger pumps) away from rotating pump shaft which makes pumping cease. When pressure in crank case drops below stand-by pressure springs force pistons back against rotating shaft and pumping begins again.

 

[Tx Jim]

I have a 2940 John Deere and steering is stiff and if you let go of steering wheel the wheels keep moving. Also the front end loader eases up by itself and is jerky when you hit the leaver. All help would be greatly appreciated.

Loader control valve and steering valve both have internal leaks which will probably require replacement of loadervalve & repair of steering valve to correct the problems. Does your tractor have one or two steering cylinders??

 

[Tx Jim]

CaptainQ
Your tractor more than likely has an internal hyd leak. Only way to determine where is by diagnostics with pressure gauges, laser thermometer and flow testing. Does your tractor have hyd hi-lo?

 

[J_J]

Sorry J_J but I must disagree.
Closed center main hyd pumps are always turning when engine is running BUT not always pumping oil. When the hyd pressure builds up to the "set standby pressure" the stroke control valve causes pressure oil to enter the pump crank case which forces the 8 pistons(on larger pumps) away from rotating pump shaft which makes pumping cease. When pressure in crank case drops below stand-by pressure springs force pistons back against rotating shaft and pumping begins again.

If you have pressure at the IN port, you must have fluid flow, no matter how little.

If the CC pump is totally destroked, you have no flow, or it is attached to an accumulator, which might supply pressure.

Excerpts:

Constant pressure systems (CP-system), standard. Pump pressure always equals the pressure setting for the pump regulator. This setting must cover the maximum required load pressure. Pump delivers flow according to required sum of flow to the consumers. The CP-system generates large power losses if the machine works with large variations in load pressure and the average system pressure is much lower than the pressure setting for the pump regulator. CP is simple in design. Works like a pneumatic system. New hydraulic functions can easily be added and the system is quick in response.


Constant pressure systems (CP-system), unloaded. Same basic configuration as 'standard' CP-system but the pump is unloaded to a low stand-by pressure when all valves are in neutral position. Not so fast response as standard CP but pump lifetime is prolonged.


Load-sensing systems (LS-system) generates less power losses as the pump can reduce both flow and pressure to match the load requirements, but requires more tuning than the CP-system with respect to system stability. The LS-system also requires additional logical valves and compensator valves in the directional valves, thus it is technically more complex and more expensive than the CP-system. The LS-system system generates a constant power loss related to the regulating pressure drop for the pump regulator:

 

[Tx Jim]

J_J
I personally hate load sensing systems. I had a Ford 5610 with that and I'll choose plain oil open-center hyd's for my tractors over load sensing. On JD CC hyd systems when pump stops pumping as I previously outlined the pressure is captured in the lines/control valves.

 

[bulldogshotgun]

JD Construction equipment have Load Sense systems that use a pump that has a cylinder on the back side of the swash plate, they maintain 300-600 psi standby system pressure over whatever the Load Sense port calls for. as LS climbs the cylinder increases the angle of the swash plate , thereby increasing stroke. they maintain standby system pressure but 0 flow with all functions in neutral.

just because you have pressure doesn't mean you have flow. remember pascals laws of fluid dynamics apply here. pressure and velocity are seperate but not independent. one will always effect the other but just because you have one doesn't mean you have the other.