Posted by Owen Aaland on July 09, 2011 at 22:13:27 from (216.47.33.237):
In Reply to: MCV Valve posted by farmall706 on July 09, 2011 at 16:19:17:
You stated that your steering is fine. If it works the same at low idle as it does at high idle that would indicate the pump, 3 GPM orifice, and the flow divider valve are working properly.
The steering has a priority flow of 3 GPM from the MCV pump. The return flow from the steering is used for the brakes, TA control, and TA lube. This oil is regulated to the 240 -270 PSI that Pete mentioned. The pressure can be checked at the bottom plug of the MCV. Since the flow is a constant 3 GPM, not dependent on engine speed, if leakage exceeds that amount of flow you will never achieve that pressure. Flow past the pressure regulating valve is what goes to lubricate the TA. If the pressure is always less than the regulator setting there will not be any flow to the TA lube circuit.
On the 806 the brake control valve has an orifice screwed in the valve under the inlet fitting. This limits the oil flow to the brakes to 1 GPM. Since the 806 uses an open center valve this is a constant flow whether or not the brakes are being used. (Later tractors used a closed center valve that allow flow only when applying the brakes.) If this orifice comes out of place the entire 3 GPM flow can go through the brake valve so that the pressure never builds to the regulator valve setting. That will keep any oil from flowing to the TA lube circuit. When you apply the brakes the pressure in the inlet side will rise until it bypasses in the brake valve. I do not know if you can apply the brakes hard enough to get the pressure high enough for the pressure regulator to start to control the pressure. The increase in pressure would be enough to begin to engage the TA clutch packs. The low side clutch should always engage even without oil pressure but once the one way clutch gets worn that is not always the case. Activating the hydraulic clutch pack may be enough to get the mechanical one to engage. That would be one scenario that would fit your symptoms.
Another place there may be a leak is the steering check valve located in the clutch housing at the bottom of the MCV. Have you have the MCV off? The check valve is plastic and is held in place with a spring, about the size of the spring in a ball point pen. The spring and valve are easy to knock out of place if you are not aware of them and it is easy to put things together without them. I have also seen the center break out of the valve and allow oil to leak through. Being an 806 any leakage is more critical than the later tractors since you only have 2 GPM of oil left for TA control rather than the 3 GPM the later tractors have.
I would recommend getting a pressure gauge connected to the steering relief valve port at the bottom of the MCV to see just what and when there is pressure in the TA circuit. It needs to be a minimum of 300 PSI gauge. The oil line coming out of the top of the MCV between the control spool and the left battery is the feed line to the brakes. You can safely cap that line to see if the brakes are what is causing the loss of oil flow. If that makes no difference it is time to remove the MCV to check the steering check valve and spring. I would also take the opportunity to go through all the MCV valve to make sure they are correctly installed and not scored.
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