Re: 706 mcv?

Posted by Owen Aaland on March 13, 2015 at 14:03:28 from (216.47.34.37):

In Reply to: 706 mcv? posted by Jrw013 on March 06, 2015 at 08:44:33:

Your tractor has two separate hydraulic systems other than sharing a common filter. The rear pump furnishes oil for the hitch and auxiliary valves. That system is continuing to function even when you have no steering.

The front pump is located on the inside of the MCV. That pump provides oil for the steering, cooling, brakes, TA control, and lubrication.

The flow from the MCV pump goes first to a flow divider valve and a 3 gpm orifice. The 3 gpm has priority so that flow is maintained unless the pump is not able to supply that much. The pump is rated at 9 gpm at rated engine speed. Any flow in excess of the 3 gpm goes to the oil cooler in front on the radiator and then returns to the MCV. There is a regulating valve to limit the pressure in the oil cooler to about 60 psi. This return flow is then routed to the rear end to provide lube for the reverse idler shaft, ring and pinion gears, and the range transmission main shaft.

The 3 gpm is routed to the steering control valve on top of the center section where it is available for use by the steering system. From the steering control valve the flow is routed back to the MCV. The steering pressure is regulated for maximum pressure by the steering relief valve. If the pressure exceeds the relief valve opening pressure the flow passes through the valve and is routed to the same channel as the oil returning from the steering control valve. There is also a steering check valve located in this channel that is normally closed. Its only function is to allow oil to be drawn into the steering system if the pump is not supplying the necessary oil flow. This allows for manual steering.

The returning oil from the steering next goes to the brakes where there is a 1 gpm orifice in the inlet fitting of the brake valve. On the 06 series tractors this is a 1 gpm flow through the brake valve whether or not the brakes are being used. On later tractors the brake valve was changed to a closed center valve so that the 1 gpm flow is only lost from the system while then brakes are being applied. After the brake system is supplied with oil the remaining 2 gpm is used for the TA control and TA lubrication. Once the oil has reentered the MCV from the steering control valve it is regulated to about 245 - 265 psi by the pressure regulating valve. The flow bypassing the pressure regulator valve is then sent to the TA lube circuit where it is regulated to about 20 - 22 psi by the lube circuit pressure regulator valve. Oil that bypasses the TA lube regulator valve is dumped to the reservoir. When the clutch pedal is depressed all of the TA pressure is dumped to the reservoir to release the TA clutches and stop the TA lube flow to eliminate drag caused by the lube circuit.

To narrow down the possibilities of what the problem may be, you said your steering was still poor. When driving the tractor with the TA lever in the direct (forward) position turn the steering full left or right so that the relief valve open. You should be able to hear the valve squeal when it opens. If the TA does not downshift the problem is most likely the steering relief valve, usually a blown O-ring. It could also be a problem with the steering hand pump or the cylinder in the front bolster but that would be rare. compared to the other areas. If the TA does downshift you are losing flow to the brakes and TA. If the problem is sever enough you may not get the relief valve to open. This could be a worn out pump, blown O-rings, or a sticking flow divider valve. I normally just remove the MCV and visually inspect everything. If the valves and O-rings are all OK then I replace the pump.

You could also have problems with leakage inside the TA but that is something you will need to uses pressure gauges to determine. Since your problem shows up in the steering the problem is in the oil flow before the TA.

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