Curt Cooke said: (quoted from post at 13:08:31 04/23/08) I'm sure you're correct Vet. My question is, "what is the light telling me?" That is, what exactly is failing to receive the oil pressure it requires? Is it a wet clutch in the torque that's not receiving the recommended oil pressure to actuate? If I take this to plow day and see that light come on, do I need to stop immediately or will it stop, even in the high side, on its own because of the lack of hydraulic oil pressure?
The greatest danger in continuing to use the tractor in this condition is the possibility of damaging the rear transmission and differential. A good test for the front pump is to check the steering operation after the system has been brought to operating temperature. If the steering does not work the same at low idle as it does at high idle your pump is not providing enough oil flow. This is the condition that puts the rear end at risk with continued use.
The oil light you see is at the end of the hydraulic system powered by the front pump. You will see this referred to as the MCV pump since it is attached to the inside of the Multiple Control Valve assembly.
A 706 has a 9 GPM MCV pump. It draws the oil from the same hydraulic filter as the hitch pump. From the pump the oil goes to a flow divider valve that gives a priority flow of three GPM to the steering and other controls. The excess oil is routed to the oil cooler in front of the radiator and then returns to the MCV where it is routed to provide the lube circuit for the rear (range) transmission and the differential. The danger for the the lube circuit is that the three gallons for the steering is a priority flow so that if you have major leakage or a weak pump you may still get enough flow to operate the steering OK but will be starving the rear lube circuit.
The three GPM priority flow goes first to the steering system. the return from the steering them goes to operate the brakes and TA assembly. If steering pressures exceed the steer relief pressure, the relief valve opens and and oil is dumped to the return side of the steering system so it is still available for brakes and TA use. The pressure in this circuit is regulated to about 240 lbs but may be higher if the springs have been changed in the MCV. The 706 has an open center brake valve that takes one GPM for the brakes leaving two GPM for TA operation and lube. Later tractors used a closed center valve so that the one GPM flow was used only when applying the brakes leaving three GPM for the TA and lube.
The TA control circuit is a closed center system so oil is uses only to apply the clutch packs. The dump valve open this circuit to reservoir when the clutch is depressed. This releases the TA clutches and dumps the lube so there is less drag on the gears to make them easier to shift. There is some leakage in the control system so in actuality there will be less than two GPM available for the TA and speed transmission lube. From the TA circuit the oil is directed to the TA lube circuit where the pressure is regulated to about 18 to 22 PSI. This is the circuit where the oil pressure sender is located. A weak pump or excessive leakage can result in this circuit not having enough flow to maintain the pressure to keep the light off.
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