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Farmall 706 Hydraulic mystery

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Curt Cooke

04-23-2008 07:48:12

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My 706 has a torque but it's out on the low side. My oil light comes on with the clutch depressed and sometimes stays on even after I let the clutch out. I seem to be able to get it to go out by pulling the torque lever back and then pushing it back ahead. Sometimes it takes several times to get it to go out. What's going on? What harm might I do if I don't get this corrected right away?

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karl f

04-23-2008 22:09:54

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Re: Farmall 706 Hydraulic mystery in reply to Curt Cooke, 04-23-2008 07:48:12

isn't the light supposed to come on when the clutch is depressed even when the system works properly?

sounds like you have some issues going on from the other info you gave though, just don't want someone else to be alarmed if they read this and think they need a new pump when they don't.

karl f

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Owen Aaland

04-24-2008 02:22:39

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Re: Farmall 706 Hydraulic mystery in reply to karl f, 04-23-2008 22:09:54

Yes, the light is supposed to come on when the clutch is depressed, but the test for a weak pump is not so much the light coming on as a CHANGE in steering effort between low idle and high idle. A lot of things can affect steering effort so comparing one tractor to another really does not tell you much. A change in steering effort between high and low indicates a loss of flow and/or pressure. This is usually a pump getting weak but can also be a leak between the MCV pump and the steering circuit or a sticking flow divider valve. Having the three GPM orifice come out can also cause steering problems but it usually is poor steering at higher RPMs caused by too much oil flow to the steering. Losing the orifice will also cause no flow to the oil cooler.

The important thing to keep in mind is that the steering gets the FIRST three gallons of flow from the pump. If there is not enough left after that you will not have enough flow through the oil cooler and then to lube the rear end.

706s do not have as many rear end failures as the newer and larger tractors, but the root cause of the failure is still the same, lack of lubrication. Running the oil level higher can help by having the ring gear run in oil so that it helps lubricate it but the main range transmission shaft, the reverse idler shaft, and the differential are all pressure lubricated with the excess flow from the MCV pump. If the MCV and flow divider valve is working properly, this flow stops when the steering gets all the oil as indicated by increased steering effort.

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the tractor vet

04-23-2008 07:58:15

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Re: Farmall 706 Hydraulic mystery in reply to Curt Cooke, 04-23-2008 07:48:12

Well there could be alot of things going on here . With the low side out the sprag in the T/A is toast and there could be a bent lube baffel or the dump valve is sticking or the MCV pump is on her way out and the list goes on . SOOooooo to cure this it is time to replace the T/A and rebuild the MCV and install a new MCV pump and while your at it now is the time to do the engine clutch and PTO input shaft with a new bearing and all the seals along with grinding the flywheel . And if this is all done correctly it will last many years .

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Curt Cooke

04-23-2008 13:08:31

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Re: Farmall 706 Hydraulic mystery in reply to the tractor vet, 04-23-2008 07:58:15

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?

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Owen Aaland

04-23-2008 14:20:09

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Re: Farmall 706 Hydraulic mystery in reply to Curt Cooke, 04-23-2008 13:08:31

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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Curt Cooke

04-24-2008 10:30:20

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Re: Farmall 706 Hydraulic mystery in reply to Owen Aaland, 04-23-2008 14:20:09

Owen, I split the tractor in the winter, I had to rebuild the engine and I put a new clutch in it while I was there. I had never as much as heard the tractor run at that point so I didn't know the torque was shot (though I suspected it was)so I didn't do anything with that. But I did fail to mark my oil cooler hoses and to this moment, I can't be sure that they're hooked up right. How do you know which hose goes to the top of the cooler and which to the bottom?

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Curt Cooke

04-24-2008 05:17:12

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Re: Farmall 706 Hydraulic mystery in reply to Owen Aaland, 04-23-2008 14:20:09

Thanks Owen!!!!! !!!!!

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Lee in Iowa

04-23-2008 22:04:39

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Re: Farmall 706 Hydraulic mystery in reply to Owen Aaland, 04-23-2008 14:20:09

This post makes me wonder about my 856. The steering in it has always been a little bit weak. Over the years its had a new MCV pump, a different upper bolster and the TA eliminated, but its still takes a little more effort to steer than my other 856. Any ideas. Thanks, Lee

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04-23-2008 19:13:49

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Re: Farmall 706 Hydraulic mystery in reply to Owen Aaland, 04-23-2008 14:20:09

Thanks Owen! I learned something here.

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