Why did my 756D quit moving

[Anonymous-0]

I was doing some light plowing with a 642, 3-16 roll over plow and twice while turning around at slow rpm while rolling the plow the tractor actually stopped moving like it went out of gear. Only for a second or two. I do swear I was in direct drive. I know how a TA works, but not the hydraulic style. Would the slow rpm combined with the plow hyds, the hyd brakes in use and power steering cause the TA to slip for lack of hyd pressure? The drive clutch is almost done but its not slipping because when I was going full load it was no problem. I know the TA is getting bad though, because I hit a hard spot and pulled it back and she stopped moving. Can the TA cause the tractor stop in direct drive too? I should plan on both clucht and TA replacement shouldn't I?

 

[Bob]

Does the tractor still have hysraulics and PTO?

 

[Redfan]

All the things you described can rob pressure from the TA and in Direct it relies on Hydro pressure to keep it engaged.
MCV pump is out and pressure checks will be needed to determine if you need a new pump.

 

[Anonymous-0]

Yes hydraulics work okay and still are. They were working when it quit moving...rolling the plow steering and braking. That's why I wondered if low pressure/flow at low rpm while turning around could cause the TA to slip in direct drive. After I got going it plowed all day without slipping. Just those two times it let go.

 

[Kevin Meier]

you should put pressure gauges on and check that after you make sure all adjustments (clutch & t/a) are correct. Loss of pressure can destroy a t/a on high and low sides. If it doesn"t move at all you probably have a clutch problem, but it also sounds like your t/a may be bad. When or if you replace the clutch you should at least check the t/a. I have the clutch, t/a or hydraullic parts in stock. Feel free to email me when you find the problem.

 

[Owen Aaland]

Your 756 has two separate hydraulic systems that share the same filter. The rear (hitch) pump provides oil to the hitch and auxilary valves. That part is probably fine since you are not having problems with the hitch or remote hydraulic cylinders

The front (MCV) pump provides oil for the steering, brakes, TA application pressure, hydraulic seat, TA lube pressure, transmission oil cooler, transmission and differential lube.

Standard flow from the front pump is 9 GPM at rated speed. From the pump the goes first to the flow divider valve. There is an orfice that allows 3 GPM to pass through, all the other oil is diverted to the transmission oil cooler in front of the radiator. Return flow from the oil cooler goes to the rear (Range) transmision and differential to lube those parts.

The 3 GPM flow goes first to operate the steering. The steering pressure is regulated to about 1900 PSI by the steering relief valve. Return flow from the steering and steering by relief valve is then sent to the pressure regulator which controls the pressure at about 245 PSI. This oil is available for the brakes, seat, and TA application. Early brake valves were open center and used 1 GPM all the time. Later brake valves were closed center and used 1 GPM only while the brakes are being used. The TA and seat are always closed center type valves.

The oil is then directed to the TA lube circut where it is regulated to 18 - 20 PSI. This should allow 3 GPM (2 GPM with early style brake valve) for TA lube. Any oil in excess of the 18 - 20 PSI lube pressure is returned to the sump.

In your tractor what is happening is that when you turn and use the brakes, there is not enough oil flow to maintain the operating pressures on the TA clutch and so the high side clutch start to slip. The low side of the TA is designed to have most of the load carried by a sprague clutch. In your case this clutch is failing which is why the TA slips on the low side. If the low side cluch were holding you would feel the TA downshift to low instead of stopping when you turn and use the brakes.

The reason it is critical to have this problem repaired is that when you are having this problem, you do not have enough oil flow in the 3 GPM circut and so the pressure drops off. Since this is a 3 GPM priority circut, when the flow drops below 3 GPM there is NO flow through the oil cooler and that is the oil that provides lube for the transmission and differential. Continued use of a tractor with MCV pump problems often lead to rear end failures.