Posted by K.B.-826 on February 27, 2008 at 11:42:05 from (24.180.130.39):
In Reply to: T/A question posted by nates90 on February 27, 2008 at 06:59:52:
Sounds like you're asking about the hydraulic TA used in the bigger (706-3688) tractors. It is totally different from the mechanical TA (Super MTA-686). In the mechanical TA, you have a clutch, a planetary gearset, and a ramp/roller. In direct drive, the clutch locks the whole unit together, and power flows straight through. When in TA, the clutch unlocks, the input shaft drives the planetary, which is where the gear reduction occurs. The ramp and roller now hold the outside of the planetary stationary, preventing it from spinning around the input shaft. The ramp and roller can only work one way though, which is why tractors with this unit freewheel downhill when in TA. With the hydraulic TA, there is no planetary. You just have two sets of gears, one for direct and one for low, like in an ordinary transmission. The gears are in constant mesh. In direct drive, hydraulic pressure causes a clutch pack to lock the drive gear to the input shaft from the main clutch. The drive gear drives the driven gear attached to the transmission shaft, and power flows on to the speed transmission (gears 1,2,3,and 4). When the lever is pulled back, the hyd. pressure dumps out of the direct drive clutch pack. The gearset for TA range works in the same way, exept there is no hyd. clutch pack locking the drive gear to the clutch shaft, this is done with a sprauge, which is a mechancal one-way (overrunning) clutch similar in operation to the ramp and roller found in the mechanical TA. The sprauge will only lock up in one direction. This too would allow for freewheeling, so a hydraulic brake pack is used to help hold the TA drive gear to the clutch shaft when going downhill. The sprauge on anything older than the 86 series was really too small, and the brake pack on any model does not stand up to abuse, such as using the TA to downshift for engine braking, as in flying in off of the road with a heavy load behind you and pulling back the TA to slow down. These two parts are the reason for most TA failures. If you have a hydraulic TA that slips in TA, the sprauge is shot, and if you have one that freewheels downhill in TA, the brake pack is shot. Be sure to double-check your linkage from the TA lever to the MCV for correct adjustment, but I'd be surprised if your brake pack isn't the problem
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