Powershift Gearbox - Transmission hydraulic circuit

Hydraulic characteristics at 2200 rpm at 60° C (140° F)

1. Clutch pressure regulating-valve .........      17.2 - 19 bar (250 - 270 psi)
2. Lubricating pressure regulating-valve .......   6.2 - 9.3 bar (90 -140 psi)
3. Filter by-pass valve – Differential pressure .. 3.1 - 3.7 bar (45 - 54 psi)
4. Oil cooler thermostatic valve opens at .............. 71° C (160° F)
5. Oil cooler thermostatic valve fully open at ........... 85° C (185° F)
6. Gerotor pump flow rate .............................. 94.5 l/min (25 gpm min)
7. Lubricating oil flow rate thru oil cooler at 85° C (185° F) . 75.7 l/min (20 gpm min)
8. Lubricating oil flow rate by-passing oil cooler at 60° C (140° F) .......... 75.7 l/min (20 gpm min)
9. Gearbox oil filter .................................................15 microns

Clutch pressure circuit
The Powershift AG150 and AG250 tractor transmissions are fitted with their own Gerotor type toothed rotor hydraulic pump mounted at the rear of the transmission. The pump is driven at engine speed by the first stage shaft. When the engine is running the pump supplies the pressure and flow rate to activate the clutches and lubricate the transmission. The oil is sucked through a strainer located in the intermediate housing. The pump discharges oil through the gearbox filter and by priority supplies the clutch solenoid valves and the pressure regulating-valve.

Once the necessary pressure and flow rate for the clutch supply is supplied, the pressure regulating-valve opens. It maintains the operating pressure in the clutches. Three solenoids are then simultaneously activated to engage the three clutches and transmit the power from the engine to the transmission. To obtain the various speeds, it is possible to activate a combination of any three of the nine solenoids. Once the required volume of oil to engage the clutches is attained and the pressure regulating-valve is open, the oil flows through the lubricating circuit via the pressure regulating-valve and the thermostatic valve.

Proportional solenoid valves
The oil reaches each extremity of the proportional solenoid valves and the volume of oil that supplies each clutch via a solenoid valve is controlled at clutch level by an electrical signal emitted by the transmission calculator (TC), this modulates the gear changes. Each solenoid valve is linked to the oil return. When the TC cuts the supply to the solenoid valve, the oil contained in the clutch returns to the tank.

Lubricating circuit
After having supplied the clutches, the oil supplied by the pump passes through the oil cooler, or bypasses it via the thermostatic valve, and is used to lubricate the clutches. The cooled lubricating oil is thus supplied to the gearbox to lubricate the shafts, clutches and bearings. All the oil supplied to the bearings is metered by a calibrated port located in the bore of the bearings and by three channels drilled in the clutch shafts. When the pump provides sufficient oil for the lubricating circuit, the pressure rises until the relief-valve opens and the excess returns to the tank.

Parts list (Fig. 1)
(1) Tank (2) Suction strainer (250 μ) (3) Suction pipe (4) Gerotor - toothed rotor pumpv(5) Pump discharge pipe (6) Main oil filter for the gearbox (7) Gearbox oil filter by-pass (8) Oil cooler (9) Oil cooler by-pass (10) Lubricating relief-valve (11) Clutch pressure regulating-valve (17 bar) (12) Proportional solenoid valve (13) Lubricating pressure pipe (14) Lubricating port
(15) "A" clutch supply pipe (16) "2" clutch supply pipe (17) Clutch return port
(18) Lubricating oil return port from relief-valve

A : Clutch "A"
B : Clutch "B"
C : Clutch "C"
H : Clutch "H"
L : Clutch "L"
M : Clutch "M"
R : Clutch "R"
1 : Clutch "1"
2 : Clutch "2"

Oil filter 

(6) Filter element (7) By-pass (19) Filter head (20) Filter bowl (21) Oil inlet port (22) Oil outlet port (23) O’ring (24) By-pass seat (25) O’ring (26) By-pass outlet to lubrication (27) O’ring (28) By-pass spring (29) O’ring

Oil filter – Oil flow

Coming from the Gerotor pump, the oil enters the head of the filter unit. It flows from the outside of the filter element to the inside. Filtered oil from the centre of the filter element then flows out through a separate channel in the head of the filter unit to supply the transmission. If the filter is blocked, a by-pass valve opens and channels non-filtered oil to the lubricating circuit. The by-pass valve operates on the pressure differential existing between the filtered oil and non-filtered oil. The differential pressure is controlled by the by-pass valve spring.
Oil filtered under pressure flows into the chamber containing the by-pass valve spring and enables the spring to keep the valve closed. If the filter becomes blocked, the pressure on the outside of the filter element increases while it drops in the centre of the filter element. The pressure of the oil on the outside of the filter element then exceeds that of the by-pass valve spring and the valve therefore opens. The by-pass valve opens when the pressure differential equals 3.1 - 3.7 bar (45 - 54 psi).

Thermostatic valve 

(9) Thermostatic valve (30) "IN" oil inlet port (31) "OUT" oil outlet to oil cooler (32) "OUT" oil outlet to tank (33) Housing towards oil cooler (34) Housing towards tank (35) O'ring (36) Support bolts. Tighten to a torque of 6.7 Nm (37) Spring (38) Guide and spring seal (39) Thermic element (40) Guide washer (41) Screw adjustment. 

Smear the screw with Loctite 290. Tighten until contact, then add an additional quarter turn.

Thermostatic valve – Oil flow

When the transmission oil temperature is less than 71°C, the thermostatic valve closes so that oil cannot flow through the oil cooler. When the oil temperature reaches 85°C, the thermostatic valve is fully open and oil flows to the oil cooler.

Pressure regulating-valve 

(11) Gearbox pressure regulating-valve (42) Return port to tank (43) Return oil pressure - Outlet to lubrication (44) Pump inlet pressure (45) Plug (46) O'ring (47) Shims (48) Outer spring (49) Inner spring (50) O'ring (51) O'ring (52) Washer (53) Regulating spool (54) O'ring (55) Plug

Proportional solenoid valve 

 (12) Proportional solenoid valve (cannot be repaired) (56) Aluminium nut - 1.7 to 2.8 Nm (57) Washer (58) Solenoid (59) O'ring (60) O'ring (61) O'ring (62) Clutch return oil port (63) Clutch outlet oil pressure (64) Pump inlet pressure

Pump and pressure circuit

The AG150 and AG250 tractor transmissions have a fully integrated hydraulic circuit. The pump (4) is located at the rear of the transmission. It is driven by the first stage shaft that runs at engine speed. The Gerotor pump has two inlet ports in order to reduce vibration within the hydraulic circuit. It can be removed by uncoupling the tractor between the gearbox and the intermediate housing. The inlet and outlet ports of the pump are fixed to the rear housing of the gearbox and oil tightness is provided by O'rings. The suction and discharge channels are moulded into the rear housing of the gearbox. The right-hand channels are suction and the left-hand channels are discharge.

During repair operations, check pump wear between the external bush and the housing, axial clearance and the splines. Replace the pump if it is damaged. The pressure regulating-valve (11) controls the pressure in the circuit used to activate the clutches. The condition of the regulating-valve can be determined by carrying out a pressure test. The pressure must be uniform across the whole engine speed range, though it may vary slightly. The pressure must be between 17.2 and 19 bars.

The pressure test can be carried out by removing the wide plug (28). Use the ATP 3018-1 service tool in a wide plug on the forward housing. The pressure regulating-valve has the form of a repairable cartridge and it must be checked for any deposits, scratches or friction points. Replace the O"rings on the cartridge each time that it is removed from the tractor transmission.

The Gerotor pump is bolted to the rear housing of the gearbox. It can be removed by taking out the six retaining bolts after uncoupling the tractor between the gearbox and the centre housing. Remove the two bolts that hold together the pump body and cover and separate the halves.

Measurement of wear on pump components
· Maximum acceptable wear between the external rim of the bush and the housing: 0.51 mm.
· Between the tip of the internal star and the external bush: 0.18 mm.
Lateral clearance of two components in relation to the housing: 0.1 mm. Examine the surface condition ace of the two components. Replace them where necessary. Maintain the relative positions of components in relation to each other, in order to avoid producing new wear areas due to accidental offset.
Reassemble the pump and fasten the cover on to the body with the two bolts. Tighten to a torque of 24
Nm. Lubricate the pump with transmission oil and fit the two O'rings smeared with miscible grease into the pump body. Install the pump on to the rear housing of the gearbox. Fix it in place with the six bolts removed previously. Tighten the bolts to a torque of 24 Nm.

Clutch circuit

The oil flow in the clutch piston chamber is controlled by the proportional solenoid valves (12) and they themselves are controlled by the transmission control unit (TC). The oil is metered in the clutch pistons by the proportional solenoid valves, in relation to the electrical signal received by transmission control. The hydraulic spools of the proportional solenoid valves cannot be repaired, except for the O'rings and the solenoid. The solenoid is fixed to the spool by an aluminium nut (56) and mustd be tightened to a torque of 1.7 - 2.8 Nm maximum.

Each clutch is fitted with a control channel located near the solenoid valve concerned. Each control channel is identified by a number or letter moulded into the housing and corresponding to the clutch. All the channels are closed by a small 7/16” plug.
Use ATP 3018-2 adaptors to check the pressure. To correctly carry out a check on a gear ratio, check the pressure in all three clutches concerned at the same time.

Lubricating circuit

The gearbox lubricating oil is supplied by the Gerotor pump via the pressure regulating-valve. Lubricating oil pressure is maintained by the pump flow and by the lubricating relief-valve. The lubricating relief-valve (10) is located at the bottom of the threaded port provided for the oil cooler connector on the left hand side of the transmission. The lubricating oil supplies each transmission shaft, lubricates the clutch plates, shafts and bearings. 

To test the lubricating pressure, use test connector ATP 3018-3. Screw the test connector into one of the medium-sized plugs (27). The lubricating relief-valve (10) may be replaced either by screw tapping the interior of the valve and removing it with the aid of an inertia extractor, or when the gearbox is disassembled by driving it out from the interior using a suitable fixture. A drilled aperture in the shaft provides lubrication for the forward bearing while a port at the end of the shaft provides lubrication for the rear bearing.