Description and operation of the variable displacement hydraulic pump

Variable displacement hydraulic pump description

The variable displacement hydraulic pump (41) Installed inside the right-hand cover plate is driven by:
- the gears (1) and (39) (GPA30 rear axle);
- the PTO clutch housing teeth (1) (GPA20 and GPA40 rear axles). The helical teeth of the gear (39) (GPA20 and GPA40) and its connected gear is replaced by spur cut teeth. It comprises a body (1) and a cylinder (3) driven by the shaft (13).

The cylinder contains nine axial pistons (4) thrust against ball joints (5) in contact with plate (6) via the pads (7). The angle of the plate is obtained by the equilibrium between the spring (8), the pushrod (9) and the piston (14) controlled by a hydraulic regulator receiving the Load Sending (LS) pilot flow pressure from the high pressure (HP) components via the single priority block or the hydraulic unit with two priority blocks.

Depending on the angle of the plate, the length of travel of the axial pistons (4) is modified, thus permitting the flow rate to adapt to the operating conditions. The distribution plate (11) at the rear of the cylinder, places the piston chambers in communication with the suction ports and the pump delivery. The ball joints (5) are drilled to provide lubrication between the pads (7) and plate (6). The oil is then sent to the housing via port O. The pressure relief valve is screwed into the upper left of the right-hand hydraulic cover plate. The hydraulic regulator is fitted with a diagnostics connector (Load Sending signal). This is linked to the variable displacement pump (41) via the ports drilled in the right-hand hydraulic cover plate. The diagnostics connector is used to control the high pressure at the variable displacement hydraulic pump outlet.


Standby position
When the engine stops, the spring (8) automatically places the pump in its maximum flow rate position.
When the engine is started, all ports are closed, and in theory no flow is required. The pressure increases in line P. When it reaches 22 bar, it pushes the hydraulic regulator spool "a" and connects with port R. The pressure acts on the piston (14) and the pushrod (9) of the pump, which moves the plate (6) into zero flow rate position. In practice, a weak 3 to 5 bar flow rate remains in the steering system, increasing the standby pressure to 28 bar.

Working position
When a function is activated (e.g.: an auxiliary spool valve), the 22 bar standby pressure drops as the system is opened. The 22 bar spring restores the hydraulic regulator spool "a" to its initial position. The piston (14) is linked to the return via S and R. The spring (8) returns the pump plate (6) to full flow rate.
The flow rate from the pump creates the LS (load sending) pressure required to activate the function.
The LS function then created balances the piston between, on the one hand, the LS pressure plus the 22 bar pressure of the spring and, on the other hand, the pump output pressure P. As long as the spool "a" has not been balanced and is not maintained by the pump supply, the piston (14) remains connected to the return and the pump plate (6) at full flow rate (pump pressure P = LS pressure + 22 bar). The closed centre system varies the pressure and flow rate in the hydraulic system. Pump delivery pressure = XLS use pressure + standby pressure (22 bar). Sufficient flow rate, determined by the inner section of the spool valve, is supplied by the pump to maintain the pressure difference at the spool valve terminals (22 bar).

Parts list
G From charge pump
P High pressure (HP) outlet to single priority block or hydraulic unit with two priority blocks
R To control piston (14)

Parts list
(1) Linkage spool valve
(2) Auxiliary spool valves
(7) or [(7) and (10)]
Single priority block or hydraulic unit with two priority blocks with or without trailer braking
(12) Variable displacement pump hydraulic regulator
(14) Piston

Maximum position - zero flow rate
When the pressure at port P reaches 200 bar, spool "b" is pushed forwards compressing its spring. The ports P and R are placed in communication and piston (14) is brought to the zero flow rate position. The hydraulic regulator (12), Installed on the hydraulic cover plate (8) regulates the variable displacement pump pressure and flow rate according to information transmitted by the LS signal. Screws C and D are factory set. Do not modify these settings.

C 22 bar adjustment screw
D 200 bar adjustment screw
O Decompression port
P Variable displacement pump pressure
R To piston (14)
S Return
XLS Signal

Pressure relief-valve (2)

The high pressure flow delivered by the variable displacement pump passes through channel P in the hydraulic cover plate and is directed towards the single priority block or to the hydraulic unit with two priority blocks (depending on version). The pressure relief valve (2) set to 230 bar ± 5 and screwed into the hydraulic cover plate, ensures the safety of the high pressure (HP) system. In case of any abnormal increase in pressure, valve (1) raises and the oil is directed to the housing via ports R in the relief valve.

Removing and install the variable displacement hydraulic pump

Remove the right-hand hydraulic cover plate.
Immobilise the driven gear (39) of the variable displacement pump (41) by inserting a bronze drift between this gear and the hydraulic cover plate. Release and slightly loosen the nut (37).
Unscrew the screws that secure the variable displacement pump to the hydraulic cover plate. Separate the pump from the cover plate.

Place the variable displacement pump (41) on a workbench. Completely unscrew the nut (37). Recover the washer (38).

Using a suitable extractor, release and remove the driven gear (39) from the tapered section of the shaft. Recover the key (40).
If necessary, remove the safety valve (45) from the low pressure system.

Clean the components. Replace those that are defective. Check that none of the hydraulic cover plate channels are obstructed. Check the presence of dowels (4). Replace the "O" rings (1) to (3).

Repeat the pump removal operations in reverse order. Inside the cover plate, apply a small bead of Loctite 574 or equivalent around the holes of screws (47) to seal these holes. Apply the same method as used with the left-hand cover plate. Lightly smear the thread of the engine speed sensor (25) (if fitted) with Loctite 5922 or equivalent. Adjust the sensor.

Adjusting the engine speed sensor

Depending on the type of transmission, the LS right-hand hydraulic cover plate (8) may or may not be fitted with an engine speed sensor. If a sensor is fitted, adjust it by carrying out the following steps.
Manually tighten the sensor (25) home, without forcing it, until it comes into contact with the teeth of the driven gear (39) of the variable displacement pump. Loosen the sensor by a ½ to ¾ turn to obtain approximately 1 mm clearance between the sensor and the driven gear. Tighten the nut to 5-7 Nm without turning the sensor.