High-pressure brake - high-pressure braking unit


The high-pressure braking unit is fitted to tractors equipped with a GTA1540 or GTA2520 transmission:
- as standard on 50 kph tractors
- as an option on 40 kph tractors


The high-pressure braking unit (1) mainly comprises:
Hydraulic part
- a 25 μ filter (3)
- a pressure relief valve set to 102 bar (4)
- an accumulator (5)
- a non-return valve (7)
- a drain screw (8)
• Electronic part
- a pressure sensor (9)
- a solenoid valve (10)


The high-pressure braking unit is connected to the block/valve assembly (13) which is fixed onto the front bulkhead of the cab and forward of the brake pedals. The high-pressure braking system may be fitted with a trailer brake mechanism. The high-pressure braking unit and block/valve assembly (13) are managed by the tractor's electronic system.

Operation


The tractor's electronic system controls the high-pressure braking system via information received from the pressure sensor (9).

Normal operation 

The hydraulic operating principle of the high-pressure braking unit (1) is to maintain a permanent pressure of between 80 and 95 bar inside the system and the block/valve assembly (13). To do this, the high-pressure braking unit is must comply with two separate operating phases: the connection and disconnection phases.

Connection phase 


If the hydraulic pressure inside the high-pressure braking system (1) drops to between 70 and 80 bar, the tractor's electronic system shuts off the electrical supply to the solenoid valve (10) via the pressure sensor (9). The solenoid valve spool then moves down to the opening position under the action
of the spring (6). The oil under pressure flowing from the variable displacement pump through the line (P) via the 25 μ filter (3) forces the non-return valve (7) to open, by compressing the spring (2). The oil under
pressure is then permitted to pass through the line (FR) to the block/valve assembly (13) and through the line (ACC) to the accumulator (5). If there is a fault in the supply from the hydraulic line coming from the variable displacement pump, the non-return valve (7) closes so that the pressure stored inside the accumulator (5) keeps the hydraulic lines (FR) and (ACC) charged and therefore provides the tractor with independent braking.

Disconnected phase 


If the hydraulic pressure inside the high-pressure braking system (1) reaches 95 bar, the tractor's electronic system activates the solenoid valve (10) (12 V) via the pressure sensor (9). The solenoid valve spool then moves upwards to the closed position, forcing the pressurised flow of oil from the variable displacement pump via the line (P) and via the 25 μ filter (3) to be reduced or interrupted. If the pressure of the hydraulic line (P) is equal to or higher than 102 bar, the pressure relief valve (4) opens gradually and the excess pressure is directed towards the return (T).


 Degraded operation

If the pressure in the high-pressure braking system and the block/valve assembly drops below 70 bar, the tractor's electronic system is informed of the fault and locks the braking system in degraded mode. In this case, the tractor's forward speed is systematically restricted. A connection time of 16 seconds is then assigned to the braking system for each action on the brake pedals. During this connection time, the accumulator (5) fills to make the hydraulic lines (FR) and (ACC) safe.

Layout of components and identification of hydraulic ports


Components 
(1) High-pressure braking unit (3) Filter (25 μ) (4) Pressure relief valve set to 102 bar (5) 1.4 l accumulator (7) Non-return valve (8) Drain screw (9) Pressure sensor (10) Solenoid valve (0/12 volts)


This drain screw is used to drop the pressure inside the accumulator (5) and the block/valve assembly system prior to carrying out any operations on the hydraulic system. The pressure sensor is used to record the high-pressure braking unit pressure in order to manage the connection and disconnection phases.

Hydraulic ports 


ACC Port to accumulator
FR Port to block/valve assembly (13) 
LS Load Sensing signal port to the hydraulic unit with two priority blocks
P High-pressure port from the variable displacement pump via the hydraulic unit with two priority blocks
T Return port

Removing and install the high-pressure braking unit

Immobilise the tractor. If the tractor is not installed with a Park Lock device, engage the hand brake. Chock between the frame and the front axle.
Remove the rear right-hand wheel. Place a safety stand under the trumpet housing.

Removal
Disconnect the following from the high-pressure braking unit:
- the harness (TR44) connected to the pressure sensor.
- the harness (L8) connected to the solenoid valve.
Disconnect the hydraulic pipes from the high-pressure braking unit. On GTA2520 transmissions, the drain screw (8) can be accessed via the face (F) of the high-pressure braking unit. This face (F) is facing towards the GBA25 gearbox housing.
Release the return hose (at the high-pressure braking unit end) without unscrewing it fully.
Remove the surrounding components which may hinder access to the high-pressure braking unit.
To prevent transmission oil draining out when disconnecting the return hose from the high-pressure braking unit, proceed as follows:
- Separate the high-pressure braking unit and its return hose from the support.
- While vertically supporting the high-pressure braking unit and its return hose in the top position, finish disconnecting the hose by unscrewing it fully from the high-pressure braking unit.
- As soon as the hose is disconnected, plug its upper end with a sealing plug. The second end of this hose, which is not to be disconnected, is connected to the bottom section of the gearbox and underneath the oil level.
Remove the high-pressure braking unit from the tractor.
If necessary, place the high-pressure braking unit on a clean workbench.
If necessary, unscrew the following from the high-pressure braking unit:
- the electrohydraulic components
- the hydraulic unions

Installing
If necessary, screw the electrohydraulic components back onto the new or re-used high-pressure braking unit.
Reinstall the high-pressure braking unit to the tractor by carrying out steps in reverse order.
Screw on the knurled section of the drain screw (8) up to its stop, in order to eliminate any atmospheric connections with the internal lines of the high-pressure braking unit.

Reinstall the wheel. Remove the axle stand. Tighten the screws and nuts to their indicated torque, depending on the version.
Start the tractor engine. With the engine at idle, wait several moments until the hydraulic and electronic functions have re-stabilized. Check that there are no system faults displayed on the instrument panel.
Move the tractor several meters at slow speed. Apply and release each brake pedal to carry out an initial operating test of the high-pressure braking unit.
Check the sealing of the high-pressure braking unit and check the sealing of the hydraulic unions.
Check the pressure of the trailer brake (if fitted) on the quick coupler located at the rear of the tractor. Carry out a road test on the brake system.