Clutch control valve of heavy duty transmission circuit

On tractors fitted with a Heavy Duty gearbox, with a mechanical reverse shuttle, the distribution block (1) mounted on the front right-hand side of the gearbox is formed of two distinct parts that ensure the control of the clutch and the four Dynashift ratios.
- The clutch part comprises a series of spools, seals, a valve and piston. These can be replaced by a kit that is found in the Spare Parts catalogue.
- The Dynashift part.

Role of the control valve
Fitted in the 17 bar circuit, this valve provides the following functions:
- 17 bar supply to the clutch when the clutch pedal is released (clutch engaged position).
- modulation of the clutch engagement pressure: 0 to 11 bar.
- starting safety when the tractor is started using external assistance. To activate the clutch it is necessary to press the clutch pedal once.
- low pressure starting safety. The clutch disengages automatically if the pressure drops below 3 bar.
- the ports O allow leaks to return to the housing.

(1) Distribution block
(2) 17 bar supply
(3) Accumulator on 17 bar line
(4) Master cylinder hose pipe
(5) Clutch supply pipe
(6) Clutch diagnostics connector
(7) Dynashift front piston diagnostics connector
(8) Dynashift rear piston diagnostics connector
(9) Accumulator for brake assistance

- The valve comprises two spools. The upper spool is controlled by pressure from the clutch master cylinder and controls the clutch supply.
- The lower spool, controlled by the inlet pressure, provides hydraulic safety for the system.

Operating phases of the clutch Valve

Neutral position, engine running 
P = 17 bar
R = Return
E = Clutch
- If the tractor is started without pressing the clutch pedal, the pressure rises to 17 bar but all the suction ports are closed and the clutch is not supplied.

Disengagement before switching the safety valve 
P = 17 bar
R = Return
E = Clutch
- The main spool (5) is pushed to the rear and compresses spring (2) under the effect of piston (19) that is controlled by the master cylinder. The pressure coming from the 17 bar circuit passes via the shoulder of the main spool (5) and begins to act on the front face of spool (4), the clutch is then linked to the return.

Clutch disengagement, switching the safetyvalve 

- The spool (4) moves to the rear when the pressure is greater than 12 bar. During full opening, spool (4) allows communication with the front of the main spool (5).

Clutch engagement, modulation phase 

- When the pedal is released, the pressure supplying the clutch is modulated by the position of the main spool (5). This position is regulated:
- at the rear by the 17 bar pressure and spring (2)
- at the front by the clutch pressure and spring (16)
- When the pedal is slightly released, the spring tension (2) diminishes. The main spool (5) moves forward and places the 17 bar line in communication with the clutch.
- The pressure in the clutch increases and this pushes the spool to the rear to find its equilibrium position.

Clutch fully engaged 

- When the pedal is completely released, spring (2) pushes spool (5) forwards and causes the full opening of the 17 bar circuit to the clutch.
- From this intermediate position, when the pedal is slightly pressed, the force acting on spring (2) is increased. The spool moves rearwards opening the communication between the clutch and the return.
- The pressure in the clutch diminishes and the spool moves forward to find its equilibrium position.
- The modulation begins after having pressed the pedal approximately 32 mm. The declutched position is reached 25 mm before the farthest pedal position is reached.

Parts list 
(1) Plug (2) Spring (3) Non-return valve (4) Safety-valve spool (5) Main spool (6) Safety-valve sleeve (7) Valve body (8) Main spool sleeve (9) Plugs (10) O’ring (11) Circlip (12) Spring (13) Push rod (14) Circlip (15) Spring (16) Modulation spring (17) Seal quadring (18) Guide segments (19) Piston
(20) Calibrated union (21) Modulation limit stop (22) Main spool limit stop

Disassembling and reassembling the valve 

Preliminary operation
- To remove the valve.

Using a locally manufactured tool, remove circlip (11) and take out plug (9) from the A assembly. Using the same method for the B assembly. Remove the A and B assemblies while visually noting their positions. Unscrew plug (1) and remove the C assembly as in the previous operation.

Check and clean the parts. Replace any components found to be scratched or distorted. Check that valve (4) and spool (5) spool freely in the sleeves (6) (8). Before reassembly, lubricate the seals with transmission oil. Replace o’rings (10) and the assemblies A, B and C using repair kits. Fit circlips (11).
Before fitting the piston of assembly A, bend guide segments (18) in a circular manner and lightly smear them with miscible grease so as to correctly position them in the grooves of the piston, the knurled faces turned towards the valve body.
- Union (20) is fitted with a calibrated port.

Final operations
Refit and bleed the clutch control valve. Check the modulation pressure of the valve.

Tightening torques 
- Plug (1) smeared with Loctite 542: 50 Nm.
- Non-return valve (3): 28 - 30 Nm.