Low pressure valve (17 bar) - Open Centre


The Low pressure valve (17 bar) (2) is screwed onto the left-hand hydraulic cover. It performs the following functions:
- it supplies oil and maintains pressure in the 17 bar low pressure circuit
- it supplies the cooling system (5 bar) and lubricating system (1.5 bar).

Parts list
(2) 17 bar valve (3) O’ring (4) Plug (5) Spring (6) Ball (7) Spool (8) Shim(s) (9) Spring (10) O’ring (11) Seal (12) Plug (13) Valve sleeve

Identification of ports 
G Brake master cylinder booster (excluding Meritor type)
O Valve supply via the steering spool valve return (Orbitrol)
P Outlet to low pressure parts
R1 Return (1.5 bar valve)
BV Lubrication to 1.5 bar valve (cold oil)
RR Lubrication - cooling system return
VR Lubrication to cooling system
PDF Lubricating line to rear PTO

Description of the 17 bar low pressure valve

Supply 


As soon as the engine is started, the return oil from the steering unit (Orbitrol) enters port O and exits by port P of the left-hand cover; then it is directed towards the low pressure circuit of the right-hand hydraulic cover in order to supply:
- power shuttle (if fitted)
- dynashift unit
- changing between Hare / Tortoise range
- 4WD clutch
- front and rear differential locks
- rear PTO brake and clutch
- front power take off (if fitted).

Operation 


A back pressure is generated in the low pressure circuit, forcing the ball (6) from its seating and thereby compressing the spring (5). The oil passes into the rear of the spool (7), creating a pressure which progressively moves it downwards by compressing the spring (9) which thrusts against the shims (8).
When one of the low pressure functions is activated, the momentary drop in pressure returns the ball (6) to its seating through the action of the spring (5). The oil contained in the chamber at the front of the plug (4) flows through a drain port located at the end of the spool (7). Leaks return to the housing via port R in the cover.

Identification of components and ports 
(1) 17 bar low pressure switch
(2) Plug (provision for outlet).
O Valve supply via the steering spool valve return (Orbitrol)
P Outlet to low pressure parts of the right-hand hydraulic cover
R Return port

Removing - install and disassembling - reassembling the Low pressure valve (17 bar)

It is not necessary to remove the left-hand hydraulic cover to carry out work on the valve.

Removal
Remove any parts around the valve that may obstruct work.
Remove and separate the valve (2) from the left-hand hydraulic cover.

Install
Check the condition of the O'ring (10) and replace if necessary.
Screw the valve back onto the cover and tighten to 40 - 55 Nm.

Disassembly 


The valve comprises several hydraulic parts (spool, springs and ball) which cannot be replaced separately. Remove the O'ring (10).
Tighten the hexagonal head of the valve in a vice with protective jaws.
Gradually unscrew the plug (12) and discard the O'ring (11).
Recover:
- spring (9),
- shim(s) (8).
Gradually unscrew the plug (4) and discard the O'ring (3).
Recover:
- spring (5)
- ball (6).
Remove the spool (7) by carefully tapping the shaft (13) against a wooden shim.

Reassembly
The hydraulic parts must be reassembled on a clean work surface clear of filings and dirt. Check:
- the condition of the springs and O’rings
- the absence of scratches or seizing on the moving parts of the valve
- the sliding of the spool in its sleeve
- the cleanliness of the ports.

Conclusion
If the valve is defective, replace it. If not, reassemble it by carrying out the disassembly operations in reverse order.

Adjusting the Low pressure valve (17 bar)
- It is not necessary to remove the left-hand hydraulic cover to adjust the valve.
- When the valve is removed, it can be adjusted inserting shims (8) between the spool (7) and the spring (9).

Operation of the cooling system

The pressure acting on the back of the spool (7) pushes it towards the bottom of the valve, pressing in the spring (9). It then allows oil to pas to port VR and the cooler. When it leaves the cooler, the oil is directed to the transmission lubricating system via the 1.5 bar valve.


5 bar valve
When the oil is cold, the 5 bar valve (1) (2) (3) opens partially. It then allows some oil to pass directly to port BV of the left-hand cover, and to supply the transmission lubricating system, without really entering the cooler.

Assembling the Low pressure valve (5 bar)

It is not necessary to remove the left-hand hydraulic cover to carry out work on the valve. Remove the pipe and union.
Place the valve (1), spring (2) and threaded ring (3) in the cover.
Compress the spring, tightening the threaded bush until it reaches a “hard point” (the bush has reached the cover shoulder).


After assembly, check that free movement of the valve and the compression of the spring, entering a screwdriver through the port provided for screwing in the temperature switch, located on the lower part of the cover.

Description of the 1.5 bar lubricating valve

Location
The 1.5 bar valve (1) is hidden behind the fuel tank, and is located at the front left-hand side of the gearbox.


Operation
It controls the oil coming from the cooling circuit, and maintains a lubricating pressure of approximately 1.5 bar in the circuit, due to the valve setting. If the pressure exceeds 1.5 bar, the spring (8) compresses, the valve (2) moves and oil flows to the return R1.


- When the requirements of the low pressure circuit are met, oil is channelled towards the booster port of the brake master cylinders in order to ensure a constant oil level.
- Considering the available hydraulic options, other assemblies can be carried out on the union (2). Example: Front PTO - Meritor brake master cylinders.

Removing - install and disassembling - reassembling the 1.5 bar valve

Removal 
Remove the part(s) that may obstruct work on the valve (1).
Mark and disconnect the pipes (5) (6), hoses (7) (8) and unions (2) (3) (4).
Remove the housing valve from the gearbox.


Refitting
Replace the O’rings if necessary.
Reinstall the valve, unions, hoses and pipes.
Replace any part(s) removed at operation.

Disassembly
Take off circlip (5). Remove the bush (2), spring (3) and valve (4) from the valve body (6).

Reassembly


The valve (1) consists of several hydraulic parts (valve, spring) listed in the spare parts catalogue. Check that the hydraulic parts are clean.
Reassemble the valve, carrying out operation in reverse order.
Manually check the free movement of the valve.