DYNA VT transmission – hydraulic


Forward / Reverse high pressure relief valves

The purpose of the high pressure relief valves is to regulate the increase in HP pressure. They protect the transmission elements (hydrostatic motors and pump).


A = Coupler and clutch functions inactive: Pressure identical in both chambers. The spool is held in closed position by the spring.
B = Coupler and clutch functions active: the pressure drop caused by the restrictor (nozzle) applies a force to the spool that is more powerful than the force of the spring. The spool moves to the right, bringing PH into contact with ES (booster).
C = HP pressure is limited by the clutch and coupler spools.
Replacing Forward / Reverse HP pressure relief valves
Drain the transmission oil


Loosen the 2 plugs with hexagonal heads located under the transmission housing. Use a wrench to remove the high pressure relief valves.
V = Forward high pressure relief valve
R = Reverse high pressure relief valve.


The high pressure relief valve is a controlled pressure relief valve.
Adjustment pressure when new: R500 + 20 bar.
Adjustment pressure after several hours: 480 ± 20 bar.
Only replace O’rings if they are damaged. Do not move the thrust collar. Tighten the pressure relief valve to a torque of 250 + 20 Nm

Scavenging valve

This valve allows to “flush” the oil in the hydrostatic loop.


Operation 
Pressure in A, B: max. 500 bar.
Pressure in T: max. 50 bar
Opening pressure: delta p = 7 bar between A and B


Transmission at neutral 
pA = pB, delta p < 7 bar
The spool is held in “middle” position by the spring.
The two channels (A, B) are closed.


“Pulling” phase 
pA > pB, delta p > 7 bar
The spool moves upwards via control port XA.
Channel B communicates with T. The hot oil coming from the low pressure side B can therefore be
“flushed” and sent by the cooler via port T.


“Pushing” phase 
pA > pB, delta p > 7 bar
The spool moves downwards via control port XB.
Channel A communicates with T. The hot oil coming from the low pressure side A can therefore be
“flushed” and sent by the cooler via port T.


Disassembling the scavenging valve 
Drain the transmission oil.
Remove the plug located under the transmission housing.


Use a wrench to remove the scavenging valve. 
Only replace O’rings if they are damaged. Do not move the thrust collars. The thrust collars are positioned facing each other. Tighten the scavenging valve to a torque of 200 + 10 Nm.
The new scavenging valve with a seal groove is provided as a replacement. Tighten to the required torques: 250 + 20 Nm

Control spool valve

Pressure relief valve - 4V4 / Y004 coupler function


The coupler function valve (4V4) regulates the increase in hydrostatic pressure (PH) in proportion to the engine speed. Due to this component the coupler effect is simulated. The coupler function valve is fitted to the control block where the hydrostatic loop PH and tank return junction is located. If this junction is not closed, the pressure in the hydrostatic loop cannot increase, and the tractor does not reach its maximum pulling power. The junction between the hydrostatic loop PH and tank is closed by the coupler function valve (4V4). The coupler function valve is controlled by the electronic control unit. The strength of the current depends on the engine speed, and changes as shown by the following table :

  Engine speed in rpm        Current supplied at A                    Maximum PH in bar              Remark
               800                                    0                                        0                        Transmission at neutral
               800                          approx. 0.46                               78                        Transmission "active"
              1200                                1.23                                    105
         from 1400                             1.71                                     500

In order for the pressure to increase in the HP loop, it is necessary to maintain the electrical supply and the tightness of the coupler function valve. The tighness of the valve seal can be ensured by a mechanical lock mechanism.


Move the control lever 
Or tighten the internal Allen screw.

Pressure relief valve 4V5 - clutch function 
The clutch function valve is fitted to the control block where the PH hydrostatic loop and tank return junction is located. The clutch function valve also allows to limit the maximum pressure PH in the 500 bar (± 20 bar) hydrostatic loop. Due to the clutch function valve, when the clutch pedal is not activated, the junction between the hydrostatic loop and tank return is closed. By moving the clutch pedal, the clutch function valve opens and the hydrostatic loop communicates with the tank return channel. This results in a drop in pressure (interruption of the pulling effort) in the hydrostatic loop, proportional to how far in the clutch pedal is pressed (comparable to a mechanical forward clutch).
Clutch pedal pressed in fully, High pressure PH = 0 bar. The maximum pressure can be achieved in the hydrostatic loop only when the clutch valve (4V5) is functioning perfectly.


Checking the control block
If maximum pressure is not reached when measuring the HP pressure, the transmission module may be defective, but it is also possible that the internal seal tightness of the control block is the cause of the problem. To be sure that it is necessary to disassemble the transmission module, it is recommended to first check the seal tightness of the spool valve block (transmission control). The pressure in the hydrostatic loop can only increase if the pressure relief valve seals are tight.
- Coupler function (4V4 / Y004)
- Clutch function (4V5)
When the following conditions are met, the coupler function valve allows the pressure in the hydrostatic loop to increase :
- Engine speed higher than 1400 rpm (PWM - Signal)
- Limp home mode activated
- Valve locked mechanically (tighten the internal Allen screw or move the lever).
The clutch function valve is closed (increase in HP pressure possible) when the clutch pedal is not activated. The tightness of the 2 pressure relief valves (4V4 / 4V5) can be controlled by the T union. 


Remove the right-hand rear wheel and its protective guard located behind it
Remove the T union. Fit a test pressure gauge to measure pressures higher than 500 bar at the PH union.


Checking procedure :
Start the engine. Activate the limp home mode by pressing in the clutch pedal fully and pressing on the button.


Apply the handbrake. Use the limp home mode lever to start the transmission.



PH
T union
Possible causes
250 bar
No oil flow to the T union
Transmission module error (shuttle valve, pressure pipe union). Remove the module.
250 bar
Oil flows from the T union
Coupler function valve (4V4) or clutch function valve (4V5) not tightly sealed. Change the valve.


Checking the clutch function valve (4V4)


Mechanically lock the valve (tighten the internal Allen screw or move the lever

PH
T union
Possible causes
250 bar
Oil flows from the T union
Clutch function valve (4V5) not tightly sealed. Change the valve
250 bar
Oil flows from the T union, but the pressure is not constant
Electrically check the coupler function valve. Change the valve if faulty


DYNA VT hydraulic tests

Supply pressure measurements

Measuring points
Reference
on
component
Description
Measuring points
Reference
on
component
Description
M1
KV
Cooler upstream pressure
M11
ZWK
PTO clutch pressure
M2
SM
Lubricating pressure
M13
750
750 rpm PTO selector pressure
M3
AS
Flushing pressure
M14
VR
Front axle clutch pressure
M4
ES
Booster pressure
M15
DS
Differential lock pressure
M5
PU
Service pump pressure
M16
1000
1000 rpm PTO selector
pressure
M6
P
Transmission system pressure
M17
Brake service pressure
M7
I
Tortoise range shifting pressure
M18
Rear axle beam lubricating
pressure
M8
II
Hare range shifting pressure
M19
Reverse position HP pressure
M9
PH
High pressure (HP)
M21
Front PTO clutch pressure
M10
PHA
Rear axle beam and braking system pressure
M22
T
Oil leak on clutch function
spool valve / coupler function
solenoid valve


Precautions to be taken during the pressure measurements


The oil temperature must be between 35 and 45°C. When measuring the transmission pressure, raise all wheels of the tractor to prevent accidents.
Set transmission ratio (Hi/Lo) to speed of 0. Release the handbrake.
Engage the front axle. Differential lock and PTO clutch are not engaged. On right-hand side, in the middle of the tractor :
Remove right-hand rear wheel and its protective guard.
Connect a test pressure gauge. Measure the pressures set out below according to the different engine speeds.
- PU Pressure (M5). Pressure measuring point located between the service pump and the pressure filter.
- P Pressure (M6) System pressure downstream of pressure filter.
- ES booster pressure.
- AS flushing or discharge pressure
- SM transmission lubricating pressure.