Mechanical reverse shuttle GBA20 gearbox – Selector cover plate

On some tractors with mechanical reverse shuttle, the selector cover plate is fitted to the right-hand side of the gearbox housing. It has two levers:
- a front lever to control the mechanical reverse shuttle;
- a rear lever to select the four basic gears and to indicate to the tractor electronic system when it is in neutral position for Hare / Tortoise range shifting. Hare / Tortoise range shifting is carried out via a pulse button integrated into the transmission lever handle. The action is validated when the lever is in neutral.
A sensor "C" fitted to the selector cover plate buzzes to warn the user that a gear is engaged when the hand brake is on.

Parts list (Fig.1)


Hare / Tortoise fork locking and gear control
(1) Plug or oil return port (2) Earthing lug (3) Screw (4) Centring screws (5) Plug (6) Stud and nut (9) Selector cover plate (13) Gear control finger and pin (14) Speed grid (15) Screw (16) Support (17) Locating ring (18) Ball bearing (19) Screw (20) Spring (21) Washer (22) Circlip (25) Seal (26) Guide ring
(27) Seal (28) Neutral position switch (31) Plunger (33) Washer (34) Toggle switch (35) Indexing rod


Mechanical reverse shuttle control
(25) Seal (26) Guide ring (36) Finger (37) Pin (38) Set screw (39) Guide ring

Parts list (Fig.3)


Hare / Tortoise fork locking and gear control
(2) Transmission lever (13) Control finger and pin (31) Plunger (34) Toggle switch (35) Indexing rod

Parts list (Fig.4)


Mechanical reverse shuttle control
(1) Control lever (2) Selector (5) Rod (6) Fork (36) Finger (37) Pin

Transmission lever (Fig.3). 
Selecting 1st - 2nd ratios
The transmission lever (2) operates lever L and engages the finger (13) in the 1st - 2nd fork. The first ratio is obtained by moving lever L upwards and the second ratio downwards. Simultaneously, the toggle switch (34) secured to pin A pivots and moves the plunger (31) thus locking the indexing rod (35) and the Hare/Tortoise fork.
Selecting 3rd - 4th ratios
In this configuration, the finger (13) secured to pin A is engaged in the 3rd - 4th gear fork. The third ratio is obtained by moving lever L upwards and the fourth ratio downwards (Fig.4). The locking principle of the indexing rod (35) and Hare/Tortoise fork is identical to that used for the 1st - 2nd ratios.

Hare/Tortoise range
Hare/Tortoise range shifting is validated when the transmission lever is in the Neutral position. If the button located on the transmission lever handle is activated and if switch (28) has confirmed the Neutral position of the lever, the electronic system of the tractor allows voltage to be supplied to or cut off from the Hare/Tortoise solenoid valve in order to obtain movement of the fork in the required range.

Hare position
The Hare/Tortoise solenoid valve, fitted on the lower part of the right-hand cover plate opens and supplies chamber "a" located behind piston (1) (Fig.5). The piston moves forward, and pushes the fork (Hare / Tortoise). The oil contained in chamber "b" is discharged into the Tortoise channel, and returns into the 17 bar system.


Tortoise position
When the pulse switch (Fig.3) is activated again, the solenoid valve is closed, and the pressure drops in chamber "a" (Fig.5). The piston (1) moves back due to the 17 bar pressure exerted on the annular face.
The oil contained in chamber "a" returns to the housing via the Hare channel and the solenoid valve.

Identification and function of Hare / Tortoise range locking elements

Toggle switch (34)
This is one of the main elements of the Hare / Tortoise range locking system. It is freely mounted on pin A (Fig.3) and pivots with it by means of a flat section.
Function - It moves the plunger (31) towards the indexing rod (35) and locks the latter during gear engagement.
- Via its opposite face it controls the neutral position of pin A (transmission lever) using the switch (28).

Indexing rod (35)
At one end there are two different locking grooves:
- the narrow groove: Hare position;
- the wide groove: Tortoise position.
The other end is designed to house the stop of the Hare / Tortoise fork.
Function - It avoids any unwanted reverse movement of the Hare / Tortoise fork in case of electronic or hydraulic malfunction.

Plunger (31)
Spherically shaped at each end, it moves between the toggle switch (34) and indexing rod (35).
Function - It acts as the system lock, whatever range is engaged.

Switch (28)
It is controlled by the toggle switch.
Function - It informs the tractor electronic system when the transmission lever is in neutral position.

Reverse shuttle lever (Fig.4)
The reverse shuttle function is activated when the lever (1) is shifted either upwards (forward travel) or downwards (reverse travel). When the lever (1) is shifted, the rod (5) and pin (37) rotate partly, the latter being secured to the finger (36). The finger (36) then moves the mechanical reverse shuttle fork (6) and selector (2).