Mechanical reverse shuttle GBA20 gearbox - Disassembly and reassembly the selector rail


Creeper gear fork

Remove the rear right-hand wheel. Chock the tractor. Disconnect the creeper unit control cable on the rear axle. Remove the selector cover plate. Take out the plugs (1), the springs (2) and the detent plungers (3) to release the selector rail (Fig.1).

1

Remove the hydraulic cover plate(s). Remove the screw (1). Pull the rod (2) and the lever (3) outwards to release the finger of the fork (Fig.2).

2

Unscrew the nut (26) and the locking screw (27) on the fork (28).
Take out the set screw (8). Turn the selector rail (7) and drive out the pin (29) (Fig.3) without dropping it into the housing.
Move the selector rail forward. Removing the pin (1) from the coupling sleeve (2) on a 2 or 4WD tractor. Depending on the angular position of the coupling sleeve (2) located between the gearbox output shaft and the pinion, the pin (1) is accessible or inaccessible.

3

2WD tractor
If the pin (1) is accessible, drive it out from the sleeve.
If the pin (1) is not accessible:
- raise the tractor rear axle using a trolley jack;
- turn the pinion to correctly position the coupling sleeve;
- drive out the pin.

4WD tractor
If the pin (1) is accessible, drive it out from the sleeve.
If the pin (1) is not accessible:
- disconnect the front differential lock control hoses;
- remove the guard. Uncouple the rear sleeve from the transmission shaft;
- turn the pinion to correctly position the coupling sleeve;
- drive out the pin.

4

Provisionally fit the set screw (8) in order to hold the selector rail in place to remove the fork (28). Slide the sleeve (2) and the coupler (1) closer together on the connecting shaft (3) (Fig.4).
Note and mark the assembly order of the sleeve (2) (Fig.4). Disengage the sleeve/connecting shaft/coupler assembly towards the outer left-hand side. Take it out of the fork (28) (Fig.4) and remove it through the opening in the right-hand cover plate. Remove the fork (28) on the selector rail (7).
Remove the lever (3) (Fig.2).
To remove the creeper unit control rod (1), turn it so that it passes beneath the selector rail (7). Discard the "O" ring (3).
Clean and check all components. Replace those that are defective.
Install the control rod and pull it as far to the right as possible. Fit a new lubricated "O" ringInstall the lever (3) (Fig.5). Tighten the screw.

5

Install the fork on the selector rail. Ensure that the finger (1) of the creeper gear control rod is pointing towards the front of the tractor (Fig.5).
Couple the sleeve/connecting shaft/coupler assembly inside the housing. Place it in the fork, turning the machined groove on the sleeve (2) towards the rear of the tractor.
Slide the sleeve and coupler onto the connecting shaft.
Install a new double pin into the sleeve.
Take out the set screw (8).
Install a new pin (29) and turn the selector rail (7) (Fig.3).
Clean the set screw (8). Smear its thread with Loctite 542 and tighten to a torque of 38 - 43 Nm.

Ensure that the ball bearing fitted at the end of the locking screw (27) returns correctly to its initial position after pressure has been applied manually by the locking screw. Position the fork (28) and locking screw on flat section "M" of the selector rail (7) (between the two locking slots (Fig.1). Fully tighten the screw without forcing it in order to compress the ball bearing. Loosen the screw by one-quarter turn. Clean the nut (26) and smear with Loctite 270 or equivalent. Tighten to a torque of 15 - 20 while holding the locking screw in place. Check that the fork locks correctly.
Turn and push the rod (2) in order to engage the finger in the fork. Fit the screw (1) smeared with Loctite 542 or equivalent (Fig.2).
Check that the rod operates correctly.
Check that the "O" rings for the internal hydraulic pipes are not damaged (see chapter 9).
Install the right-hand cover plate.
Attach the electrical harnesses with a retainer.
Install the trailer brake valve (if fitted, open centre), checking that the "O" rings are not damaged. Tighten the screws to a torque of 25-35 Nm.
Reconnect the pipes and hoses via the hydraulic cover plate.
Install the strainer, support and filter (open centre).

If necessary, on 4WD tractors:
- couple the rear sleeve (5) of the transmission shaft (3);
- Install the guard (1);
- reconnect the front differential lock control hoses.
Install the plugs (1), the springs (2) and the detent plungers (3) (Fig.1). Install the selector cover plate. Reconnect and adjust the creeper unit control cable.
Install the wheel. Remove the axle stand and the jack.
Tighten the wheel nuts to a torque of 400 - 450 Nm.
Top up the oil level in the rear axle and gearbox. Start the tractor.
Check the correct operation of the electrical and electronic circuits. Check the tightness of the cover plate mating face and the hydraulic unions.
Carry out a road test of the creeper unit control and 1st, 2nd, 3rd, 4th gears of the Hare/Tortoise ranges.

Hare / Tortoise fork and lock- 1st / 2nd - 3rd / 4th fork assembly and selector rail

Remove the right-hand rear wheel. Chock the tractor.
Remove the selector cover plate. Remove the right-hand hydraulic cover plate.
Remove the plugs (1), the springs (2) and the detent plungers (3) (Fig.1). The detent plungers are now longer (Fig.1). This modification improves their guiding into the forks (5) and (6) and also into the plugs (1). To optimise operation, a decompression spring on the detent plunger drives out the oil enclosed in the spring chamber (2). This design allows the plunger to enter and exit the fork more smoothly, thus improving gear shifting.
Take out set screw (8) (Fig.6).

6

Drive out the cup plug (9) forwards by striking the rear end of the selector rail (7) (Fig.6).
Place the Tortoise and Hare fork (20) in the Hare position.
Remove the adjustment screws (19).
Remove the screws (16) and the cover plate (15).
Take out the Hare and Tortoise piston (13) and discard the seals.
Remove the cylinder (10) and discard the seal.
Turn the pump suction pipe (1) upwards.
Move the selector rail (7) to the rear in order to release the forks. Slide the selector rail under the rear axle housing reinforcement.
Take out the 1st-2nd and 3rd-4th gear forks.
Extract the Hare and Tortoise fork.
If necessary, remove the locking stop (18).
Take the selector rail out through the selector cover plate opening by passing it through the Hare / Tortoise cylinder bore.

Clean and check all components. Replace those that are defective.
Install the selector rail through the selector cover plate opening and the Hare / Tortoise cylinder bore. Install the forks.
Check that the hydraulic ports in the Hare / Tortoise cylinder (10) are not obstructed. Install the cylinder fitted with a new "O" ring (11) (Fig.7). The cylinder (10) is held in position (Fig.7):
- at the front, by the shoulder machined on its outer diameter;
- at the rear, by the cover plate (15).



Slide the selector rail forwards into the Hare / Tortoise fork and the 3rd-4th and 1st-2nd forks.
On the piston (13), fit the "O" rings (12) (14), and then insert the piston into the cylinder bore (10) and onto the selector rail (7) (Fig.7). Turn the adjustment screw holes towards the tapped holes on the fork. Temporarily lock the piston with a screw (19).
Install the cover plate (15) with the screws (16). Tighten the screws to a torque of 25-35 Nm.
Clean the set screw (8). Smear its thread with Loctite 542 or equivalent. Tighten to a torque of 38 - 43 Nm.
Smear the cup plug (9) with Loctite 542 or equivalent. Install it flush with the housing.
Install the detent plungers, springs and locking plugs. Tighten the plugs to a torque of 50 - 70 Nm.

Adjusting the clearance between the Hare / Tortoise fork pads and the synchroniser sliding coupler

The positioning of fork (20) is obtained by adjusting the difference in the dimension between the centre line of the tapped holes "Y" and the centre line of the indents "Z" on the piston (13). A different fork movement is obtained either by adjusting the front screw or by adjusting the rear screw according to the adjustment required (Fig.8).

8

Position the control piston (13) and the sliding coupler "C" in the high range (Hare) (Fig.9).
Hold the sliding coupler manually against gear "L" of the high range (Fig.9).

9

Adjust the position of the fork (20) using the two adjustment screws (19) previously cleaned in solvent and lightly smeared with Loctite 221 or equivalent, in order to obtain a clearance of J1 = 0.3 mm between the rear face of the pad and the sliding coupler (Fig.9). For this adjustment, use the makeshift tool.
Position the control piston (13) and the sliding coupler "C" in the low range (Tortoise) (Fig.10).

10

Check that there is a clearance of J2 = 0.3 minimum (this value is determined by the clearance J1) between the pad and the sliding coupler when the latter rests against the gear "T" of the low range (Fig.10) Alternatively and progressively tighten the screws (19) to a torque of 25 - 35 Nm taking care not to modify the adjustment. In the Tortoise position, if face "X" of the fork pad (20) is driven against the sliding coupler "C", increase the clearance J1 (Fig.9). Check that the fork operates correctly.

Adjusting the Hare / Tortoise fork locking stop

Locking is adjusted by an adjustable stop (18) located on the Hare / Tortoise fork. This adjustment is obtained using a tool  whose role consists of conveying to the locking stop the positioning of the indexing rod, mounted on the selector cover plate. The aim of this adjustment is to make the Hare/Tortoise mechanism safe during operation, by mechanically locking the fork in the Hare or Tortoise position. Before adjusting the locking stop (18), it is essential to adjust the clearance between the Hare/Tortoise fork pads and the synchroniser sliding coupler.

On the cover plate (Fig.11)

11

Legend
L Mechanism synchronisation in Hare range
X Tool dimension measured on the selector cover plate and set on the gearbox (3) Gear engaged (e.g. 3rd gear) (18) Locking stop (31) Plunger (35) Indexing rod


Place the indexing rod (35) in the high range position (Hare).
Engage the 3rd gear ratio.
Install the tool on the selector cover plate (Fig.12).
It is essential to equally distribute the clearance between the indexing rod and the plunger (31) (Fig.11, detail A).
Using the tool, measure dimension X according to Fig.12 and Fig.11.

12

On the gearbox (Fig.11)
Engage the 3rd ratio. Using the tool, set dimension X on the gearbox.
Apply manual pressure on the fork towards the Hare range.
Adjust the locking stop (18) (Fig.11) by placing it in contact with the tool.
Lightly smear the thread of the screw (17) with Loctite 242 or equivalent. Tighten to a torque of
40-50 Nm.

13

Check
Check the operation of the Hare and Tortoise ranges.
Check the smooth operation of all gears.
Install the right-hand hydraulic cover plate.
Install the selector cover plate.
Reconnect and adjust the creeper unit control (if fitted).
Install the rear wheel. Tighten the nuts to a torque of 400 - 450 Nm.
Top up the oil level in the housings. Check it using the gauge at the rear of the centre housing.

Carry out a road test.
- of gears from 1st to 4th and vice versa;
- of the Hare and Tortoise ranges;
- of the creeper unit (if fitted).
Check the tightness of the cover plate mating faces and the hydraulic unions.