GPA20 rear axle - Hitch and linkage

The linkage cover plate is fitted on the upper face of the rear axle housing. Via two rings (8), it supports the shaft (4), which has the two linkage arms (5) (11) splined to it. A cam (9), screwed on the shaft, inside the cover plate, shows the different positions taken by the linkage arms. This position information is recorded and sent by a sensor (26) to the electronic linkage controller. To ensure a correct operational clearance between the linkage arms and the cover plate, the shims (12) are fitted on the left-hand end of the shaft. Two friction washers (30) prevent the arms coming into direct contact with the cover plate. The rear face of the linkage cover plate supports the auxiliary spool valves. The forward speed sensor (15) is screwed to the front of the cover plate, which also supports the 4-speed PTO control rod (21) (if fitted).

Parts list
(1) Washer (2) Screw (3) Lock washer (4) Linkage shaft (5) Right-hand linkage arm (6) Nylon ring (7) "O" ring (8) Ring (9) Cam (10) Linkage cover plate (11) Left-hand linkage arm (12) Shim(s) (13) Ring (14) Set screw (15) Forward speed sensor (16) Nut (17) Cup plug (2-speed PTO)
(18) Screw (4-speed PTO) (19) Control finger (4-speed PTO) (20) "O" ring (4-speed PTO) (21) Rod (4-speed PTO) (22) Screw (4-speed PTO) (23) Elbow union (24) Screw (25) Nut (26) Position sensor (27) Cup plug (version without linkage) (28) Threaded plug (version without linkage) (29) Seal (30) Friction washers


- the trailer electrical connector (1)
- the spool valve control cables (2)
- the hydraulic unions connected to the spool valves (3)

- the trailer brake connector and its support, by undoing the unions at both ends,
- the supply pipes for the lift rams.

- the forward speed sensor (15) harness
- the position sensor (26) harness
- the PTO control cable (4-speed PTO, if fitted)
Remove the four screws from the spool valve support.
Remove the spool valve support with the spool valves.
Remove the upper pins from the rams and lift rods.
Remove the screws (24) from the linkage cover plate (10).
Sling the cover plate and remove it.

Place the cover plate on a workbench.
Remove the elbow union (23).
Unscrew the nut (16) and remove the forward speed sensor (15).
Unscrew the nut (25) and remove the position sensor (26).

For tractors fitted with a 4-speed economy PTO

Loosen the Allen screw (22) of the rod (21).
Remove the screw (18).
Remove the control finger (19).
Remove the "O" ring (20).
Unlock the screws (2) for the linkage arms.
Loosen the screws.
Remove the washers (1).
Remove the shims (12).
Remove the linkage arms (5) (11) and the washers (30).
Remove the nylon rings (6) and the "O" rings (7).
Remove the set screws (14) from the cam (9).
Extract the linkage shaft (4) from the cover plate.
Remove the cam (9).

Remove the rings (8).
Remove the cup plug (17) (2-speed PTO).

Fit the rings (13) up against the face "A"

Check and clean the components. Replace any defective parts.
Clean the mating faces of the linkage cover plate and of the spool valve support. Fit the rings (8) in the cover plate.
Fit the plug (17) smeared with Loctite 542 level with the cover plate (2-speed PTO). Fit the linkage shaft (4) and the cam (9) in the cover plate. Ensure that the fitting direction of the shaft and the position of the cam are correct.

Tighten the set screws (14) smeared with Loctite 241 to a torque of 5 Nm. Protect the shaft splines. Lubricate the "O" rings before fitting.

Grease the splines of the shaft (4) (with Anti-Seize Grease or equivalent).
Fit the linkage arm (5) (sensor side), the washer (1), the lock washer (3) and the screw (2). Grease the face of the cover plate (with Anti-Seize Grease or equivalent) before fitting the arm.

Tighten the screw (2) to a torque of 360 - 470 Nm. If work has been done without the need to carry out the shimming J1 (e.g. replacement of seals) it is essential to fit the shims (12) at the left-hand end of the shaft (factory fitting) to preserve the correct positioning of the cam and to avoid having to adjust the sensor (26).

Carry out shimming of the shaft (4) to obtain the clearance: J1 = 0.10 to 0.20 maximum.  Fit the arm (11), the washer (1) and the screw (2).
Tighten the screw to position the linkage arms (5) and (11) correctly on the shaft. Measure the distance between face "A" of the shaft and face "B" of the arm using a depth gauge.
By measuring dimension "X", determine the thickness of shims "Y" needed to obtain:
J1 = 0.10 to 0.20 maximum
Y = X + J1
Fit the shims (12) selected previously, the washer (1), the lock washer (3) and the screw (2). Tighten the screw (2) to a torque of 360 - 470 Nm.

For tractors fitted with a 4-speed PTO

Refit the finger (19).
Fit the "O" ring (20) from the external side of the cover plate.
Smear the screws (18) (22) with Loctite 241 and tighten.
Check that the control functions smoothly.
Fit the elbow union (23).

Clean the mating face of the cover plate on the rear axle housing.
Smear the mating face with a sealing product (Master Joint 510 Loctite or equivalent).
Sling the cover plate and refit it. When refitting the cover plate, check that
the differential lock supply pipe and the 4-speed PTO control finger engage properly in their respective positions.
Fit the PTO control cable support (depending on option) and fit the screws (24) for the cover plate (10). Tighten to the following torque:
• screws Ø 12 = 72 - 96 Nm
• screws Ø 16 = 160 - 200 Nm
Refit the upper pins for the lift rams. Refit the trailer brake valve support if fitted. Retighten the pipe at both ends. Refit:
- the differential lock supply pipe on the union (23)
- the control cable (21) (4-speed PTO, if fitted)
Fit the forward speed sensor (15) with Loctite 577 Sensor Sealing or equivalentScrew home the speed sensor without forcing it until it comes into contact with the differential ring gear.

Unscrew the sensor 3/4 of a turn to obtain a clearance of approx. 1 mm between the sensor and the ring gear.
Tighten the nut (16) to a maximum torque of 5 Nm.
Connect the sensor harness.
Secure the harness with a retainer.
Clean the mating face of the spool valve support cover plate.
Smear the mating face of the spool valve support cover plate with a sealing product (Master Joint 510 Loctite or equivalent).
Refit the spool valve support.
Refit the 4 screws for the spool valve support after smearing their threads with Loctite 510. Tighten the screws to a torque of 50 - 70 Nm.
Refit the parts that were removed during step.
Adjust the PTO control (4-speed).
Smear the thread of the position sensor (26) with a sealing product (Hylomar or equivalent), then screw the sensor in a few turns. Start the engine.
Using the external controls, ensure that the linkage arms are in the raised position (continuous pumping). Stop the engine.

Screw in the sensor (without forcing it) up against the cam (9). Connect the sensor to the female connector of a makeshift harness. Connect the other test harness connector to the tractor harness. Connect terminals 1 and 2 to a multimeter. Start the engine. Lower the hitch by 3 to 5 cm, measured between the hook and ground.
Unscrew the sensor to obtain a voltage of between 6.92 and 6.96 volts. Reconnect the tractor harness. Stop the engine.
Tighten the nut (25) to a torque of 25 Nm using the makeshift wrench.
Check tightness:
- of the mating faces of the cover plate and the spool valve support
- of the hydraulic unions

Version without linkage
• The ports of the shaft (4) are plugged with the cup plugs (27) sealed with Loctite 542.
• The position sensor (26) is replaced by a threaded plug fitted with a seal (29).

Rear hitch

The hitch comprises several components, depending on option and country:
- a top link with a ball joint or hitch
- two lower bars with ball joints, automatic hooks or telescopic end fittings (depending on version)
- two levelling units
- two tube type stabilisers
- a multi-hole drawbar

Presentation of the hitch

 • Top link:
- (1) top link with ball joint
- (2) top link with hook
- (3) draft sensor
• Levelling unit (4) 

• Lower links:
- (5) lower link with hook
- (6) lower link with fixed ball joint
- (7) telescopic lower link
• Telescopic stabiliser (8) 
• Multi-hole drawbar (9) 

Disassembling and reassembling a lift ram

Run the engine and lower the lower links to their maximum using the tractor's electronic system. Disconnect the supply hose. Remove the ram and drain its oil.

Remove the elbow union (1) fitted with its "O" ring.
Pull the rod from the ram (5) until the snap ring (4) appears in port O of the union.
Insert a screwdriver in this port and offset the snap ring into the adjacent V-shaped groove of the rod
while turning the rod at the same time.
Remove the rod and the snap ring (4). Drive out the wiper seal (2) and the seal (3) (always discard these three parts).

Dry off all the components using a jet of compressed air.
Lubricate the assemblies with clean transmission oil. Assemble the ram.

Lubricate the upper and lower pins of the ram with "Anti-Seize" grease and refit the ram. Reconnect the supply hose.
Run the engine. Using the linkage electronic controller, raise and lower the linkage arms to their maximum positions several times.
Check that the hitch operates correctly and that there is no leakage of oil around the rim of the ram.