Check the condition of the front coupling dogs of the gears. They must be in perfect condition and with no sign of wear on the edges of the teeth.
The idler gears must rotate freely on their shafts.
When refitting, make sure the circlips are correctly positioned.
Check the needle roller bearings for wear.
The threads and splines of the shafts must be in perfect condition.
Also check that the component parts of the gear selector mechanism are in good condition.
Engage

Engage the gears and check that the gearchange mechanism does not stick (selector fork - gear groove, and fork pin - desmodromic drum groove) due to incorrect end float. Restore the correct end float by shimming the gearbox shafts and the selector drum with suitable shims.
For the total gearbox shaft and selector drum end clearance values, refer to sect. 3 - 1.1, Gearbox.
Disassembly of the gearbox shafts
Inspection of the gear selector forksHorn not working
Fault codes
Dds: horn diagnosis -> short circuit to ground (s.C. Gnd).
Dashboard: the error "claxon" (horn) is shown on the service display. The eobd
warning light activates.
Wiring diagram
Db dashboard connection, s horn button. 5 Blue/white - b/w, 1 red/blue - r/b,
db 19 purple/bl ...
Removal of the licence plate holder
Disconnect connector (5) of the number plate holder wiring from the main one.
Release the number plate holder light cable from the ties and the cable grommets
as indicated in sect- 7 - 6, flexible
wiring/hoses positioning, since the cable is together with the rear abs sensor
cable.
...
Specific tools for the engine
88713.2011.Tool to lock crankshaft at top dead centre
88713.2676 .Wrench for tightening cylinder head nuts
88713.2103 Stand for cylinder head assembly
88765.1657 Timing check tool
88713.1821 Wrench for tightening timing belt tensioner pulley shafts
88713.2861 Camshaft oi ...