GEARBOX SPLINE FAILURE
THIS IS actually a problem with the gearbox mainshaft which engages and drives the transfer box input gear. It was common a few years back, and most of the affected transmissions will, by now, have been rebuilt or at least fitted with the later, improved transfer gear. It is no longer a common occurrence, but worth being aware of on older vehicles.
The problem was attributed to lack of lubrication on the mating splines of the mainshaft and transfer gear. This was sorted in the later R380 gearbox by fitting a transfer gear that was cross-dilled, allowing oil to flow through the drillings to reach the splines.
The symptom is a distinct knock from the central transmission area when taking up the drive, but especially if the vehicle was rocked back and forth by moving from first to reverse gear. A knock can be produced by plenty of other components though, including worn radius arm bushes, propshaft universal joints, A-frame joints and bushes. So a good check around is vital before getting worried about the mainshaft. If you hear a knock and suspect the mainshaft splines, investigate by removing the Power Take Off (PTO) cover from the rear of the transfer box. There’s no need to drain the oil before removing the cover, unless you suspect it is over-filled, in which case remove the filler plug first to see if any excess drains out. If you see rust and fine metal shards in there, the debris will be from the mainshaft’s worn splines. If the vehicle is put into first gear and the handbrake released (on level ground with wheel chocks to prevent excess rolling) you may be able to see the movement by nudging the vehicle back and forth from underneath. The transfer gear can be removed here to inspect the mainshaft splines.
The answer is usually a new mainshaft and a new transfer gear (of the crossdrilled type). As this work demands a strip of the gearbox, it’s usually cost-effective to fit a recon gearbox.
A gearbox serial number with suffix F suggests a cross-drilled gear is fitted, though many earlier boxes have been retrofitted with the improved gear. This is typical damage caused to an early LT77 gearbox mainshaft by wear and corrosion of the splines.