The selectable four wheel drive system of Series Land Rovers is mechanically complex, with multiple potential routes to strife.
For a problem with the yellow knob selector, unscrew the knob, remove the spring and mounting bracket and then the gearbox tunnel. Check the linkage is not bent and lubricate the joints.
To diagnose an internal selector problem, you will
need to remove the floor panel to check if the selector shafts are sticking. If sticking, remove the cylindrical dust cover at the end and lubricate the shaft and wriggle it until free.
If the shaft cannot be freed, then it could be a seized locking pin or damaged selector spring. Some further dismantling is required to fix the problem.
For a problem with the red knob selector, check for a correctly fitting rubber boot or anything else restricting its forward or reverse movement.
A variety of freewheel hubs are available for the Series Land Rover and these can be checked by jacking up each front wheel in turn. With the handbrake on and four-wheel drive not selected, turning the wheel should rotate the front propshaft when the hub is engaged. De-selecting should allow the wheel to rotate without rotating the propshaft. If there is a problem it's worth dismantling the freewheel hub to check for damaged/replaceable parts. Fairey freewheel hubs have three locating pins inside which can easily be damaged. They are standard size roll pins .
Series Land Rovers have a reputation for breaking half shafts. Failure for the propshaft to rotate, when the free wheel hub is engaged, means a broken half shaft (or possibly a broken differential). Replacing a broken shaft is not difficult once the broken inner half has been removed.
To diagnose a broken rear half shaft, Repeat the wheel test with the gearbox in neutral. Failure of the rear propshaft to rotate means either a broken half shaft on that side or a differential problem.
To check that four-wheel drive is fully functioning, remove a rear half shaft, select four-wheel drive and if the vehicle can still move forward then everything is OK.
If your Series Land Rover is jumping out of four-wheel drive, perhaps its low transfer lever boot is fitted incorrectly, or is under tension. It can push the lever into neutral, disconnecting four-wheel drive.
A further cause of jumping out is a faulty selector spring. Remove the threaded plug on the top of the transfer box (near the oil filler plug) and check the spring for weakness or damage. Renew the spring if necessary.
If jumping out of low transfer only, it could be too much end float in the intermediate gear. Check by removing the bottom cover plate on the transfer box. Float should be 0.004 to 0.008in (0.1 to 0.2mm). The intermediate gear is the largest gear, consisting of two cog wheels separated by an integral smooth shaft. To adjusting the end float using shims you will need to remove the transfer box and strip it.
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