gears blocked

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hey guys, i have a 1992 cab chassis rocky. F70 i think. Recently i started the ute, and all the gears were blocked. I stopped the car, the gears went in fine, i then re-started the car and everything worked as per normal. A week or two later however, and the problem has re-occured but this time it seems more permanent. The gears will go in without the clutch when the ute is not running, and it will start in gear and creep forward, but all gears are locked when the vehicle is running. The clutch doesn't seem to have lost any pressure whatsoever. Any help would be greatly appreciated, richard

Geared

Two things spring to mind immediately, the clutch is shot, or the gear selector mechanism is faulty. As you can get gears when the engine is stationary it would suggest a clutch problem.
Check through the side of the bellhousing, where the clutch operating lever goes, remove the rubber boot to look inside, ensure the thrust bearing is intact, and has not pushed through the pressure plate. Check the fork has not become partially detached, and the spring clip holding the thrust bearing is located correctly, and still in one piece. Check for wear on the friction plate, problems like this can come on suddenly when the plate wears.
Check the clutch cable, if this is slack it will not fully engage the clutch; to adjust, where it comes through the bulkhead pull it through, there is a metal section with ridges machined into it, this has a spring clip on it, pull this off and move up the cable and push back on.