"Electric" gear selection? Check fuses, relays, and wiring connections.
Early one with manual trans. Check linkage for wear preventing full movement of selectors. Otherwise, problem could be with worn selector lever end(overrides gate) selector fork wear, sticking interlocking plungers (prevents engaging two gears at once) or damaged or worn splines/sliding dogs.
Try removing the lever ball on the gearbox and lining up the selectors with a suitable screwdriver or bar. Check the lever tip, ball, spring and selectors.
Mr. Toad, September 2008
