a problem like that (i.e won't change gear, or will only change gear when the engine is revved a lot) is reasonably likely to be a problem with the fluid levels.
if the fluid level is low there won't be the required amount of fluid or preassure to change gear, over revving the engine may compensate an amount for this.
really there are two things you can do.
top up the transmission fluid and see if this helps, or change the transmission fluid and see if this helps.
changing is probably the best, again a good service manual will let you know where exactly the drain plug for the transmission is.
also (more crucially) it'll let you know what the capacity should be.
in this instance I think you'd do better to drain completly and refill to the volume that the book tells you to.
this may, or may not, fix the problem. but in reality, even if it doesn't fix the problem you'll have wasted what? half a day, a empty container, a trip to the dump to get rid of the oil and about $10 - $20 on a can of transmission fluid...
but you'll have gained some experiance, which is probably worth the waste even if it doesn't work.
if the fluid level is low there won't be the required amount of fluid or preassure to change gear, over revving the engine may compensate an amount for this.
really there are two things you can do.
top up the transmission fluid and see if this helps, or change the transmission fluid and see if this helps.
changing is probably the best, again a good service manual will let you know where exactly the drain plug for the transmission is.
also (more crucially) it'll let you know what the capacity should be.
in this instance I think you'd do better to drain completly and refill to the volume that the book tells you to.
this may, or may not, fix the problem. but in reality, even if it doesn't fix the problem you'll have wasted what? half a day, a empty container, a trip to the dump to get rid of the oil and about $10 - $20 on a can of transmission fluid...
but you'll have gained some experiance, which is probably worth the waste even if it doesn't work.