The transmission uses two solenoids to command all the gears. Here is the table:

1-2 on, 2-3 on -> 1st
1-2 off, 2-3 on -> 2nd
1-2 off, 2-3 off -> 3rd
1-2 on, 2-3 off -> 4th

I can't think of any one solenoid causing your problem. If 2-3 didn't work, you wouldn't have second but you say you do. But, if the other solenoid was the problem then you could still use third.

I dunno, maybe this will help.