Nissan: Sentra Transmission, nissan sentra, forward gears

Question
QUESTION: I own a 1991 Nissan Sentra, 1.6 Automatic (RL4F03A). The problem is that the car runs fine when cold but when the car warms up it only goes reverse, no forward gears.  Transmission fluid looks ok.  It can run forward all day until I stop and move the shifter to reverse and back to drive that's when I loose all forward gears.
Thank you for any information you can provide me.
ANSWER: You have a problem in the valve body. I would check that first.

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QUESTION: I replaced the valve body and still doing the same thing.  Runs fine when the transmission is cold but not forward gears once it warms up. Any ideas?
Thanks!

Answer
This may be beyond your abilty and I dont mean this in a bad way. You need pressure to operate the valve body and the clutch packs. If you replaced the body then most likley it came with solenoids??? You need to find out where your line pressure is going when you move the shifter. For some reason the low clutch is not engaging. It might be easier to simply swap it out with a good used unit. This trans is not really a poroblem unit.